# External Brake resistor

Helo

Can we drive a brake resistor with the extra half bridge. Like When the voltage is positive on both sides, there should be no current flow right. And no shorts. When shorted to ground the resistor dissipates the peak Back EMF spike. ?

This requires a voltage divider. Will current flow and fry the MCU pin if using a simple voltage divider ?

I have to say Iâ€™m not quite following you hereâ€¦ whatâ€™s the proposed circuit?

Hmmmâ€¦ dynamic brakingâ€¦ I must say I still donâ€™t understand the question about the voltage divider, but anyway I think I must defer this topic to someone more knowledgable than me.

My approach would be to take a look at how ODrive and/or VESC solved it, and see if I can understand their solution.

You need to trip maan!

Ehemâ€¦ what I ment was. God sir, ye shall know when to send the current ontu the 4.7 ohm resistor

That is a bad idea and it is not going to work. You will short your power-supply or battery once you turn on that transistor. The idea of dynamic braking is to return energy to a battery when braking.
If you use a power-supply instead of a battery you will need some kind of crowbar circuit, because a power-supply cannot absorb current.

Its basically a huge pull-down resistorâ€¦ When the bridge goes low, there is gonna be GND on both sides of the resistor. When the bridge turns on, to VIN, the resistor will dissipate the energy as heat. Its more or less a way to do a controlled short, thatâ€™s true.

The TVS Diode does the same. I guess you anticipate the spikes and do some current bleeding.

I think you can compare it to ABS braking in cars, just with current.

From the little I know Iâ€™d say one can differentiate regenerative and dynamic braking - the first tries to recover the energy while the second just dumps it as heatâ€¦

You can not compare it to ABS braking.
What ABS does is keep the deceleration of a rotating wheel constant, by regulating the eneryloss in the brakes.
What your circuit does is absorbing the regenerated energy that is produced by active braking.
ABS doesnâ€™t do regenerative braking and your circuit doesnâ€™t care about deceleration, it just tries to protect the powersupply.

Does someone know the kind of BEMF to expect when driving eboard outrunners @ 24V and max phase current of 35amp peak?

It is not relevant for regenerative braking, but you can calculate that yourself.
Just enter RPM / Kv.
Phase current has no meaning. It is the same for any motor with the same Kv constant.

ODrive comes with one of those power resistor (for brake energy dissipation) but no idea how they use it

Somewhere on the odrive GitHub there has to be the schematic for the odrive 3.5 i would look there how they are doing it.

I do made something like this before.
Basicly, when the motor running in the generator mode (e.g. negitive iq), it crams power into the bus.
When theres a battery, its just charging and everything is all right.
But PSU cant absorb power and it goes into the cap and produce deadly high voltage killing the MOSFET and everything.
So a â€śbreak resistorâ€ť is needed to limit the bus voltage.

Its basically a huge pull-down resistor

Its just like that.
The way to control it is using a pid controller or just bang-bang controller.

Its just like a buck conventer without inductor.

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