XT90S Anti-spark Loop Key Polarity

Hey gang, I’m placing the loop key on the negative line, going from the VESC back to the battery. Does the polarity of the XT90 matter? I’d imagine it’s positive terminal is on the VESC side, with the negative terminal running back to the battery - just want to confirm! Thanks

It doesn’t matter. But for cleanliness, put the - side of the connector to the - side of the battery.

one more question? does it matter if the anti-spark loop key is on negative or positive wire of the battery ???

In theory its the same (think of your wiring as a circle O, current just “flows” around the circle, wherever you cut that circle its the same. In practice, its better to cut the positive side because of common ground. But for this application, where the loop key will be directly connected to the battery either positive or negative is fine.

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If it is an xt90s then yes the polarity can matter. Some have reported melting connectors cause the resistor was on the wrong side.

That was a myth and I still think they simply made mistakes by for example not inserting the plug correctly. There is just a resistor in there - why should polarity matter?


Not sure. @sl33py or @barajabali (can’t remember who) said that when they did it the other way it worked. I think the resistor is on one side so that the current from the batteries is limited when it comes in, not when it goes out.

Its a loop, doesnt matter where the resistor is, also, resistor has no polarity.


I know that resistors have no polarity. I’m just confused as to why some people’s issues have been fixed when they swapped it around.

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maybe because it was a different issue from the start :wink:

Well, i have experienced it like @lox897 said

My wiring was correct but my loop key got destroyed when i plugged it in the first time…
I changed the loop key with a new one and plugged it in the other way. Since then it is working.
My thread to it

exactly - “you destroyed it the first time you plugged it in” perfectly describes how you most probably have not plugged it in correctly and just concluded from there that something bigger must be wrong. After the initial mistake the resistor died and you always got a spark. If you had used your new plug in your old configuration it would have also worked. Also everybody in your thread told you that the polarity does not matter.

How can i not plug in my XT90-S correctly?
Or what was wrong with my wiring when i have not changed ANYTHING except a new XT90-S in another orientation?

Would be good to know!

For example the way I linked directly above: not inserting it all the way. Your wiring was right and it would have worked with a new plug no matter the orientation you had used. You can’t conclude it was a polarity issue solely on the fact that you turned it around and it works now. That is not the way to go. You would have to retest after making sure the plug works correctly on its own (like you did now). I am sure that changing the polarity now will not make a difference in your system.

Me too.

you are right :wink:

I disagree!
When i plug it in half way that only the resistor is touching the plug my XT90-S should not be melted! The resistor refuses the energy to flow with high current. But when you just plug it in halfway to turn the board on (not riding) there is no high current needed. Why should the resistor blow?

Just accept the fact that it was human error ( or faulty product - less likely ).

There is a voltage drop voltage drop across the resistor ( depends on the current and resistance ).
If the current flows, heat level increases until it’s fucked up.

That could be a reason… everytime…[quote=“IDVert3X, post:16, topic:11780”]
There is a voltage drop voltage drop across the resistor ( depends on the current and resistance ).If the current flows, heat level increases until it’s fucked up.

The XT90-S is designed for the current. So why should the resistor of the XT90-S blow?

The resistor is designed to handle the current for very short period of time.
Heat accumulates and small package like this can’t handle much heat.
That’s why you have to always push it all the way in.

Let’s do the math. You have 100R resistor for example.
If using 42V, it will limit the current to 420mA.
420mA @ 42V gives you ~ 17W, and that’s A LOT of heat!
It’s designed to handle this current for a fraction of a second when pushing the connector in.


Maybe many ppl are to shy to plug it in completly in half a second.
Thanks for the clarification.

also these are not hightech connections/solderings inside that xt90s, so if you have a weak connection due to a bad production, it could melt down in a few seconds if you don´t push it all the way in directly.
the next connector might be better and have a more solid connection, so that it handles the heat longer.