# DIY break chopper - protection against overvoltage - no cutoff

Folks I’ve found back the break chopper circuit made by Sawyer !! @Sawyer93 is it you by chance ?

Short word it is the end of HVC cut-off (like if you coast downhill and want to regen brake while battery is full, well you can still go at it).

We’d need to replace the halogen bulbs he uses to dry the excessive power by something more discrete IMHO. A big shunt or something similar ? A rack of high power LEDs to give it post-apocalyptic look?

Anybody is able to decipher it better than I do ? @b264 @Deckoz what do you guys say ?

And a video of how it works / what it does (see the gauges in left corner) :

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maybe you could wire this to a big break light

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Would be a killer look if the light is mounted on the rear truck for example

Here’s the brake chopper circuit in full simplicity

Just replace the Brake resistor with another resistive load…

And actually they made a microcircuits which is at the end of that video… With the schematic to be able to customize the clamping voltage

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This is probably overkill.

Probably not, you might be dumping 1-2kW into this thing. Also look at this:

I dunno - were not talking about instantaneously dumping all the kinetic energy of the traveler. This guy uses a 225W array of bulbs to dump the excess current, and i doubt he’s shedding the maximum load that array can emit.

You’re saying that 100W resistor will be overkill. From the link in first post:

Braking 100 kg from 30km/h within 30m should need ~1900W ( time to brake = v/2 / s, E=mv²/2, P=Delta E/Delta t )

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It’s my opinion that this should be handled by the BMS and not an add-on circuit. The balancing circuitry in the BMS would be best suited for this, maybe have a place for a large resistor or energy sink to hook up. @SimosMCmuffin

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Thank s for the replies guys. Yep à BMS managing this would be killer (was thinking of the FLEXIBMS) ; still would be nice to get a spare pcb to simply hook up to your battery?

Even at only 20w, those 100w resistors get burning hot in seconds. So hot they burnt the piece of wood they were mounted too. You’ll need a lot more than just one, hook them up to a large heatsink, and probably still need a fan (a big fan).

Generally think the issue with this idea is finding something that can absorb the load. 225w is only like an extra 6a of breaking power…

Mellow has this in a 1000w resistor…inside a tube of aluminum…

If it’s built right, it’s only braking power. If it’s built wrong, it might be breaking power.

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Heh, I just googled this and ended up right back here. There’s also some info on their website.

I’d be interested to see this, all the big resistors (1000w range) I came across were huge, as in the size of my forearm. Anyone pulled apart a Mellow board?

edit… Nevermind they have a rundown on their site too.

I’m not suggesting that a 100w resistor could effectively bring you to a dead stop. I watched the video, and based on that use case (light, short braking) a 100w resistor would be fine.

I know how hot they can get. We used to use them to make heated beds for repraps back in the day.

Well, that should do!

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I suppose it could be done on the Switch-module. I don’t see any real reason it couldn’t. Might be actually possible to just use the BQ76200’s Charge FET output to directly control a N-channel FET going to a brake resistor. Heat dissipation might be the real problem, as you need to dissipate all that extra power somewhere else than the battery.

Heat dissipation wouldn’t be the BMS’s problem

If you want to hook up an external dissipator though, it’d be interesting if the BMS had a spot for it. Maybe even a power resistor bolted to the truck hanger or something, dissipate heat on that…

I was about to ask about the type of resistor you’re thinking to use. My first thought was something located on the switch PCBs itself, but you seem to be thinking of a completely external resistor. I suppose the easiest could be to just leave wire hookup points available on the board.

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As we can see above, it may not even be a resistor. I wouldn’t assume what it is at all. Just somewhere to dump energy, if it’s hooked up…