2
\$\begingroup\$

I fly model aircraft as a hobby. On some model aircraft the battery connectors can be reversed especially the cheap JST ones with a broken or missing polarity locking tab. Or, the batteries can be mis-wired or a soldering mishap can lead to the poles being swapped. The classic solution to a problem like this would be a diode, probably a Schottky one.

But on a system where currents can exceed 100A, a diode is simply not practical; cooling and size constraints come to mind, as well as the wasted power being approximately 0.4 x I. If the power to an ESC is reversed, it may cause the body diode of the MOSFETs to conduct, leading to very high currents, immediately destroying the ESC. That sucks, of course. But it may be worse; if the ESC has a BEC output (a voltage regulator), the reversed polarity could break down the regulator and fry attached devices, such as servos, receivers, cameras and the like. Which would make for a really bad day.

Being an RC aircraft application, this needs to be reliable, crash-resistant (i.e. relays are probably not an option due to their mechanical nature), small in size, and low in cost.

So what solutions for this problem are there?

\$\endgroup\$
  • 1
    \$\begingroup\$ The best reverse-polarity protection option here (weight-constraints, size-constraints, etc...) is to just get a connector with good polarization protection. Dean's ultra connectors are nice, and good for 100's of amps, and there are other similar connectors out there. Any other option is going to be pretty bulky, and waste some energy. \$\endgroup\$ – Connor Wolf May 25 '11 at 2:01
3
\$\begingroup\$

Any reverse polarity protection you add will add to the complexity, weight, loss and unreliability of the system, so it's probably not worth it.

In short: you can't really add reverse polarity protection in this case without compromising normal operations, so it's better to be careful to never reverse the polarity in the first place.

Make sure you use polarized connectors like Deans Ultra plug in stead of crappy ones that can be reversed with a little man-handling.

Be careful to put the connectors on the batteries correctly.

\$\endgroup\$
  • \$\begingroup\$ I came to post the same exact answer. Fix the issue, don't make a detector for when the issue happens. I've replaced all mine with Deans Ultra. Like a 20A US plug, there is no way to get it in backwards. \$\endgroup\$ – Joe May 24 '11 at 13:28
2
\$\begingroup\$

I don't have any experiences with RC aircrafts, but I understand your concerns. I have seen some ideas about a polarity protection circuits using MOSFET's or even something called HEXFET's on battery controlled applications. I know these would, at least, last longer than diodes emitting heat.

(just another thought) I was trying to think of a way to add a LED as a reverse polarity indicator so you could quickly unplug it, but nothing came to mind.

I hope these can help point you in the right direction:

HEXFET

MOSFET - This one is specifically for guitar effects petals, but uses similar concepts.

\$\endgroup\$
  • 2
    \$\begingroup\$ Yep. Ideally, change the connector. If you can't change the connector, then the MOSFET protection is the way to go. It won't waste as much energy as the diode. Note that the body diode needs to be connected in the direction of current flow so it doesn't defeat the whole thing. See Reverse Current/Battery Protection Circuits \$\endgroup\$ – endolith May 24 '11 at 15:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.