Use a resettable fuse, so you don’t blow yours.

A few years ago, my dad entrusted me with his E-Flite Blade CX2 R/C coaxial helicopter, because I became interested in making it fly autonomously. I had a free project in one of my CS courses and decided to build a stabilizing auto pilot for the helicopter around an Arduino Mini, but more on that in a later post.

The helicopter is fairly easy to control indoors, as soon as I figured out to stay way from the ground, the ceiling, or any furniture. Flying too close to any such obstacles creates turbulence and backwash, which make the helicopter difficult to control. This also makes it almost impossible to fly outdoors, even in low wind conditions. The helicopter is powerful enough to lift itself off the ground, but can carry only a small payload of extra electronics. However, it is a lot of fun to fly!

Since the helicopter belongs to my dad, I made sure to take good care of it, but small crashes are inevitable in test flights around the living room in a studio apartment. The worst crash that could happen is a power-on blade strike, where the blades strike an object while the throttle is not at 0%. Either the blades could shatter on impact, or worse the blades are stuck, which cause the DC motors to over-current the 4-in-1 control unit. As Murphy’s law dictates, just that happened to me.

The short in the control unit from the over-current remained well hidden, since there was neither a spark nor a small puff of smoke: the #1 sign for any electronics geek that something just broke. I didn’t know something went awfully wrong until the next time I tried to fly the helicopter.

Plug the battery pack into the control unit. Go. Turn on the transmitter. Go. But the transmitter wouldn’t associate with the receiver in the control unit, which simply blinked red and green. Ok, let’s try it again. Unplug everything. Plug the battery pack in and turn on the transmitter. Same red and green blinking. Oh, no. Panic. Let me read the manual. Rebinding? I tried to rebind the transmitter and receiver with the bind plug, but that didn’t fix it either!

I spent the afternoon frantically searching the Internet for a solution, until I came across a discussion about Blade CX2s and the ‘over-current’ problem. I soon learned that the control unit was lost and this could have been easily prevented with a fuse between the 4-in-1 control unit and the DC motors. A $5 fuse? Why wasn’t this included with the helicopter in the first place? Needless to say, I was a bit furious that I had to replace the $60 control unit.

So before you have to spend $60 to replace a destroyed 4-in-1 control unit, invest $5 in resettable fuses (EFLH1206 Over-Current Protection/PTC Fuse Harness). These positive temperature coefficient (PTC) devices are non-linear thermistors, which at some specified level of current break the circuit and disconnect the motors from the control unit. The fuses reset once the current drops off on the wire.

The resettable fuses need to be installed between the control unit and each motor (not the servos). Check the photo below to see what it should look like installed on your helicopter. The fuses will add some length to the wiring, but you can easily stow them away in the cockpit above the control unit.

If you’re lucky, you may have bought one of the newer Blade CX2s, which now ship with the fuses already installed. Either way, next time you get unlucky with a power-on blade strike, you won’t blow your fuse over having to replace the $60 control unit.