Multimeter is one of the first items you should acquire when getting into FPV. In this tutorial I will explain how to use a multimeter and how to “test your drone” with it. A modern multimeter is also sometimes called a DMM (Digital Multi-Meter).
Both multimeter and “smoke stopper” are excellent tools for performing electrical checks on FPV drones, to prevent electrical damage and magic smoke when powering up for the first time.
Here is my tutorial on how to build an FPV drone from scratch.
Buying a Multimeter
You can buy a basic multimeter for as cheap as $5. But this is something you are going to use for the many years to come, I recommend spending a bit more on a good quality DMM.
- BSIDE ADM08A (recommend, feature-rich and good value):
- INNOVA 3320: https://amzn.to/2QkwGss (cheap and basic)
A DMM can be used to measure voltage, resistance and current. Modern multi-meters even have advanced features, such as continuity test, diode test and measuring capacitance.
A basic multimeter has two probes and a dial in the middle for selecting function.
There are usually 3 to 4 ports for connecting the probes depending on the function. The black probe should always be plugged into the black port as the ground (COM). The red probe can go into one of the red ports depending on what you are measuring.
Don’t be daunted by the amount of functions/modes that are available, we normally only ever use 4 or 5 of them when testing an FPV drone. Here I will explain how to use these basic modes.
How to Test Continuity
The first thing you should do when finish building your drone, is to “check the drone with a multimeter”, this basically means “continuity check”.
Continuity mode checks for short circuit between two points, and if there is a short circuit, the multimeter will beep.
Before powering up your quadcopter for the first time, you should check for continuity between the positive and negative wires of the power lead.
It’s also useful to check solder joints that looks sketchy, you can perform continuity checks to see if the pads are accidentally shorted together.
You can also validate if solder pads are connected together on a flight controller.
Make sure your quad is powered off when doing continuity check. Sometimes the multimeter might beep for a split second then stop. That can happen if there are capacitors between positive and negative. When you touch the pads with your probes, it charges the capacitors up so the meter will think there is a short and beep, but when the caps are charged the beep will stop. That’s normal and nothing to worry about, it’s not considered a short circuit if the DMM only beeps for a second or less and doesn’t continue to beep.
- Check if the positive and negative of XT60 lead are shorted
- Making sure motor screws are not too long
- For finding solder pads that are connected on an FC, or which wire is which in a long harness
How to Measure DC Voltage
Before connecting power to a device, you should always make sure the polarity and voltage are correct. You can verify both with a multimeter.
Rotate the dial to select a voltage range that is definitely higher than the voltage you are measuring. If you are not sure what voltage to expect, just start from the highest range and work your way down. By using the lowest possible voltage range, your reading will be more accurate.
If you get a negative sign, it means the probes are the wrong way round. You can use this property to find out the positive and negative of a battery and any DC voltage source.
- Verifies voltage
- Checks for positive and negative
How to Measure Resistance
You rarely need to measure resistance, I mostly use it to confirm the value of a resistor when I am too lazy to look up the color codes.
Actually continuity mode is based on resistance measurement. If your DMM doesn’t have continuity mode, you can just measure the resistance instead. A short circuit has close to zero resistance.
Same as measuring voltage, you can start from a higher value, and work your way down if you don’t know what to expect. Using the smallest possible resistance range gives you better precision.
You can use a multimeter to determine how much current a component draws. To perform a current measurement, the probes should be connected in series with the device you are measuring.
There are usually two connectors on the DMM for current measurements – one for lower current (milli-amps and micro-amps), the other one for higher current (amps). Again, if you are not sure how much current you are measuring exactly, you should start with the higher range and work your way down.
Note that a multimeter is not designed to measure high current, typically not higher than 10 or 20 amps. Anything higher you should be using a power meter or a clamp meter instead.
With a DMM it’s okay to measure low amp draw devices such as VTX, FPV camera, or RX, but for anything that is power hungry, like motors, you should NOT use your multimeter or it might blow up the fuse.
A clamp meter (buy: http://bit.ly/2L7Q9t1) is a much easier way to measure high current. It allows you to take measurement instantaneously by clamping the jaws around a wire, which means you don’t need to break into the circuit to take measurement.