Fail safe BEC?

My CC BEC just decided to stop regulating voltage and sent full battery voltage to all my components. So my new spektrum Rx, hitec servo, and cooling fan are fried.
Of course, CC will not cover anything beyond the BEC.

Anyone know how to add an additional simple circuit (like a zener diode/resistor shunt) that can clamp the voltage to prevent this from happening again? 300$ is now toast due to a 20$ little BEC. Seems to me BEC's should be designed to fail open, not closed.

[BrianG]

Wow, that does suck! Yeah, some type of circuit would be nice. But, shunting devices are designed to handle very brief pulses of voltage, not steady-state overvoltage. Doing that would just make them blow after a sec or two, and then you'd have full V output again.

I would probably opt for a voltage controlled switch instead. Maybe use an op-amp (typical LM-741) set up in comparator mode. The - input would go to the BEC v output, and the + input would go to a resistor/zener (~8v) divider, also tied to the BEC output line. The op-amp output would feed a PNP transistor in switch mode that is always on provided the BEC output V is lower than the zener v. If the v on the - input exceeds the zener v, the output goes low switching off the transistor. Since the transistor is operating in switch mode (full saturation or off), there should be VERY little heat/power loss. I haven't worked out the details, but such a small simple circuit should be able to be easily added in-line between the BEC and your load.

EDIT: Moved thread to the CC section. Maybe the right people will see this thread...

Brian, thx for the reply. However, I am a dumb mechanical engineer so what you mentioned is beyond my abilities. If you ever devise such a device, sign me up for a pre-order.

[BrianG]

I'll see what I have in my parts bin and rig up a little prototype. Beta version is "coming soon". (Taking a page from CC's book - j/k Castle guys... I just HAD to say it. )

Why can't I run two BEC's in series (fist at 7V out, second at 6V), with maybe an LED/Resistor in series between the two set up such that the LED won't light unless the voltage out of the front end BEC is over some critical point?

[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.

sign me up for one, Brian. You will be rewarded.

[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

[BrianG]

Cadima: OK, I'll see what's in my parts bin and do some prototyping. Be warned, the result (being through-hole components) will be about as large as the BEC itself. Might be a week or two before I get sufficient time to work on it as there is a lot of things going on right now...

Patrick: Thanks for the input!

I was figuring more like 0.2-0.4v across the transistor because it is being run into saturation. But yeah, a FET would work better I'm sure, I just never worked with those too much. Heck, even a relay would work for this application, but I'd rather stick to a solid state solution.

BTW: Is the issue where the BEC sends full batt voltage on failure something you are aware of? Your BEC is a nice small unit, it would be a shame to have to add a circuit (which would be physically about the same size) for protection...