Current is the rate of electricity flow in a circuit. Using the same water analogy as before, higher pressure (voltage) and a bigger pipe (lower resistance) means a greater volume of water per second (current) will flow. This simple relationship is represented by the equation known as "Ohm's Law":
Where: 'I' is current, 'V' is voltage and 'R' is resistance.
Another important equation is the one for power.
Where 'P' is power (measured in Watts), 'I' is current and 'V' is voltage.
Watts is a measure of work, or the conversion of electrical energy into some other form such as heat, light or motion. As the equation implies, it takes both voltage and current to do work.
If there is not enough current, your circuit may not be able to do the work it was designed to do. Logic circuits may not function reliably, displays may be dim, motors may stall.
On the other hand, if there is too much current, things will heat up and components may be damaged. In extreme cases there may even be smoke or flames.
Reasons for measuring current in a circuit include:
Determining circuit power requirements
Verifying correct circuit operation
- Testing power supply performance
Verify that batteries are charging or discharging at a safe rate
Estimating battery life or recharge time
Diagnosing circuit problems
Most meters have several current measuring ranges. Choose one that is good for AT LEAST the maximum current you expect to be measuring. If in doubt, choose the next higher range. There is usually overlap between the ranges and you can always go back to a lower one after you have verified that it is safe to do so.
- microamps (uA)
- milliamps (mA)
- amps (A)
Multimeters contain sensitive circuits capable of precision measurements of tiny currents and voltage. These circuits can be damaged or destroyed by high current flow. That is why most meters have a separate jack for high current measurements. This jack is fused for safety. If you are using the high-current setting, be sure to use the right jack.
Also note the warning labels indicating maximum safe levels for each jack.
To measure the current, you have to make it flow through the meter. To do that you need to make your meter part of the circuit. You need to break the circuit at the point where you want to measure the current and insert your meter in the middle. Before connecting your meter to the circuit, double check your range and make sure you have the leads plugged into the right jacks.
For this example, we are going to measure the battery supply current going into a MintyBoost:
First we need to break the circuit so we can insert our meter. In this case we will just unsolder the battery wire from the Mintyboost circuit board
Now we connect the meter between the battery lead and our temporary lead. We'll use some alligator clips to hold it all together.
The positive meter probe is connected to the positive battery lead. The negative probe connects to the temporary lead we soldered to the Mintyboost circuit board. This closes the loop and makes the battery current flow through the meter.