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Accidental polarity inversion

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Old 04-26-2019, 04:24 PM
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Default Accidental polarity inversion

Hi all - I just made the one mistake I thought I would never do... had my SMC 4000mah upside down and plugged in the negative, then proceeded to connect the positive, which sparked loudly when I touched the copper. It only lasted a fraction of a second.

I immediately recognized the mistake, and connected the battery the right way and for my surprise the esc turned on like nothing happened. Steering works, but throttle is not working. I hear the esc buzz when I hit the throttle and if I start spinning the wheels with my hands, the throttle works normally if the wheels are spinning. It just won't start accelerating from a stand still.

So is my esc toast? Or is it possible that something went wrong on the motor? I was thinking in replacing the bank capacitor to see if it helps, but my gut is telling me that it won't fix the esc.

ESC is a Hobbywing Justock, Reedy S+ 17.5 motor.

The worst part is that I do have all the countermeasures to avoid this, positive wire is red and won't reach the negative. But I held the battery in my hand for a quick test of the servo - quick test that is probably going to cost me a new ESC

Let me know if you guys got any input!
Thanks
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Old 04-26-2019, 04:48 PM
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We’ve all done it. It’s just one of those things. Most just don’t admit it.

You would smell the esc if it cooked. Theirs a distinct smell of burnt electronics. I did it 3x last year. Too many vehicles with different batteries. Cooked 1 esc. The other two just broke the solder connections.
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Old 04-26-2019, 04:49 PM
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You have probably blown the filter capacitor, it's cheap to replace it and it won't run right without it. You may have caused additional damage to the ESC but you won't know if you don't try replacing the cap first.
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Old 04-26-2019, 04:50 PM
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There are two types of people who run bullet plugs, those that have plugged a battery in wrong and those that will. It happens bummer but in the end we are all human and make mistakes.

When I did it myself the esc worked ok that day, however at the next event when I turned the esc on it popped.
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Old 04-26-2019, 05:48 PM
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On the battery that has holes on both sides, cover the unused holes with tape or shoo goo the holes shut so you can't plug any bullets in there by mistake. Shoo Goo can be removed down the road if needed.
Good chance the ESC is done.
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Old 04-26-2019, 08:00 PM
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It really is a bummer... but like you guys said, it happens!

I will get a new capacitor and see if it runs, if not, I will just get a new justock.. good that they are not expensive ESCs.

Thanks all for the great info!
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Old 04-26-2019, 09:21 PM
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The esc is most likely done. But the bright side is that you shorted out a really cheep esc.
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Old 04-26-2019, 11:21 PM
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Originally Posted by m3rcfh View Post
I hear the esc buzz when I hit the throttle and if I start spinning the wheels with my hands, the throttle works normally if the wheels are spinning. It just won't start accelerating from a stand still.
Sounds like you have blown at least one phase of the MOSFET H-bridge, so nothing happens when it needs to run that phase to get going from a standstill. Once it is turning the other 2 phases are enough to keep it going. Time for a new ESC.

Originally Posted by Billy Kelly View Post
You would smell the esc if it cooked. Theirs a distinct smell of burnt electronics.
Not always but most of the time.
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Old 04-26-2019, 11:59 PM
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Originally Posted by LzREngineering View Post
You have probably blown the filter capacitor, it's cheap to replace it and it won't run right without it. You may have caused additional damage to the ESC but you won't know if you don't try replacing the cap first.
Can you explain why? Maybe I am still missing some basics of electronics alhough I work with it almost 40 years but the guy says the servo is working but no throttle. The capacitors are parallel over the battery connection and the ESC can still work w/o them so I can not imagine the capacitors are the reason why throttle is not working.
I can tell you that the reversed diodes inside the FET's will be blown in just short moments of reversed polarity.
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Old 04-27-2019, 05:59 AM
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Originally Posted by nbTMM View Post
Sounds like you have blown at least one phase of the MOSFET H-bridge, so nothing happens when it needs to run that phase to get going from a standstill. Once it is turning the other 2 phases are enough to keep it going. Time for a new ESC.
Thank you, now I know for sure that I need a new ESC and won't event try fiddling with it to see if it works.

Do you think that is it even worth of sending the blown ESC to Hobbywing for repair? Or is it going to cost as much as a new ESC?
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Old 04-27-2019, 03:52 PM
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Originally Posted by Roelof View Post
Can you explain why? Maybe I am still missing some basics of electronics alhough I work with it almost 40 years but the guy says the servo is working but no throttle. The capacitors are parallel over the battery connection and the ESC can still work w/o them so I can not imagine the capacitors are the reason why throttle is not working.
I can tell you that the reversed diodes inside the FET's will be blown in just short moments of reversed polarity.
I'm just guessing, but in the original post it stated that the motor would run with a kickstart but not from standstill. This seems like a sensor circuit issue rather than destruction of the drive FETs.

I was only suggesting to change the cap as it's a very easy thing to try, and the cap may well have been destroyed by the reverse polarity.

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Old 04-27-2019, 07:43 PM
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save the good fets for next time
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Old 04-27-2019, 09:04 PM
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From what I've seen, both the ESC and Sensors in the motors can be fried if polarity is reversed. The ESC provides voltage to the motor's sensors, and if reversed, it could fry those sensors in the motor itself. This could cause problems, even after replacing the ESC.

Its a darn shame that polarity protection circuits, which could be implemented in ESCs, would incur significant performance losses due to voltage losses, or incur significant cost increases to mitigate.

Put red paint on the wires and plugs to indicate polarity. 10 cents of paint could save $300 of ESC and motor. You gotta pay attention, or pay.
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Old 04-28-2019, 08:15 AM
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Originally Posted by m3rcfh View Post
Do you think that is it even worth of sending the blown ESC to Hobbywing for repair? Or is it going to cost as much as a new ESC?
Probably not. It's a fairly cheap ESC so would probably take more than it is worth to ship it, diagnose the exact problem, repair it and ship it back. I would hazard a guess that if hobbywing "repair" any they just confirm that it is broken, toss it in the bin and ship a brand new one to the customer.
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Old 04-28-2019, 06:35 PM
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Ok, interesting development today on this story guys... I went to the LHS and they suggested to start with new sensor wire, and so I did. Did not solve the issue.
Then the techs said it could it either be the motor or the ESC that is toasted, no way to tell which one without replacing. Since I don't have spares, bought another Justock and planned to install it later this week.

When I got home I though I would give it a shot unplugging the sensor wire from the motor and hitting the throttle. It worked normally (with some throttle oscillation as expected for not running the sensors). So is the issue here the sensor board on the motor that got blown? Or can it still be the ESC?

Last edited by m3rcfh; 04-28-2019 at 06:37 PM. Reason: Forgot some words
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