First power-up of a new FPV build: multimeter check, smoke stopper and a props-off bench test

None of this needs expensive gear. A multimeter with a continuity beeper and a smoke stopper cover almost everything.

Before anything is powered

Start with your eyes and the wiring diagrams for every part in the build. GetFPV's electrical checks guide lists the most common build mistakes as soldering a component backwards, to the wrong pad, or to a pad with a higher voltage. A careful visual pass catches most of them: neat joints, no solder splashes between pads, wires going where the diagram says. The author of that guide credits this step with catching a capacitor he had soldered on backwards.

Two mechanical items belong in the same pass, both from Oscar Liang's new-build checklist:

• Motor screws. Carbon fiber conducts, so a screw that is too long can reach the stator windings and short them to the frame. Oscar Liang's rule of thumb is a screw about 2 mm longer than the arm is thick, for example 7 mm screws on a 5 mm arm. To check, put one multimeter probe on a motor screw and the other on any of that motor's three wires (they are connected inside the motor). A beep means the screw is touching a winding. Repeat for every screw.
• VTX antenna. Fit it before the build gets power for the first time.

Continuity check across the battery lead

With the quad unpowered, set the multimeter to continuity mode and touch the probes to the positive and negative pins of the XT60 (or XT30) lead. You want silence, or a resistance that keeps climbing.

A short beep that stops on its own is normal. Capacitors between positive and negative charge up from the meter, which reads as a short for a moment. Oscar Liang's guide says a beep of a second or less that does not continue is not a short circuit. A continuous beep, or a resistance close to zero that does not rise, is a short. Do not plug in a battery until you have found and fixed it.

Continuity mode also helps with joints that look doubtful: check whether neighbouring pads are bridged, and if they are, or you cannot tell, redo the joint.

What a smoke stopper does not catch

A smoke stopper protects against a short or a reversed connection. GetFPV points out that it does nothing for a component wired to a pad with too much voltage, for example a receiver on a battery-voltage pad instead of 5 V. Only a voltage measurement catches that. If your flight controller has selectable-voltage pads, measure the pad in DC voltage mode before soldering a receiver, camera or VTX to it. A negative reading simply means the probes are the wrong way round, which also makes the meter a quick polarity check.

Smoke stoppers: bulb or electronic

A smoke stopper sits between the battery and the quad and stops a short from drawing full pack current. There are two common kinds.

Bulb type. The classic DIY version puts an automotive bulb in series with the positive lead. The bulb only lets through as much current as it needs to light up. Oscar Liang estimates the limit as bulb wattage divided by battery voltage: a 27 W bulb on a full 3S pack (12.6 V) allows about 2.1 A. His build uses a 3157 dual-filament bulb (27 W and 7 W) with a switch to pick the filament:

• Start on the 7 W filament. It passes about 500 mA, enough to boot the flight controller and initialise the ESCs. That can be too little for a power-hungry VTX or bright LEDs. If the bulb lights up and the FC and ESCs keep restarting, disconnect the LEDs and VTX and try again.
• Once the quad works on low current, switch to the 27 W filament with everything connected to check the arm switch, motor spin and stick response on the bench.

A short shows up as a brightly lit bulb. Because the bulbs are rated for 12 V, Oscar Liang recommends a 3S pack for testing. The designer of the circuit told him up to 15 V is fine, so an empty 4S pack might also work. He also stresses that the bulb limits current but does not stop it: if something is wired wrong, a small continuous current can still damage it.

Electronic type. Products like the VIFLY ShortSaver 2 use an electronic fuse that cuts power completely instead of limiting it. VIFLY's product page lists:

That voltage range covers 6S builds, which a 12 V bulb does not. The V2 also has a power button, so you can connect the pack first and switch the quad on when you are ready. When it trips, the manual says the red LED comes on in place of the green one.

Picking the threshold takes a little care. The manual recommends 1 A for most builds. In his review, Oscar Liang suggests 2 A for a 5-inch quad or anything with a powerful VTX, and 1 A for 3-inch and smaller builds if the VTX is not too hungry. The inrush current when you connect a pack can also trip it falsely. For that the V2 has a trip time setting (a long press on the power button enters it, according to the manual). If a 6S build gives false alarms, Oscar Liang suggests a longer delay, starting from the lowest setting and working up, ideally on a quad you already know is good.

A smoke stopper is also worth using when flashing ESCs. Oscar Liang notes that an error during flashing can switch on all the FETs at once, which is a dead short.

GetFPV adds one limit: do not arm and fly with a smoke stopper attached. The current a quad draws in flight will burn it out.

The power-up sequence

1. Props off. Betaflight's Motors tab documentation says it bluntly: always remove props before connecting a battery and testing motors.
2. USB only. Plug the flight controller into the computer, no battery. In the Setup tab, move the quad by hand and check that the 3D model follows it, with the arrow pointing to the real front. If it does not, set the board alignment in the Configuration tab. Betaflight's troubleshooting guide notes this check needs no battery.
3. Battery through the smoke stopper. Plug the pack into the stopper, then the stopper into the quad. A brightly lit bulb or a tripped electronic stopper means stop and unplug. A healthy build boots: the FC comes up and the ESCs initialise.
4. Motors, still without props. With the pack connected (on the bulb's high-current filament, or straight from the pack once the build has passed step 3), open the Motors tab, tick the box confirming the props are off, and raise the master slider slowly until all motors just spin smoothly.

Checking motor direction and order

Betaflight assumes "props in" by default: the front props spin in towards the camera. If you fly "props out", tick "Motor direction is reversed" in the Motors tab so Betaflight knows. The setting does not change how the motors spin; you still have to make them match.

• Direction. Feel each spinning motor against the diagram in the app (a strip of tape on the shaft makes it easier to see). The Motors tab has a "Motor direction" tool that spins motors gently and lets you reverse them. It works only with DShot ESCs and needs the battery connected. Betaflight's guide also lists reversing direction in the ESC configurator, or swapping any two of the three motor wires.
• Order. Raise each slider on its own. The number on the slider has to match the motor that spins, according to the diagram. If they do not match, the "Reorder motors" dialog in the Motors tab spins the motors one at a time and asks you to click the one that is turning. Betaflight's older troubleshooting guide does the same job with the resource command in the CLI.
• Telemetry errors. With bidirectional DShot on, the Motors tab shows a DShot telemetry error rate (E) for each motor. A powered ESC should show 0% or very close to it. The docs say 1% or more usually points to a hardware problem.

Last comes prop direction. The leading, higher edge of each prop faces the direction of rotation, and opposite corners take the same prop. Betaflight's guide is clear on this point too: never check prop direction while the battery is plugged in. Props go on only after motor direction and order are confirmed.