In direct current (DC) circuits such as we’re discussing here, Watts (power) and Volt-Amps (Volts × Amperes, VA for short) are equivalent. For example, 12V DC at 2 Amps equals 24 VA or 24 Watts. That’s not the case with alternating current (AC) circuits and inductive loads…but in the USB-PD realm, it’s all DC and Volt-Amps, though easier just to say “Watts.”
- Whatever voltage(s) a USB-PD source supports, the maximum current that might be supported at that voltage is 5 Amperes (5A), period. This is a hard limit of the specification.
- THEREFORE, the only way to actually achieve that claimed “100 Watts!” feature of USB-PD is at 20 Volts.
- Without USB-PD negotiation, the maximum power is 5V 3A or 15 Watts, but even that isn’t guaranteed. Check what’s printed on the source.
- To access more current (Amperes) below 20 Volts, a second component (in addition to the USB-PD trigger) is a DC-to-DC or buck converter. This drops one DC voltage to a lower voltage, maintaining close to the same overall wattage. For example, 20V at 5A (100W) could be converted to 5V at 20A (100W/5V), or 12V at 8.33A (100W/12V).
- This conversion is not perfectly efficient; about 5% of power is lost as heat (the actual efficiency may be a couple percent lower; 95% is good enough for “napkin calculations”). So you can’t quite access a full 100 Watts when down-converting this way, but it’s close.
- 100W-rated buck converters are relatively bulky and expensive. If a project doesn’t require a full 100 Watts, it’s fine to use a lower-spec unit!
25W and 100W-rated buck converters. These ones accept 12–24V input and produce 5V output at inverse-proportional higher current.
Notice the metal bodies and fins…when running close to full capacity, they do get warm.
- In the radio control hobby, a small buck converters is also called a BEC or UBEC ([universal] battery eliminator circuit), allowing a single battery to power a vehicle’s motor and receiver. In the past, each required separate batteries at different voltages. RC hobby UBECs are usually selectable to 5V or 6V output.
- Some projects can achieve a slim profile by “fanning out” power from the PD trigger to multiple smaller buck converters. A 100W portable NeoPixel project, for example…the LEDs could be powered in four groups, each from a compact 25W-rated buck converter. This only works with certain projects and topologies like the aforementioned pixels…never combine the outputs of multiple buck converters for more power, as each will actually deliver a slightly different voltage and may back-feed into others.
- The 5 percent or so lost in conversion is not a lot of energy, but it’s concentrated in a small area and can be hot to the touch or could melt some materials. Buck converters should be situated away from plastics, skin and curious fingers, but also not tightly sealed…a ventilated enclosure is ideal.
- There are other small inefficiencies in such a system; a couple percent lost in cables and ports. Use high-quality USB-C cables that are rated for high current. If anything is warm to the touch, it’s inadequate for the task.
These power-measuring USB-PD cables are not precision instruments, but they’re in the right ballpark and usually good enough for this sort of work.
They’ll tell you how much power a device is presently using, which may be less than in the product specifications or the capacity of any included charger. Those numbers are usually upper limits, not average use.
This multi-function USB tester works similarly, with additional information distinguishing between voltage and current…and can also measure from USB-A ports (though without all the USB-PD goodness there).
It’s highly unlikely that you need this, but if you’re pushing the limits or just really curious about the end-to-end power and efficiency of a USB-PD system, a load tester can be gradually ramped up to see where a source really hits its limit and shuts off. This sort of safety feature is one of the benefits of USB-PD over “naked” lithium-polymer batteries.
Here a couple of large power banks are run through a 20V USB-PD trigger, through a 100W buck converter to 5V, and into the load tester. One source was able to deliver about 95W, the other closer to 90W. About what we’d expect given various ratings and efficiencies.
- In practice, you really should not push any part of the system to its limits for any length of time. Allow an ample margin of overhead on sources, cables and DC-DC converters. These things do get warm when “redlined,” and that shutdown feature might put an end to the fun when least needed.
*slaps roof* This baby could power so many NeoPixels!
If you actually are planning a large portable NeoPixel project this way, these guides might be informative:
- Sipping Power with NeoPixels has tips for stretching those watts even further.
- The Adafruit NeoPixel Überguide is our soup-to-nuts reference for all things NeoPixel. Especially relevant here are pages for “Powering NeoPixels” and “Best Practices.”
- 1,500 NeoPixel LED Curtain has some information on distributing power through large installations.
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