“Once you let the magic smoke out, you cannot put it back in”. That quote was the first and perhaps the most important piece of information from my electronics 101 course. Using a multimeter and smoke stopper to perform FPV drone electrical checks is an excellent way of preventing electrical damage and magic smoke loss to your craft. Completing FPV drone electrical checks also helps to make the hobby a more enjoyable and cost-effective experience. This article will aim to outline the correct method of using these electrical testing devices and discuss a series of tests to help prevent the magic smoke from escaping.
Series and Parallel Circuits
FPV drone electrical checks require the multimeter probes to be placed either in series or parallel with other electrical components to measure them. Parallel means setting the multimeter probes across or on either side of a component. Basically, the circuit will continue to function with and without the multimeter probes attached. As seen in the diagram below, the 5V output on the flight controller (FC) would continue to supply 5V to the camera, even if the multimeter probes aren’t connected. Parallel is used for almost every multimeter measurement (including voltage).
Series means putting the probes in line with the circuit so that the circuit would be open or disconnected without the probes attached. As shown in the diagram below, the multimeter is placed in series with the video transmitter (VTX) and the 5V pads on the FC. Series is used mostly for measuring current.
Multimeter Units of Measurement and Range
A multimeter has a main dial which is rotated around to configure the measurement mode and range. Measurement modes include voltage (V) and current (A). The range of a multimeter is the maximum value it will display in a set mode. For example, the multimeter will measure a maximum of 20V if set to the 20V mode as pictured below. I would highly recommend purchasing a multimeter from re-sellers such as Element14 if you do do not already own one.
My trick to choosing the right multimeter range is to pick a range that is slightly larger than the expected reading. An example is selecting the 20V range to measure the voltage of a charged 16.8V 4S LiPO.
Except for current, putting a multimeter into a specific range will not blow it up if you attach the probes to a larger measurement. I’ll explain this further for each separate multimeter mode.
Multimeter range is measured using the SI prefixes. These units come before the electrical SI units to specify a tolerance. This is done as multimeters are only able to measure 3 to 5 significant figures with their screens. A table of prefixes and electrical units is shown below for the most common multimeter measurements.
SI Electrical Units
|Voltage (V/volts)||Current (A/amps)||Resistance (Ω/ohms)|
|200mV: 200 milli volts/0.2 Volts||200μA: 200 micro amps/0.0002A||200Ω: 200 ohms|
|2000mV: 2000 milli volts/2.0V||2000μA: 2000 micro amps/0.002A||2000Ω: 2000 ohms|
|20V: 20 volts||20mA: 20 milli amps/0.02A||20kΩ: 20 kilo ohms/ 20,000 ohms|
|200V: 200 volts||200mA: 200 milli amps||200kΩ: 200 kilo ohms/ 200,000 ohms|
|1000V: 1000 volts||10A: 10 amps||2000kΩ: 2000 kilo ohms/ 2,000,000 ohms|
Voltage is the potential energy difference between two points of an electrical circuit. It is measured in parallel with power connected to the drone. There are two voltage symbols on a multimeter V~ which represents alternating current (AC) voltage and V (sometimes with vertical lines next to it) which represents direct current (DC) voltage. Most primary electrical systems on the drone such as the flight controller use DC, however, the ESC’s operate the motors using AC. I do not recommend performing any AC voltage FPV drone electrical checks. Most multimeters can measure from 0.002V to 1000V. Drones use 3.3V to 16.8V electronics, so the 20V range is often all that is required.
Resistance measures a material’s ability to oppose changes in current and is denoted by the symbol Ω and measured in ohms. For example, high motor resistance will cause hot motors due to a high opposition to current changes. General resistance checks are best done in the standard 200Ω tolerance. Resistance is measured in parallel with no power connected to the drone. Observing the diagram below, the multimeter is set to 20K setting and reading a value of 9.83 on the resistor. This means that the resistor has a value of 9.83K/9,830 ohms.
Current is the amount of charge moving past a point over a given time. Current is denoted by the symbol ‘A’ for amps on the multimeter. The current of a circuit is always measured in series with power connected and with the red multimeter probe plugged into the 10A current socket as shown in the diagram below. For all other electrical checks, the red probe should be plugged into the socket marked with volts and ohms symbols. The black plug is always plugged into the common ‘COM’ or ground socket. Because multimeters cannot usually measure more than 10 amps of current, low current components should only be measured. These include all components but the motors, ESC’s or power distribution boards.
Continuity checking tests whether two points on a circuit are connected. This is useful when determining if two close solder pads have been connected or not. I also find it helpful in finding alternate solder pads if one is pulled from the board. Continuity is measured in parallel with no power connected to the drone.
FPV Drone Electrical Checks
FPV drone electrical checks are a vital part of a multirotor build. These simple checks could potentially save you a blown up brand new $40 flight controller or damage to other pricey electronics. When performing electrical checks and building a drone, it is vital to have the wiring diagrams for all electronics on hand to simplify the wiring processes whilst reducing the chances of any magic smoke escaping.
When building a drone, the most common mistakes are soldering a component backward, to the wrong location, or to a higher voltage pin. With these FPV drone electrical checks, these mistakes can be remedied before causing any severe damage.
Datasheets and Specifications
If checking an electrical component, I recommend that you first download its datasheets, wiring diagrams and the specifications of the components you have chosen to build with. This information provides reference material when checking the wiring of electrical components and helps to eliminate any obvious electrical issues.
Motor Resistance Check
Unless your multimeter has a lower resistance setting than 200 ohms, it will most likely not be able to measure the motor resistance accurately. Resistance FPV drone electrical checks can determine whether a motor has burnt out or has shipped with a defect. Turn the multimeter to the lowest resistance settings and connect the probes onto each wiring pair. Most motors should measure around 50 milliohms. Anything lower and you have yourself an excellent quality motor. Anything higher and you have a burnt out or low-quality motor. Because the motors must be disconnected from the ESC’s to perform this FPV drone electrical check, it is best to complete this before building your drone.
Before plugging a battery in or soldering components together, I like to perform a quick visual inspection in case there are any easily noticeable defects. After soldering up all components, I also like to perform another visual check, before plugging a battery in to make sure that the solder joints are neat, wired correctly and that no solder has splattered or connected to an unwanted location. If this visual check is done to a high standard, and with reference to the wiring diagrams, most potential issues can be spotted here. This check has been especially useful to me when identifying a capacitor which I had soldered on backward.
Before soldering electronics to a power distribution board (PDB) or flight controller (FC), I like to measure the voltage at power pads on that components will be soldered to. This helps to prevent soldering components backward or to a higher voltage power pad. This FPV drone electrical check is especially critical when an FC or PDB has selectable voltage solder pads as providing a receiver with 12V rather than 5V (or usually 3.3V for team Spektrum) will result in some magic smoke, courtesy of your receiver. Voltage checks are completed in parallel as the circuit will remain operational with or without the multimeter probes attached.
Current FPV drone electrical checks can be used to determine how much current a component is using. When performing current FPV drone electrical checks, the probes are connected in series, and the drone should be powered. I will also remind you that it is not wise to measure motor, ESC or battery current as it can result in your multimeter blowing up (or at least its fuse) due to the 10A measurement limit. You should only perform current FPV drone electrical checks on the low voltage electronics (VTX, FC, camera, etc.) unless using a high current multimeter.
Low current components will usually draw from around 100 milliamps (100mA) to 1 amp (1A) however you should check your datasheets for each components specification. The multimeter also needs to have its probes moved to the current plugs as demonstrated in the diagram below. It is also not a good idea to try and measure voltage while the multimeters’ probes are connected to the current plug otherwise this can also blow it up. The diagrams below demonstrate the correct multimeter setup to measure the current draw of your VTX.
Continuity FPV drone electrical checks are the best way of determining whether a wire is disconnected or if a rogue solder blob has made it between two solder pads. Usually, I perform a visual inspection of my solder joints to check for bridging between joints, but if I spot anything remotely sketchy, I use the multimeter to perform a continuity check. Continuity checks are also useful for validating that a solder pad is grounded by putting one probe on the black battery wire and the other on the suspecting pad.
Setting the multimeter to continuity mode, it will beep or display a number on the screen other than 0 or 1 when it detects that the two multimeter probes are connected. Continuity checks should only be performed when the flight controller is not powered.
This check is also useful when determining where a wire has become disconnected. I have been caught out before when my 4 in 1 ESC was plugged into the FC, however, a pin was not connected correctly. This lead to a motor not operating. Using the continuity check along different sections of the ESC wire and connectors, I was able to find this issue and swap the connector to remedy it.
Using a smoke stopper is the final one of the FPV drone electrical checks. A smoke stopper is a type of fuse. When a circuit is plugged in backward, infinite current tries to flow around which ultimately fries your electronics. A smoke stopper breaks the circuit before the high current begins to flow around the loop thereby keeping your electronics in working order.
The ideal smoke stoppers include resetting fuses and can be used on multiple occasions without having to swap a fuse or light bulb if you miss-solder something. This is the type that GetFPV sells.
Plug your battery into your smoke stopper and your smoke stopper into your quad. If the drone’s lights are still on after a few seconds and the fuse or bulb on the smoke stopper is operational, then you have yourself one working drone and all FPV drone electrical checks have been passed. When using a smoke stopper, you shouldn’t arm or attempt to fly your quad with it attached as the excess current draw will burn out the smoke stopper. Another important note is that a smoke stopper will only prevent broken electronics due to short circuits or reverse polarity electronics. A smoke stopper will not prevent electronics connected to a high voltage wire from smoking. For that, you must use a voltage measurement mode on your multimeter.
Smoke stoppers and multimeters are cheap tools for performing FPV drone electrical checks and can potentially save you a more significant sum in the long run from unbroken electronics. Just in case an electrical component blows up, having spare electronics is a good idea. To read more about that, see Jon E 5’s article on why spare parts and backup gear is essential for FPV.