Relay ON/OFF Switch With No Visible Spark

Hi Guys, Just wanted to share my discovery with you. I recently rebuilt my board with a new battery and enclosure and I wanted a new (proper) switch.

I discovered a latching relay that was rated for 120A and has a coil voltage of 24v. I then found a ‘centre off’ rocker switch which I wired up to the relay. I fed 8s 33.6v (from my main battery) to a linear voltage regulator that steps the 33.6v down to 24v in order to switch the relay.

Rock the switch one way, it connects and the battery voltage & Amps are allowed to flow through it, rock it the other way and it disconnects. The beauty of it being a latching relay is that it doesn’t use any power while it’s allowing the current to flow, it’s a one-time, power to flick the switch one way or another then it’s at rest.

The best part of this, is that (I took the top cover off the relay to expose the components and copper), there’s absolutely no visible spark or arc. Fair enough, it doesn’t feed the power slowly to the VESC like the XT90 anti-spark switch does, so I could still be potentially damaging the caps, but I was under the impression, if there was no spark or arc, there was no issue. Correct me if I’m wrong.

Anyway, I’ve been using it like that for about 6 or 7 rides now, turning it off and then on again when stopping for a rest or a snack and it’s shown no signs of issues.

What do you guys think?


its hard to find an actual relay that is not crap in the intertwebs without spending too much. If you have the space and actually have a quality relay go for it… But this takes up 2x or 3x the space of an antispark ?

Personally I think relays,ssr and stuff is over engineering and not needed :slight_smile:

The relay is only the L shaped black object with the white writing at the top of the photo. It’s about the same footprint as a vedder anti-spark PCB, but it is a bit thicker.

Yep, but the step down regulator also takes up some space

I have looked into relays as the BMS I have is designed to use two of them. They are available in 36v coils. I have noticed when you start calling them contactors the price goes down. I thought that the caps where used to smooth out the voltage ripple not protect the inrush current. Please correct me if I’m wrong.

With batteries there is no ripple to smooth out. It is as DC as it can be :smiley: The caps are used as external decoupling for the battery wires.

The step down regulator is tucked underneath the switch there, in the blue heat shrink, it’s very small indeed.

So why, for example, does the xt90 plug let a low rate of current through, then almost immediately the full current/voltage? Is it literally just so that it doesn’t spark? I thought it was to protect the components from a surge of current?

Yes it’s literally just so that the spark doesn’t vaporize the metal on the connectors and cause them to go bad over time, from my understanding of it anyway.

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Wow, I didn’t know that. Thanks :thumbsup:

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Yeah that is right, it damages the connectors. At 10S it just destroys them :smiley: every time i connected it before i made a proper switch the spark just burnt a piece of my connector :smiley:

Hey I was wondering how you wired your relay .i have been trying to do the same thing

like that but no luck : I was wondering if you could tel, me if I’m doing anything wrong

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what about if you have a bms

Doesn’t matter, you still have a power source the same…

Here’s a better option; solid state relays. I figure an ssr is what the anti spark switch is, if anyone has more info please inform me. The ssr takes the contact breaker fully out of the equation, as an analog relay is still just a heavy electromagnetic contact breaker. SSR’s still have a minute voltage leakage of around 5mA, but for as little as 25$ you can get one up to 200A, just be sure you find a dc to dc SSR. If you’re still concerned about voltage leakage, install a battery cutoff switch before the ssr, switch that on first then power up the ssr. With tiny current, no sparking will damage the cutoff switch, and the ssr will open current with no spark. One thing I’m not sure of is it’s been mentioned the anti spark switches turn current on gradually to prevent overloading the capacitors in your vesc.

Switches are cheap, the real concern is the effect of high voltage/current being switched on through an analog switch, dumping voltage and creating arcing, which destroys contacts but more importantly can damage your vesc.

Anyone with more info on SSR’s feel free to pipe in, I’m contemplating using two 25A SSR’s in parallel on my current build with a battery isolator before the SSR’s, but for 75$ a proper anti spark switch is cheap compared to a new vesc