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Melting loop key? SOLVED

What does exert too much mean? There is a 5.6Ohm resistor in there - At 42V there will be a current of 7.5A going through that little resistor which will heat up quickly. They are not made to hold that constantly but just for the initial connection. If you exert it too much and the current flows through the resistor constantly it will obviously melt.

Edit: Sorry I thought exert meant to pull the plug - just looked up the word and it means something different.

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the resistor shouldnt be involved once you plug the XT90 in completely. just the tip has Business with a resistor or capacitor or whatever magic device it is. :stuck_out_tongue:

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The resistor isn’t bearing any of the load ( or so it shouldn’t be after the initial connection) it maybe it a. Problem of me not plugging the key in far enough maybe…

yeah mixed up the words - I am sorry

Maybe you could increase resistor size like someone did on endless:

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Now that we know in theory AND practice that it is not polarity. It must be something else.

Honestly it might be that I didn’t plug it in all the way. LOL it’s a tight fit and the last mm or 2 is hard to get all the way in. I totally didn’t realize that that actually matters with anti sparks. Whoops. Will try that tomorrow.

I think the little resister is taking the load which is obviously can’t bear much of without melting.

Thanks buddy that will be my plan B :slight_smile:

Hahaha I regret to inform you all that it worked all along… It just needed to be filed down a little to plug 100% in. Those last mm’s actually made all the difference.

My dumb ass. Making my mistakes all public and shit.

Thanks anyway everyone!


I dont think its a resistor problem, because that shouldnt really play a role after the xt90m is fully plugged in. maybe its really the fine copper strand silicone wire. the high voltage sag/jump I saw in the vesc live Monitor (clearly inductance effect!) made me think it must be a loop size thing, but after I played around with I saw that the inductance of 20cm straight wire is much higher than a 2cm loop.

however, if you decrease the wire thickness the inductivity rises and its proportional to the wire windings, so many many small windings in that loop would actually increase the inductance by a lot!

what I would try next is, using one solid copper wire for the loop and check if the voltage sag/heat thing persists. if thats the case, then there must be something fishy going on with the small strands in the silicone wires.

hm, ok :stuck_out_tongue:

Oh and I tested it with about a 2 inch diameter loop just for shits and giggles and it works fine. And my loops are much smaller than 2 inches in diameter. So that shouldn’t be an issue
The wire was made of 10 gauge super worm wire FYI

im not satisfied with this outcome - I know that my loop key was plugged in :confused:

Let’s keep this thread alive to find a solution because I know you’re not the only one with this issue…

Maybe the actual connection inside the connector wasn’t fully engaging despite being fully plugged in. What @barajabali experienced seems like the only logical reason why this should happen.

Path A with resistor connects first, then Path B with ~4mm bullet plug doesn’t fully connect sending most or all current through Path A, resulting in poor motor performance and melted loop key. Nothing else left in the equation, right?


I respectfully think you’re over-thinking things here.
The only way you can burn an XT90 is by applying more current than it can handle.
If it’s not plugged in all the way, the small resistor cannot handle the full current draw for very long.
When it’s plugged in all the way, an XT90 can handle over 90 Amps safely.
Inductance of the loop key has nothing at all to do with it.


If I understood correctly these loop keys are passive components, a single resistor inside. First it looks an easy option but then…
Easy because they are cheap, small, no polarity to mess up, etc.
…but don’t forget the dark side:

  1. You must be sure that the key itself can handle the current that will flow through the key and the loop you add.
    E.g. Use a short high-Amp piece of wire, e.g. what was recommended above.

  2. You must “operate” it as quick as possible. Don’t worry, you won’t be faster than electrons, but you must plug the loop key together as fast as possible and properly.
    Why? Because it has two stages. First the contact is made only with the resistor that sucks up the spark. Then on full engagement contact is made directly with the loop moving the stress away from the resistor. So if you not fast enough and let the current flow through the resistor too long or you do not engage the full contact because of poor-quality plastic finish, you can easily burn the resistor.
    Yeah, but how fast? Well, let the caps charge through the resistor before the full contact - see next point.

  3. Make sure the built-in resistor is powerful enough to handle - even only for a sub-second or a second - the current peak.
    If you have a lots of capacitance in your setup - e.g. many and/or large capacitors, like a dual VESC - and you are not pre-charging them, those caps will suck current from your battery when you engage your loop-key. The resistor must be able to handle the current the caps suck out from the battery for long enough the caps are (almost) charged.

@barajabali may had the issue described in #2 - not fully engaged contact stressing the resistor?

R, J.


What ecxactly did you file down to get it to plug in 100% ?

My loopkey is identical to whitepony’s when he had it melt ,…mine is an interupt on the POS of Battery before esc…never had a problem ?

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I’m still waiting on my cnc shop and some more connectors for the charger, I might get an extra pair of originals just in case hah, xt90 connectors. Or should I just check the connection before the DC is live to make sure they connect 100% ? I wonder if my 44v is too much and 10000mah, it was stated it can only handle 7amps and my connectors are going from the battery to my esc therefore 10 amps is too much right?

Somebody needs to launch a poll asking how many people are actually having problems with a XT90s loop key.
Not a lot I expect.
That should kill all this panic.

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You’re gonna be fine just make sure to plug it in all the way sorry bout the confusion

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