Frequently Asked Questions
It works! But then the iPod/iPhone/device gets drained...what's up?
This is a common problem with iPods/iPhone/iTouch. These devices have very large batteries and they have an annoying bug that if they -think- they're being charged they'll stay on and not 'go to sleep' conserving power. That means that if the AA batteries die before the device is done charging, the device will stay on at full power because it thinks it is plugged into the wall. This can cause it to drain the battery. The best suggestion is to use fresh/charged batteries
and don't leave it plugged in overnight
This is especially true of iPhones, as they suck out all the battery power in a little less than an hour in order to 'quick charge'. This reduces the capacity of the batteries, but means that your phone will be up and running faster.
Once you see the charging icon change from a plug, the mintyboost is 'dead' and you must unplug it. For more information, see the user manual.
How come my iPhone/iTouch doesn't completely charge?
iPhone/iTouch's have batteries that are so large there is no way to fully charge them on 2 AA's. You can only charge 1/2 of a battery because the AA's will die before finishing
. You should use the Mintyboost to 'boost' it, but not as a way to fill the whole battery to full.
Will the MintyBoost charge my device?
Very likely! Check out our compatibility lists for more information about your favvy gadget
. If you find a new device that it does/doesn't work with, please post on the forum or contact us (thanks!)
Nearly all USB-charging devices and any device that charges with 5V power (up to 500mA+ with v2 or v3 of the kit) can be used.
If you are having problems with your device, see the other FAQ's below.
ARRGH! It doesn't work!
Don't panic! Post to the forum
(don't send email) about your problem.
A high resolution photo of the top and bottom of your kit will be extremely helpful in debugging your problem.
I get the right voltages but it doesn't charge my favorite gadget!
Is the kit a "USB Specification" Charger? If not, how can I make it into one?
Some devices will only charge from 'dedicated USB spec' charger. You may want to convert your Mintyboost to a 'USB spec' charger, which is very simple to do, follow the instructions here on how to do it
. This mod will work on any MintyBoost version (v1 v2 or v3) Then try again!
Can I buy a pre-made/pre-built MintyBoost?
We don't sell assembled MintyBoosts, only kits
. However, if you post on the forum, people sometimes offer to sell manufactured kits at low cost.
What does the MintyBoost kit come with?
The kit comes with a PCB and all components necessary to build the kit. It does not come with an altoids tin or tools or batteries. The tin isn't necessary but MintyBoost is designed to be placed in one for protection and aesthetics.
How many charges/hours of use can I get out of a MintyBoost?
This question is hard to judge because every device has different power usage. However, a simple way to calculate approximate run-time is: take the size of the internal Lithium-Ion battery (for example, many iPods have 750mAh batteries) and divide 1000 by that number.
So for a 750mA battery, 1000/750 = 1.3
The MintyBoost will fully charge the device about 1.3 times, as a best case.
Remember, this is only
an approximation and has a lot to do with the quality of the batteries you use (expensive alkalines v. cheap rechargables) and the internal circuitry of the device for recharging the battery.
For a detailed mathematical analysis, check the user manual under "How many recharges will I get?"
I want better performance, should I attach 3 or 4 AA batteries? How about a 9V?
Check out our detailed battery power tutorial!
OK but what if I want 4 AA's, 2 sets of 2 in parallel?
Check out our detailed battery power tutorial!
When I use the kit, the chip/batteries seems warm or hot...Is this OK?
It's normal for the chip and batteries to be warm or hot, especially when charging a device that is nearly drained. The chip should not get hot or very warm when nothing is plugged in.
The maximum temperature for the mintyboost chip is over 100 degrees C, that is hotter than boiling water. If you are worried, lick your finger (a little) and touch the tip of your finger to the top of the LT1302 chip. If you don't hear a sizzle (of the water boiling away), it's just fine.
However, if the batteries
are so hot that its painful to touch, start to smoke, burn or melt or leak fluid....something is wrong! Unplug it immediately and remove the batteries if it is safe to do so.
But it's really hot and I'm worried!
Yes, this is normal. When the chip is working to charge your gadget, it's going to get hot. Normally you never get to see or touch the chips in your chargers so you don't realize how hot electronics get.
Really, it's normal for the Mintyboost to get hot!
If you're concerned, you can always do the finger test above. If it's too hot, it will sizzle, otherwise, it's OK.
When I plug in a device, sometimes theres a 'hum', 'hiss', 'squeak', 'whine', etc noise...is this normal?
Yes, sometimes the inductor resonates with the boost converter and that resonance leaks into the audible range. While it's not good for it to always vibrate, it does happen occasionally and is not harmful to the charging device. you can try moving the inductor a little with your finger but the noise is electrical in nature so it's hard to stop completely.
Another thing is that it depends on how much power the device is drawing and what kind of batteries are inside.
If this is a charger, why are the data (D+ and D-) lines used?
Why doesn't the 3Gs work with the Mintyboost v2, but the older iPhone does?
What are the pullup/down resistors used for, and which should I use?
We have a full tutorial all about Apple gadget charging, check it out!
Does the v1.x work with iPhone/iTouch? Does the v2.0?
Version 2.0 pretty much works with iPhones/iTouch devices but the v1.0 cannot because it is not powerful enough. To get it working you will need the LT1302-5 which can provide 500mA (compared to 200mA from the MAX756).
We have a full tutorial all about Apple gadget charging, check it out!
Can I convert a v1.x kit to a v2.x? Can I use a MAX756 in a v2.x or a LT1302-5 in a v1.x?
No, they are completely different designs requiring new PCBs are components, they are not drop-in replaceable at all.
Why doesn't the v1.2 kit come with R1-R3 and LED1
These parts are optional: they are for the low battery indicator. There wasn't enough space to add them to the PCB using through hole parts so they are surfacemount and on the bottom. They're an 'extra' capability which you may add if you feel experienced with SMT soldering.
I'm trying to make/breadboard this kit on my own and it's not working! HELP ME!
It is pretty much impossible to get good ouput from the kit if it is breadboarded. A custom PCB or proper perf-board is essential. Most components cannot be swapped, and must be used as indicated, even if they're difficult to find in your area. All documentation is on the site.
I can't provide any support or help to 'DIYer's because of the time and difficulty in debugging these projects. Please endeavor to work out your project on your own.
Do you have any high rez photos?
Mintyboost® is a registered trademark of Adafruit Industries
Basic User Manual
Here is a basic instructional manual that will cover pretty much everything you need to know about the mintyboost you just built.
So, does it work with your gadget? First you can always visit our compatibility list which we keep updated. We have a list for versions 2.x
of the kit and 3.x
. We try to keep the list updated as much as possible - if something is not on that list then it doesn't mean it doesnt work, just that we dont know. Almost all devices charge fine!
One thing you can always do is try it out! Wait until the battery is halfway drained, then plug your gadget into the USB port and wait a few minutes and see if the battery inches up or if the charging light goes on. You can also feel the mintyboost to see if it's warming up which would indicate it's providing power to the gadget.
Which batteries to use?
Basically, get the best batteries you can. New Lithium or Alkaline 1.5V batteries work great as do fully charged NiMH rechargables. Old, worn out or dead batteries will not work at all.
We have a very detailed tutorial about batteries in case you want a lot of details
How to charge
Easy, get a USB charging cable for your gadget, nearly all come with one these days, you can also usually get one for cell phones or games at an accessory shop. Plug in the USB end into your mintyboost and the other end into your device. That's it! The gadget will draw power to charge up the battery.
While charging your device, you may notice the Mintyboost and batteries getting very hot. This is normal, it's because of how much power is being delivered to the gadget! The Mintyboost will cool down as soon as you unplug it. The maximum temperature for the mintyboost chip is over 100 degrees C, that is hotter than boiling water. If you are worried, lick your finger (a little) and touch the tip of your finger to the top of the LT1302 chip. If you don't hear a sizzle (of the water boiling away), it's just fine.
If the mintyboost is getting really hot when it's not plugged in, there may be a short! Remove the batteries and check to make sure there is insulation between the mintyboost PCB and tin.
You may also hear a 'hiss' or 'squeak' coming out of the chip. Sometimes the inductor resonates with the boost converter and that resonance leaks into the audible range. While it's not good for it to always vibrate, it does happen rarely and is not harmful to the charging device. You can try moving the inductor a little with your finger but the noise is electrical in nature so it's hard to stop completely.
Another thing is that it depends on how much power the device is drawing and what kind of batteries are inside.
When to stop charging
The mintyboost will charge and charge until the batteries give out and the voltage output isn't 5V anymore. At this point the batteries are toast and you won't be able to squeeze any more power out of them. However, some phones and mp3 players (especially ones that start with an 'i') are not very smart and even if there is no more power coming out of the charger, they stay on, which means they slowly drain back down.
Basically, the best way to avoid this is to not leave the mintyboost plugged in overnight. Just charge for a few hours or while you're using it!
For Apple products, you can also use the charging icon as a hint, while the Mintyboost is still working, you'll see this icon:
When it's done, you'll see a plug icon - the device may also beep or buzz when this happens. At this point, you should unplug the mintyboost and get new batteries for it.
User manual - Choosing batteries
The most important part of using your Mintyboost is choosing and installing the right batteries. Poor quality batteries will cause frustration, and using the wrong kinds can damage your kit!
Please read this guide which will cover all the kinds of batteries you can use.
How the mintyboost works
The mintyboost is a voltage source which means it tries its hardest to make sure the output is at 5 volts no matter what the current draw (by the target device) is. The device you are charging (mp3 player, etc) is a resistive load. That means it basically sucks as much power as it wants out of the mintyboost.
The device you are charging is almost certainly very 'stupid' - it doesn't know or care that you are trying to use AA batteries to recharge it. Most devices assume that you are plugged into the wall or into a computer (which is plugged into the wall) and suck as much power as possible to quickly charge up so you can listen to music or make phone calls.
This, of course, is a bit of a tradeoff. The higher the current draw, the harder the mintyboost has to work, and the AA's have to work. They'll get hot, they lose efficiency, the batteries drain and droop. There is no way for the mintyboost itself to fix this, it is completely up to the phone or music player!
For that reason, we need to have good batteries, that will be able to work well while being drained fast!
We suggest using two AA batteries to run the Mintyboost - that's why we include a 2xAA holder in the kit! AA batteries are common and inexpensive and provide a good amount of voltage and current. (See below for detailed information about voltage and current guidelines) However, not all batteries are made the same! Picking the right kinds of batteries will give you the best results.
In approximate order of effectiveness, here are the non-rechargeable batteries you will want to use:
- Lithium 1.5V 'high drain' AA batteries (2900 mAh)
- High quality (Duracell/Energizer) 'high drain' alkaline batteries
- High quality (Duracell/Energizer) alkaline batteries
- Non-brand Alkaline batteries
- Other kinds of non-rechargeable AA batteries (zinc, etc)
Higher quality batteries will give you longer boost times, cheap or dead batteries are the leading cause of 'flaky' behavior so be sure to get good batteries.
Don't "mix" dead and non-dead cells, or different kinds of cells. If you buy 2 batteries in a pack, keep them together. This will get you the best performance.
You can use rechargeable batteries in the Mintyboost, as well! Here in approximate order of effectiveness, are the rechargeable batteries you will want to use:
- High-capacity (~2500mAh), high quality NiMH batteries (See the standard in testing, the NiMH shootout!)
- Non-brand medium capacity NiMH batteries (See the standard in testing, the NiMH shootout!)
- NiZn batteries (? We haven't actually tested these so we are not 100% sure if they perform well but they should be OK)
- NiCad batteries
Good quality NiMH batteries work fantastic, but be careful of using really old cells. Don't "mix" dead and non-dead cells, or different kinds/brands of cells. If you buy 2 batteries in a pack, keep them together, and charge/use them at the same time. This will get you the best performance.
Alkaline or rechargeable?
Which is better? Well, that's a question that depends a lot on the battery quality. In general we always suggest rechargeables as they are less wasteful and are not much more expensive. However, sometimes you're in a bind or need the light weight of Lithium AAs. Either type will do fine, but try to stick to #1 thru #3 in the preferences above.
The tradeoffs are:
- Alkaline batteries are available everywhere, don't need to be charged before use, can be less expensive
- Alkaline batteries have a higher 'nominal voltage' of 1.5V so they provide more power just because of the voltage increase
- NiMH are very good at providing a lot of current, often better than Alkalines (although perhaps not as good as Lithium 1.5V cells)
- NiMH cells other than Eneloop and similar 'low self discharge' will slowly run down even when just sitting around
Because the run time completely depends on how the device charges, what its battery size is, etc, nothing beats timing and testing the first time you use the Mintyboost to charge your device.
Bigger batteries? More batteries?
Let's say you want more power out of your mintyboost - the best way to do this is to upgrade the batteries from AA to something beefier!
Let's start with alkalines. Please refer to this Duracell alkaline battery capacity chart:
For example, upgrading a set of Duracell AA's (2850 mAh) to C (7800 mAh) or D cells (15000 mAh!) will increase the capacity 3x (C) or 5.5x (D) !
If you want even MORE power, you can add a third battery in series to increase the input voltage from 3 volts (2 x 1.5v) to 4.5 volts. Three AA Alkaline batteries has approximately 1/3x more power than 2 Alkaline AA's, three C's have 4x as much capacity as two AA's, and three D's have 7.5x more capacity.
, there is a limit to how many more batteries you can add. For example, after 3 Alkalines, one would think you should go for 4. But 4 Alkalines is 6V nominal (and actually may be as high as 7V) - since this is higher
than the 5V output, it is not safe or good for the mintyboost or your device. For that reason, use only 2 or 3 alkaline batteries, not 4
OK, now what about rechargable AA's? First thing to remember is that rechargables C or D cells may not have more capacity. Sometimes they are really AA batteries in a large plastic shell! Assuming that you can get real rechargable C or D cells, you should be able to get 3x or 5x more capacity just by going with bigger batteries.
The other option is to add more batteries in series. You can definately go with three NiMH cells, which will boost power by 1/3 and as long as you are using NiMH/NiCad rechargable you can also try 4 AAs
. Because NiMH cells do not run at 1.5v (they run at 1.2v or so) four cells will be under 5 volts. Of course, do not use 5 or 6 AA's as that will go well over the 5 volt limit.
Lithium Ion Polymer
If you are an advanced electronics geek and want an ultra light rechargable mintyboost you may be thinking about going with a lipoly or liion battery. The Mintyboost works great with these, but be sure to watch for the following:
- Use only single lithium ion polymer cells - 3.7 to 4.2v. Don't use 7.2v-8.4v cells! those are way too big.
- Use only lithium ion/poly cell with a protection circuit. The mintyboost draws a lot of current and will drain the battery all the way down, unprotected raw cells will almost certainly be damaged, either bursting, catching on fire, leaking fluid, etc. Really, we mean it!
- Use only a proper recharger for the cells, lithium ion/poly's are delicate and cannot be charged in a NiMH charge.
- Do not parallel lithium polymer cells on your own. Power packs come with a 'balancer' circuit that must be used to keep them from discharging into each other.
Basically, don't start noodling around trying to invent your own lithium polymer cell pack!
How many recharges will I get?
This completely depends on what kind of batteries are in the mintyboost and what kind of battery is in your device. Both are energy buckets, but the size of the buckets will tell you if you can fill one with the other.
There are few things that make a difference:
- The first thing to keep in mind is that the Mintyboost converts power from low voltage to high voltage (5V) but it is not perfect, there is charging efficiency loss. The conversion efficiency can be approximated as 80% - it may be higher or lower but it's pretty close.
- The device battery capacity is mAh. (The mWh is not useful here, because the charger inside is linear.) If your gadget has a 1000 mAh battery inside, that tells you the size of the bucket you are trying to fill.
- The mintyboost battery capacity. For fairly good batteries, this is going to be about 2800 mAh.
- The fact that the most AA batteries you get have their capacity specified for low-drain use and this is high drain use. For alkalines you can probably consider the capacity to really only be 75% of what they say it is. For NiMH, maybe 80%, and for Lithium Polymer it's 100% (they are really good at this sort of thing).
- The mintyboost battery voltage. For alkalines this is 1.5, for NiMH its going to be less, 1.2V
Let's say we are using Duracell 1.5V Alkaline AA's.
First, figure out what the milliWatt-hours of the mintyboost power supply is:
mintyboost mWh = Battery voltage (V) * Battery capacity (mAh) * Capacity derating
2 * 1.5V * 2850mA * 75% = 6400 mWh
This is the battery's capacity. Next we calculate the milliAmp hours the mintyboost will output at 5V:
Output mAh @ 5V = Battery mWh (mWh)/ 5 (V) * Boost Efficiency
6400 mWh / 5V * 80% = 1026 mAh output
This how much current it can provide, and for how long. Now divide this by the size of the battery it's charging. Say you are charging an iPhone 3G (which has a 1200 mAh battery inside):
# of full recharges = MB output mAh /Device Battery Capacity
1026mAh / 1200mAh = 0.85 times
So, at best, two Alkalines can get you about 85% charged up (not more than one charge!)
Lets try again with Sanyo AA NiMH 2700mAh
(the best NiMH batteries there are):
MintyBoost mWh = 2 * 1.2V * 2700mAh * 80% = 5184 mWh input
Output mAh @ 5V = 5184mWh / 5 * 80% = 830 mAh output
# of iPhone 3G recharges = 830 mAh / 1200 mAh = 0.70
These batteries will give you 70% charge up. Less than Alkalines because the nominal voltage (1.2V) is less than Alkaline (1.5V)
Finally, lets do a 1200mAh lipoly battery
with 3.7V nominal voltage:
MintyBoost mWh = 3.7V * 1200mAh * 100% = 4440 mWh input
Output mAh @ 5V = 4440mWh / 5 * 80% = 710 mAh output
# of iPhone 3G recharges = 830 mAh / 1200 mAh = 0.60
These batteries will give you 60% charge up on your iPhone.
Note that these calculations will give you a reasonable approximation and an upper bound. As the batteries age, their capacity decreases. The internal charger of whatever you're charging may also not be 100% efficient. Keep this in mind but don't be surprised if it's an 'upper limit.'
Detailed guidelines - Voltage Requirements
The Mintyboost converts a low voltage to 5V USB standard. Because of this the first thing you must be aware of is that you cannot put a higher than 5v battery on the inputs to the circuit. If you do, it could damage the kit. So make sure your battery unput is always below 5V. Do not use 9V batteries under any circumstance!
Another important thing to realize is that the chip inside the MintyBoost also has a minimum voltage requirement, which is 2V. So do not try to run it off of a single AA battery or similar.
Your best bet is to keep the battery input between 2V and 4.5V
. Under 2V, the chip won't work, above 4.5V the chip becomes less efficiant (for extremely technical reasons that we won't go into right here) and above 5V it will not be able to keep the voltage at a steady 5V.
Detailed guidlines - Current Requirements
Many devices now charge at 500mA or higher rate, which translates into a 1000 mA draw from the batteries (because the voltage is boosted, the current is also boosted!) Getting batteries with low internal resistance and high mAh capacity is key.
Try to aim for at least 2000mAh capacity of your 1.5v batteries. For Lithium ion/poly 3.7v cells, 1000mA or higher is good as well. Larger capacity batteries also have a positive side effect that they dont droop or sag as much under high loads and have lower internal resistance!
Tons of info!
How to: How to
This page details how I went through the process of coming up with the idea, hardware, design, etc. for this project. It's not 100% correct but it's pretty close. Since this project only took 2 days (on & off) to design/test/release, it's a lot easier to keep track of than something enormous like the x0xb0x
This tutorial is quite old and the Mintyboost has gone thru many revisions since then
. We suggest that after you read this you go on to read our "Apple Charging Secrets" tutorial which will take you from v1 (here) to the latest v3
OK so where does an idea come from anyways? It's the only important question & the most difficult. I guess I'd have to say it was prompted by looking at these half-dozen projects:
OK, there's probably even some I'm missing. So what's the overarching theme here? Almost all use 9V batteries and a 7805 (an extremely common linear 5V regulator: makes a solid 5V from 7-18V input). This design works great because, well, 7805's are awesome and 9V's provide 7-9V depending upon how 'dead' they are.
However, there's one thing about 9V's that I've learned (from lots of bad experiences). One is that they don't have a lot of amp-hours: that is, how much current (amps) they can provide and for how long (hours). A duracell 9V provides -about- 500mAh over its lifetime. That's 500 mA (or .5A) for one hour or 100mA for 5 hours. That number is somewhat idealized but it's a good starting point.
Another problem is that they don't like to supply a lot of current, because they have high internal resistance (~2ohms), but basically that just means that if you want a lot of current (say to resuscitate a drained device) the 9V wont provide all 500mAh, but maybe more like 400. (Say you're drawing 250mA, then .25A*2ohm = 0.5V lost to internal resistance. For more info on 9V, read the duracell datasheet
Another problem with the 9V+7805 scheme is that a 7805 is a linear regulator. That means if you want 100mA at 5V (basically, USB power) then you're taking 100mA at 9V and then losing the 4V*100mA = 400mW (.4W) difference as heat.
As the battery wears down to 7V the heat loss goes down to (7-5V)*100mA=.2W but you're still getting bad efficiency. At best the efficiency is 72% (5V/7V) and at worst it's 55% (5V/9V) That means you're losing about a third of the battery power to heat!
I'll also throw out that the 7805 itself has a quiescent current of about 5mA so you're always losing 5% (5mA/100mA) efficiency just for regulation! (& that's at least since if you're trickle charging the battery at 50mA then the 5mA quiescent is 10%)
OK so basically the 7805+9V solution works but the efficiency is startlingly low, say 60% or so, and provides only 300mAh at 5V.
We can engineer better!
From experience, I know that AA's are great. They are cheap, have lots of power, very low internal resistance and are easily available everywhere. Whereas a 9V has 500mAh (for a total of 9*500 = 4.5Wh power) two AA's have 3000mAh each for a total of 2 * 1.5V * 3000mAh = 9Wh, about twice as much power. The only problem is that 2xAA's provide 3V and what we need is 5V. With a 9V battery we can use a linear regulator because 5V < 9V but, sadly, we cant use a linear regulator to turn 3V into 5V. Instead we will need to use a boost
regulator (also known as a DC/DC switching/step-up regulator).
The process of how a boost regulator works is somewhat beyond the scope of this document, suffice to say they work great but are a little more annoying than linear regulators because you have to pick out an inductor and wire up some extra parts. You can get a lot more info about Boost Converters at wikipedia
Case in point
So at this point I start thinking about enclosure and size. Most people think of this last, and that's a bad idea. If there's one thing I've learned from hacking on electronics, it's that you should try and select the case first because it dictates a lot of the electronics and interface.
I know that the parts for the kit must be all through-hole (no surface mount) and easy to work with. I also want AA batteries, 2 is good although I know from experience that most boost converters will work with any number from 1 and 3 just fine. I have a predilection for Altoids tins and I also know that I can fit ~2 AA's into a gum tin so I pull out a tin and take some measurements:
The PCB-mount one seems to be pretty good, it doesn't have a switch but I don't need one anyway (see quiescient calculations, below).
So I take some measurements...
Looks like I have about 1.25" x 0.7" semicircular PCB space at the top for the circuit board.
I also try out another battery holder I have, this gives me more space, 1.25"x0.85"...but the batteries go in sideways so one would have to remove the holder to change the batteries. I'd prefer that you can just take them out directly, so I don't go with this one (it also turns out I don't need that extra space).
I now do a little hack to turn the PCB mount 2xAA battery holder into a wire-lead one. Like this product photo (for "HOLDER BATTERY 2CELL AA 6"LEAD" from Digikey).
Basically I just solder on red and black 6" wires and clip off the PCB through-hole leads. This is actually a little difficult because the plastic melts and you have to sort of keep it in place while you solder. Its not suggested :)
OK looks good.
Now that's done I'm ready to think about what I can cram into that space.
So now it's time to design the boost supply. Since I don't have much space, I'm going to try to make my circuit as tiny as possible but still be easy to solder. That means I want a boost chip that is 8-DIP (smallest though-hole), with an internal MOSFET switch (1 less part) and is high frequency (to keep the inductor small). I also need to be able to supply 100mA at 5V and it should run on as low as 2V input. Also I want to be able to buy it online from a common supplier.
- 8-DIP package
- Internal FET switch
- 100mA output @ 5V
- 2V minimum input voltage
OK, lets search Digikey. I start with "DC/DC converter 8-DIP" and check "items in stock."
I then select 1 output, 8-DIP (to differentiate between 18-DIP) and select all the current-outputs >=100mA and apply the filter. There's still about 40 options. So then I select the all voltage input ranges that start with 2V or less. Also I select all the Adjustable, and 5V-inclusive output voltage options.
Looking over this list, it looks like I have a lot of options so I'm going to go back and select only the chips that can be preset to 5V (as opposed to adjustable ones that use 2 resistors to set the voltage). 5V is very common so every reasonable DC/DC chip will be available with such an option.
Now there are about a dozen options.The LT1073, LT1111, LT1173 and LT130x as well as the MAX751 & MAX756. They're all pretty much the same, so I basically make my choice based on price at 100 pieces (since I'm planning to kit it up). I also know that Maxim is great about sending samples so I decide to go with the MAX756 (datasheet
) which is $2.32/100. Note that I could have gone with any of them, so this a somewhat arbitrary choice.
According to the datasheet, I can supply up to 200mA @ 5V, run off input voltages as low as 0.7V and the efficiency is about 85% with 2 AA batteries. The chip also runs at 500KHz which is pretty fast and means that the inductor can be pretty small (~22uH) Anyway, I've used this chip before and it's worked out well for me.
The next step is to choose an inductor. This can be a bit of a pain, and there is a lot of math you can throw at the problem. However, the datasheet suggests (under "inductor selection") to get a 22uH inductor, with a ~1.2A saturation limit, and DC resistance of 0.02 ohms.
What we want is through-hole, which actually means it's going to be hard to find an inductor; almost all inductors are surface mount. But I'll take a look at what digikey has to offer. I search for "fixed uH inductor ~smd ~smt" which means I don't want SMT/SMD (surface mount) and I want a non-adjustable inductor that is in the uH range (not mH or nH). I then filter out inductors with 1-3A current and 18-27uH inductance.
That filters it down to about a dozen choices. The SLF inductor is actually surface mount, and we're going to outright ignore the ones that cost more than $2.50. Inductors for small electronics like this should cost around $1-$2, as a guideline. That leaves us with the DN7418-ND "INDUCTOR 27UH POWER AXIAL" and the 6000-220K-RC "INDUCTOR HI CURRENT RADIAL 22UH." Both of these look good, with about ~1.5A saturation current and 0.07 ohm DC resistance.
I also check out Mouser
. The online search for mouser isn't as nice as Digikey's so I end up looking at the paper catalog instead. I only found one inductor, really, the 18R223C (22uH radial power inductor) and/or the 18223C (axial version) that also has plenty of power capacity and a 0.03ohm DC resistance.
So, order 2 of each of these.
Rapid prototyping #1
In reality, what I did was look through the Digikey catalog, where I only found the DN7418 inductor (the other one was somewhat hidden in the RF inductor section). And it showed up before the Mouser box, so I spent an hour or two making up a prototype.
The circuit itself is simple, I want one large electrolytic cap for low frequency smoothing on the battery, and an output cap pair (electrolytic and one ceramic cap for high freq. smoothing). I also need the chip, a reference voltage capacitor, the inductor and a schottky diode to finish off the boost regulator. I happen to have some 1N5818's, which are often used as schottky diodes in boost regulators. I also need a USB type A female jack, of course, and two holes to solder the battery pack into.
All these parts must fit into the space left over from the battery pack. I make EagleCAD library parts for the inductor and chip (the rest are already there) and lay out the board. I'm not going to detail making library parts in eagle or pcb layout, others have done so already. Use whichever software you want, I like Eagle because there's a free version available for download if you're just making small PCBs.
Since I am know this is just a prototype version, I make the PCB single sided -- for easy etching. I also make the traces really large.
I print out a paper version of the PCB and punch the parts through to verify that they're the right shape/package.
I get my etching setup together, turn on the heater for the etching tank, and print out a bunch of tiled PCB layouts on toner transfer. I transfer the toner onto a single sided PCB and etch it in the tank.
Then I clean off the toner transfer, drill the holes with a dremel drill-press with carbide drill bits, and cut out the shape.
Then I solder the parts in, and fit it into the case with the battery pack, using double-sided foam sticky tape to hold down both the battery holder and the PCB without shorting the PCB to the metal tin.
Testing the prototype
Now we test to see if it works! With the two batteries inside, I measure the voltage on the USB connector: about 5V, which is good. I send off this version to a friend with once of each kind of iPod, including the newest 4G video iPod, for real-world testing: Both to verify the iPod will charge and also how long it will run with the additional pack.
Numbers as soon as the QA dept gets them back to me...
Verifying the math
So, in theory, we should be able to calculate the efficiency of the boost converter from datasheet info. We're basically boosting 2.5-3VDC -> 5VDC at around 50mA-100mA. Looking at the MAX756 datasheet, this is the efficiency graph:
So we should be getting around 85% efficiency, perhaps a little more. I think the only thing that can really change this number a bit is the inductor. (Below, I verify I'm getting 82% efficiency)
If we're getting 82% efficiency conversion from 2 x 3000mAh Duracells, that means we get (2 * 1.5V) * 3000mAh * .83 = 7.38 Watt hours. Compare that to a single 9V as we calculated before: (1 x 9V) * 500mAh * .65 = 2.93 Wh. So we're going to get about 2.5x more power out of these two AAs than a single 9V.
With rechargeable batteries, we get (2 * 1.25) * 2200mAh * 81% = 4.45 Wh (about 50% more than an alkaline 9V and 3x more than a rechargeable 9V)
Next, lets verify the efficiency using test equipment, and try out the different inductors to see if they make a difference. Instead of using batteries, I'll provide 3V from a bench supply that will also tell me how much current is being drawn. And instead of an iPod I'll fake the load with a resistor. Since the standard USB current draw is 100mA from 5V, that means I need a 5V/.1A = 50 ohm load. I can't just use a tiny resistor because 5V * .1A = 1/2W and most resistors are 1/4W. So instead I take two large 100ohm 'power' resistors, and twist them together. I also check the resistance to verify that together they are 50ohms. I also find a 20ohm power resistor. This will allow me to not only test a 100mA load but also a 250mA load.
I perform 4 tests with 2 inductors: 100mA load for both 2.5V in and 3V in (rechargeable and disposable batteries) and 250 load for both. All the images are up on Flickr for viewing, here is one example one...inductor #1 (DN2474) with 100mA load and 2.5Vin.
Here are my results, summarized:
It looks like inductor #2 is little more efficient, probably due to the fact it has a lower DC resistance (30 milliohms instead of 70mohm of the other inductor). It's also a bit cheaper so I'll go with that inductor.
Regardless, it looks like the efficiency is around 82% which is about what I expected.
Another thing to note is that I don't put an on/off switch in like you'd need with a 9V+7805 regulator. That's because the quiescent current of the MAX756 is very low, on the order of 100uA (0.1mA). I measured this myself and got about 75uA.
That means that the self-discharge rate is ~2000mAh / 0.1mA = 20,000 hours, more than 2 years. Most batteries don't last that long! Therefore we don't need a switch, when nothing is plugged in, almost no power is being used.
So now that I've verified that the project works, I have to figure out whether I want to sell it, how many I expect to sell, and how much I want to charge. Lots of people have different techniques for this. I tend to go with my 'gut' which usually means there's a lot of information I use but it's difficult to express it.
I tend to decide whether I want to sell something based on how popular/useful/easy it is. I think that this kit will be pretty popular and useful because lots of people have stuff that charges/powers over USB. Also, it seems like other people are selling similar things (like the 9V + 7805 type charger, or Griffin's 9V charger
, or Belkin's 4xAA charger
) It's easy to make because all the parts are through-hole and there's not a lot of them.
I'm going to basically assume I'll sell 200 or so within a few months, and I'll order parts in batches of 100, so I should budget that way. (I often buy more than 100 PCBs at a time because of the scale economies involved in PCB manufacture, as I show later.) It turns out so far that I can sell a couple hundred units of a kit in a few months, particularly if it gets picked up by a blog or web site. This may or may not be true for you, however if you can't afford to make 25 kits at once you're going to find that it's hard to make any money in the process.
To figure out how much to charge, I make up a table with different quantity prices:
||Price / 1
||Price / 50
||Price / 100
|Boost chip MAX756
|2 x 0.1uF caps
|2 x 100uF caps
|Sticky tape squares
To calculate the PCB costs, I used Advanced Circuit's
These prices are for 2 PCBs, which I'll cut in two, because it's cheaper (probably because they don't like dealing with very small circuit boards). I usually go with 2 week turn prices. Note that the PCB quote doesn't include the $150 one-time tooling NRE fee, which adds $3 to the /50 price and $1.50 to the /100 price. Advanced Circuits is a little expensive, but they're very good on quality and they're good at catching mistakes. Anyways, you can try going with a cheaper shop but I can only vouch for these guys.
There's also shipping prices included, maybe $1/per. In general, I double the parts cost to come up with the 'retail' cost. In this case, I'll charge $19.50. Anything less than $10 or $20 is great because $20 are considered to be stuff/food coupons, really.
There's a bit more work to do. First, I redesign the board since I'm going with a radial inductor instead of an axial one:
I actually do another etch test, to verify eveything one last time. Then I tile two boards together (cheaper) and generate gerbers.
I use gerbv (free software) for viewing and verifying the gerbers. On windows, I use GC-prevue.
I always check the boards with www.freedfm.com before I ship them off to be made. I used 4pcb.com so it's the same company but even if you don't go with 4pcb.com as your PCB manufacturer, it's a neat service.
A week later (depending on your turn time) A box shows up with the circuit boards!
Then I sit in front of a computer and do a lot of website stuff. I also take a lot of photos. A good photo setup will make documentation easy. I have a simple 150W ECT bulb + diffuser setup at EYEBEAM. A tripod is key!
Mintyboost® is a registered trademark of Adafruit Industries
The mysteries of Apple device charging
TL;DR? Watch this video which not only talks about iPhones and charging but also hot air rework.
Want more details? Read below!
Quick introduction to USB
Every computer has a USB connector on it, and all the connectors are the same, with 4 pins. One pin is Ground, two pins are Data (D+ and D-) and the last pin is 5V power. The Ground and 5V pins are used to provide power to whatever is plugged in - keyboard, mouse, USB key, etc. The two data lines are used to transfer information back and forth - what keys are pressed, saving files to the USB drive, etc.
Shown above, the 4 wires. Red is power, black is ground, and the white and green wires are the data lines.
Using USB as a power supply
Some inexpensive USB toys (say a USB fan or mini soda cooler
) don't have any data transfer, they just suck the power from the USB port to run. In this case, they do not use or connect to the Data pins (they are left to 'float').
USB ports can provide up to 500mA of current output, and technically
every device is supposed to perform a basic data transfer to the computer (called enumeration
) where it says "hey, I'm about to drag 500mA out of the computer, just so you know" and the computer can say "go ahead" or "no can do" (this is called the power negotiation
However, we've found that every device that does not require to do any data transfer (say, USB fans or USB battery chargers) don't bother to warn the computer and just go ahead and grab up to (or even more!) than 500mA from the USB port. As long as they aren't going thru an unpowered hub, this seems to be just fine. All computers have a resettable fuse on the USB port so that if more than 1000mA is drawn, the power will be disconnected. This protects against short circuits in your $5 USB fan that would take down the entire computer!
Knowing the above, we designed the first Mintyboost to not have anything on the data lines - we assumed that nearly every charger and device would just ignore those pins as they tend not to be used
For example, in the CAD file of the first Mintyboost, the USB connector is the top square. The four ovalish pins in row as the USB wires. Pin #1 and #4 are used for power (blue lines are connected) but there are no traces on the middle two data pins
- they are floating.
When we first released the v1.0 of the MintyBoost oh so long ago, we quickly got feedback from people who owned all sorts of Apple brand gadgets. It turned out that older devices worked fine but some of the newer ones, such as the iPod Mini, were not charging. Hmm! Time to experiment!
We figured there was something simple that would make the Apple device charge, and it definately had to do with the data lines (the power lines are fixed at ground and 5V). We thought "is there a enumeration chip inside every charger?" but since that's expensive and kind of overkilly we decided instead to read up on the USB protocol (go Jan!). In particular, in her fantastic book there's a part about the low level signaling states. Since you want to get the iPod charging, but NOT make it try to enumerate, we figured that we should see if there was some sort of special state you could put the data lines into that would say "no computer is attached but there is power". Turns out there is! It's called the SEI and occurs when BOTH data lines are at 3V.
Now to test. We stole an iPod from a friend and cut open a cable so we could mess with the data lines. We tried 3 options each - connected to ground, connected to 3V and not connected (float). At the same time we measured the current draw going thru the power line. We found the following:
OK, so yes when the pins are floating no charging occurs. The next thing to note is that whether the pins are pulled up or down effects the current draw. Since this was the first Mintyboost, using the MAX756, we wanted to use the lower 100mA rate so one pin pulled down and another pulled up. This would be more efficient for the battery use and keep the chip from getting hot (it could provide 250mA but didn't like it much).
Thus was born Mintyboost v1.1!
Note that because the mintyboost runs from 2 AA batteries, we can get 3V directly from the batteries so we connected the pullup right to the battery input.
The iPhone and larger iPods
This worked ok for about a year, then people starting getting the newer iPods and noticed that the Mintyboost didn't work anymore.
Hmm, looks like we need to have two pull ups after all. We made a new version that now had either a pulldown or pullup on the D+ line.
This worked for a bit until the iPhones came out. With enormous batteries, the iPhone was not happy charging at 250mA - it wanted 500mA or even 1000mA to charge! We sought out an upgrade to the MAX756 and found the LT1302 which could provide 500mA no problem.
Using 100K pullups on the data lines worked pretty good and all was happy! Then the iPhone 3Gs came out and...
Apple stopped being as 'lax' with the charging interface and started being very picky about having the official chargers. We still doubted that there was an enumeration chip inside each charger - too expensive and complex. So there must be something else going on in those data lines.
Time to sacrifice an official Apple iPhone 3Gs charger!
Taking it apart, desoldering the 4 data line resistors and measuring them on our multimeter, we found the following as shown in the schematic:
The four resistors create a voltage at each of the data lines that's not 3.3V but rather 2.8 and 2.0 (or so) volts. The problem is that when you do this, the iPhone starts to draw as much as 1Amp! Way more than the LT1302 and a couple AA's can provide. We were a bit sad and thought that there was no way to get the Mintyboost working with an iPhone when we took at trip to J&R and found an item called the TuneJuice. The TuneJuice is an iPhone charger that uses 4 AAA batteries. This is very interesting because there is no way to get an Amp out of AAA's - they are just way too small. That means there must be something ELSE going on in that TuneJuice charger to keep the iPhone from gobbling up the batteries. So we took apart the charger!
And found the following! (We substituted the closest 1% resistor values)
This time both voltages on the data lines are = 49.9K / (49.9K+ 75K) * 5.0V = 2.0V
We did some experimenting (see the video up top) and determined that in fact the different voltages/resistances did effect the charging rates! Using the 2.8V&2.0V setup resulted in a 1 Amp charge rate and the 2.0V&2.0V setup resulted in a 500mA charge rate.
This made us very happy, because 500mA is within the capability of the MintyBoost chip. We redesigned the PCB to allow us to have 4 resistors on the datalines and put two 75K and two 49.9K resistors in each kit. So far we have had no problems charging any of the latest Apple devices. Hooray!