So in the rush to get ready for SVR this week, I wired the leads on one of our batteries backwards. I didn’t notice and no one else noticed, so when we plugged it in apparently our breaker made a clicking noise and may have tripped at some point. Regardless, it was left plugged in backwards and on for roughly 10 seconds.
The bad part - the damage. Apparently our VRM is shot, our PCM may be nonfunctional, and our PDP only supplies power to certain rails. Is this a problem that someone else has ran into and had to fix? Or are we just out of luck?
Interestingly, none of these are the failures I would expect.
The VRM is reverse polarity protected. The PCM is not as explicit in the documentation, but I thought it was as well.
The PDP outputs are just copper, to damage it like that you have described, you would have had to fry the traces off the board. Are you sure it isn’t the devices attached to those rails that are damaged?
Your motor controllers are the most likely to be damaged by a reverse polarity event.
Devin,
To check the output of the PDP disconnect all your loads and try a known good device connected to each output. As pointed out above, there is nothing in the distro that can go bad. However, there is a lot of electronics in the PDP that will be affected by reverse polarity. My suggestion (I know you don’t want to hear this) is recycle everything. If it isn’t bad now it will fail when you need it the most. The clicking you heard is the resettable breakers.
Are you sure your PDP is shot? Talons/motor controllers burn out super easily; I would check those before replacing the PDP, as it’s a pretty nice piece of equipment that is well protected. I wouldn’t be surprised if your VRM and PCM needed to be scrapped though; sorry about that.
I know it’s prohibitively difficult to put reverse voltage protection on the PDP input. But what about adding an inexpensive (~$2) 70dB buzzer with a diode so that it screams when you apply power backwards? That might give the user a chance to open the breaker quickly enough to avoid frying everything.
Ever since we fried a D-link in my first year as a Mentor, all the dlink/camera connections are tested for polarity and voltage before first use.
Similarly, a few years ago, our team acquired a Battery Beak to test batteries before use. It has the added feature of testing for polarity. When a battery is first wired, we use the Battery Beak to test for correct polarity.
That only protects against a backwards-wired battery. I’m suggesting something built in to the PDP, which would detect any polarity swaps from the battery through the SB-50 connectors all the way to the PDP input itself. I imagine it could be added to the “smart module” with the CAN connectors.
I deal with a lot of robots each year (about 240 this year after this weekend (though probably only 160 unique teams), and I have not seen this on the battery before that I can recall. That obviously doesn’t mean it never happens, but I would say it is not that frequent.
I do/did see frequently where leads get mixed up on motor controllers for the inputs. This was much more prevalent with the old victors/talons than the new ones. This would burn out certain controllers. Most often this occurred during a rushed add or wiring job. One team had burned up the current and spare before having me take a look at their robot.
I think most of he vulnerable items would fry at nearly the speed of light; realistically too quickly for a reaction to help.
A big honkin’ diode across the inputs - reverse biased normally - would act like a short in a RP situation and perhaps trip the main breaker while dropping the voltage to a less harmful level. Needs to handle several hundred Amps for many milliseconds.
An interesting cost/benefit analysis… assuming that this worked, adding $2 to the cost of every PDP would cost more than replacing the occasional one that is hooked up backwards.
Sorry to hear about the damage… we hooked up our speed controllers the wrong way in 2004 and let the smoke out of one of them. It has NEVER happened again… I suspect this lesson will remain with your team for just as long!
The moderately expensive screw-ups we make in school and FRC are good learning for preventing the truly expensive screw-ups we can make later in life.
Jason
A reverse diode across the input to the PDP would be huge and the short reverse voltage is likely to still produce damage in downstream electronics. Such a diode would need to withstand 600+amps and might allow a volt or more in reverse. The main breaker may still take a second or two to trip at that current. Considering the normal human reaction to the trip, this must survive at least two events before team members will go looking for the actual problem. Inspecting I have been lucky to find two batteries in team’s pits that had been wired backwards but never used and one that was being wired backwards while I was in the pit.
Honestly I think this might be the best solution. Train team members to check newly wired/acquired batteries with one of these before plugging in for the first time. Cost: a couple of dollars in parts.
That’s why I think a warning circuit at the Main Switch would be cheaper and more effective. If voltage is potentially going the wrong way through the switch, it could warn you (buzzer) before you close it and cause damage.
There is only one polarity of wiring at the Main Breaker.
If you close the breaker, death will occur.
I would rather know before I put the battery in the robot. But you know, training will help this situation. Everything is already color coded, red battery terminal, red wire and a big “+” sign on the connector (both sides).