Save your electronics, build a SmokeStopper!
We’ve all been there… Working all day on a new build, you finally finish, you give your new beast one final look-over and don’t see anything wrong. So you plug it in and TZZZT! SNAPP! POP! and instantly, your plans for new-found aerial bliss disappear literally in a puff of smoke.
You tear it down and find a blown ESC caused my a motor with a too-long screw into a winding or a little blob of solder that you SHOULD have caught before you put the new shrink-wrap on, but DIDN’T.
Now you have to tear it apart and replace the toasted ESC before you can even see if the motor is salvageable. You already know you’re going to be ordering a new one shortly; the only difference is whether you’re going have to wait for it to come in the post or if you ordered enough to have a spare and will need to replace it.
Enter the SmokeStopper –
Nothing exciting; quite the opposite.
This is a little gadget that hobbyists have been using for nearly a century; a light bulb as a current-limiting device to provide power to a device under test. The wattage of the light bulb is chosen such that if there’s a dead short in the device under test, instead of allowing massive current to flow and letting out the Magic Smoke, the bulb will glow and only the amount of current needed to illuminate the bulb will flow. This only needs to be enough to allow the device to initialize; not enough for it to operate at full power.
This can prevent fatal current surges from ever hitting many electronic components; saving the part and allowing you to get back to troubleshooting the original failure instead of having to replace burnt-out parts and THEN troubleshoot.
In our case, since we’re dealing with a small quad, we need 2-3A with a 3S pack to be able to power up, arm, and start the motors with props on at just above idle. As luck would have it, a common everyday 3157 automotive taillamp will flow approx 2A through the heavy filament, and another 450-500 mA through the lighter filament. If we connect them in parallel, we get a Current-Limiter in the 2.5A range.
CAUTION: Use a little common sense!
This video shows me sticking my hands in running props on a 250 quad as a test exercise. Remember this is a TOY-size quad with plastic props. Even with the torque-limiting provided by this device, you can probably still get hurt by larger quads and Carbon or Wood props. If you build a SmokeStopper with larger wattage or multiple bulbs for higher current to be able to slow-spin up larger quads, it will DEFINITELY allow enough power to flow through your motors that you can get hurt doing this!!!
Here’s how it works.
The concept is simple: Make this device; build it with the power connectors YOU use (Mine will use the super popular XT-60s) to make it so convenient you won’t NOT use it. Then, EVERY TIME you work on anything where you move or work on anything electrical/electronic, you plug this in between your battery and the Quad to test for shorts.
If you have an old 3S battery that you just use for bench-testing, leave the SmokeStopper plugged into it so you don’t even have to think about using it. Or… make a long set of leads to power from your 12V Workbench Power Supply; you’ll NEVER be without it! Just remember not to use higher than 3S/14V power for testing; you don’t want to blow the bulb if you do have a short.
Remember, this device needs to be sized to the aircraft. This one works well with small 250-sized quads running 1806 sized to 2206 sized motors. It will slow-spin up a single 3548-950KV with a 14 inch prop on my 72″ Piper Cub. I’ve tested with my 2212-1050 equipped 450-size FPV Spider; it will arm and just spin the motors above idle nicely.
Even if you have a larger craft that can’t slow-spin up on the current it provides, the single 3057/1157 SmokeStopper can STILL save you from smoking stuff if there’s a random dead short present.
This is a device that you NEED on your bench, no matter WHAT you fly! Or DRIVE, for that MATTER!
If you try to used this on a brushed nano or micro-quad, it will still probably flow enough current to blow transistors off the board if there’s a short. Look for a 5-7 watt bulb for this application, or use only the smaller filament from a 3057/1157.
Here’s what we need: a 3157 taillight bulb, 3 leftover ESC wires, a pair of the connectors YOU use all the time, some Red & Black heat shrink tubing to match (+) & (-) on the connectors and Epoxy to pot the wires onto the bulb once everything is soldered. The big heat-shrink is optional if you pot the wires properly. (FYI – A 4057, 3057,3357, 1157 bulb will also work; the 3157 is just what I had handy. This isn’t rocket surgery; the tolerances are pretty broad.)
The first step I’m going to assume you already know how to do; making a main power input for a quad. On the XT-60s, it helps keep the connectors properly aligned (the plastic housing tends to get soft during soldering) if you plug the female connector into the end of the male connector before soldering. Let the soldered poles cool completely before moving on to the other pole and again before unplugging the connector.
Here I’m tinning the (+) pole of the female connector; note that I’ve kept the male connector and the wires on the other end to act as a heat sink; again, to keep the pins aligned properly so they plug/unplug easily.
I originally made this using thin 20ga wires from a small ESC; I found those were not strong enough to stand up to tugging on the XT-60 to pull it apart. This version, I’m making with 16ga ESC wires.
Don’t forget to put the heat-shrink tubing on the black wire BEFORE you solder it to the (-) pole on the female connector. Note that I’m placing the wires so they overlap; this it to help keep the Red wires oriented correctly.
Now a little clear heat-shrink to keep the red wires in a “Y” configuration…
…and pull the Red wires up behind the loop of the black wire to keep things tidy.
Use a thin blade to lift BOTH contact wires on one side out of their slot; it doesn’t matter which side you start on.
Now pull them both out straight…
…and bend them over the housing to the other side, next to the contact wires already there.
Now, tin both pairs of contact wires together like this.
Don’t forget to place the big heat-shrink tubing BEFORE you solder the Red wires!
And here it is, all soldered up. I’ve found that the contact wires here can be a bit weak to hold up against handling, so I’m going to pot the wires with Epoxy.
You want to use plenty of Epoxy…
…you need to completely encase the wire to provide strain-relief or they’re going to break in no time. If you don’t mind the look, you can omit the Heat-Shrink; I just like the look of the finished product better.
I prefer to use Epoxy for this purpose over Hot-Glue; light bulbs ARE a heat source. You can probably get away with Hot Glue if you’re just using this to protect your gear at first power-up; it should never be on long enough to get hot. However, if you intend to use it for motor testing or as a discharge load (As I intend to do), you should pot with Epoxy so it doesn’t matter if it runs lit up for extended periods.
Well, there you have it. Test with your other quads; you may be surprised at which ones it will actually work for. I’m thinking anything above 450-size will need at least 5A to arm and run up to just above idle; so probably two 3157 (or equivalent listed above) bulbs in parallel will yield enough for them. I have NO IDEA how much current will be needed for Hex or Octos; they’re a bit out of my range at the moment.