Switching BEC protection

[BrianG]

Due to my recent experience with a UBEC failure taking out a servo and receiver, I started researching methods to help prevent collateral damage if/when it happens again. I came up with a tiny, simple, and cheap "sacrificial" circuit that is designed to blow before the servo(s) and receiver does.



As you can probably tell, this would be installed between the BEC and the receiver. It is designed to be used with ESC integrated BECs, or external ones.

The circuit consists of a TVS zener diode rated for 7.5v, and a 3A "picofuse". I opted for zener-based protection because zeners have a tendency to fail "closed" (basically become a piece of wire). And the particular zener I am looking at has a response time measured in pico-seconds. The 7.5v rating is to provide a little voltage headroom, yet clamps dangerous voltages. I chose a "picofuse" (non-glass type) due to the environment. You may think 3A is a bit low, but this type of fuse can conduct more current than its rating in bursts without blowing.

Operation is simple. If the BEC is operating normally, the zener and fuse are basically transparent; very minimal voltage drop and full current available to the load. However, if the BEC decides to fail and send full battery voltage to its output, the zener clamps the voltage to 7.5v and shorts the excess voltage to ground. This will generate quite high current and blow the fuse. And even if the zener blows before the fuse does, it becomes a short (as mentioned earlier) so it helps the fuse blow even faster.

It is important to note that this will not protect the BEC. If it fails, it is already too late. The best we can do is try to protect the other devices downstream. So, yeah, if the BEC fails, you'll have to replace it (or the ESC if the BEC is integrated like the MMM), but certainly cheaper than replacing servos and receiver as well. And like I said, this is a sacrificial circuit, which means once it blows, time to get a new one.

Yeah, there are probably more elegant ways to implement protection, but I feel simple is usually better, and it doesn't get much simpler than this.

Ok, all that said, I want to gauge interest. I plan on making an order for the parts necessary to do some testing and make a few for myself. But I am curious if anyone else would invest something like $7-$10 for such a thing. And I will not be selling these myself, but may end up in a certain storefront. The most expensive part of this, believe it or not, is the stupid 6" servo extension lead. No matter what, I don't plan on making much (if any) money off this, but I'd like to break even and that's not even including my time. This project is for the benefit of the hobby, not my wallet.

Maybe someday, manufacturers will build in some type of protection into their BECs, but for now, something likw this will have to do.