Fail safe BEC?

[Pdelcast]

Originally Posted by BrianG

I suppose you could, but it would be kinda clunky, not to mention $$$. The LED thing could be done, but it's not "active protection" (meaning: it still relies on the user to notice something and stop running). I still think an overvoltage switch is the best idea since it would disconnect if it saw anything over a certain voltage.

I drew up the idea I was thinking of:



U1 is pretty much any Op-Amp that can work off a single supply as low as 5v and as high as ~30v (for if the BEC sends full v). The LM324 looks like a decent candidate.
D1 is a Zener diode to set the trip reference voltage.
R1/R2 is a voltage divider to fine tune the "trip" point in case the zener diode is not a perfect match.
Q1 is a PNO transistor set up as a switch. Power lost should be very minimal so no heatsink is needed.
R4 limits Q1's base current.

Since the output current will be limited by the op-amps output X Q1's hfe, you'll probably need a darlington pair. A single transistor that can handle ~10A usually does not have high current gain.

All these parts can be obtained at RadioShack for around $10-$15 at a guess.

Using a darlington pair would mean a voltage loss of about 1.2V across the pair -- at 2-3 amps that's a significant amount of heat to dissipate on the PNP darlington. It would need a heat sink.

OR, you could use a P-channel MOSFET in the same circuit -- a good FET would not require any heat sinking, and wouldn't lose any output voltage. Add a pull-up from source to gate. Something like:

http://www.fairchildsemi.com/ds/FD%2FFDD8778.pdf

about $.25 or so...


Patrick