I want to protect my dc motor controller from voltage spikes due regenerative braking. Battery is 48v (54.6v at full charge) li-ion chemistry 100A peak, with "cutt-off protection BMS" from overcharging.

Connection is:

  1. 48v/15Ah Battery
  2. 200A contactor with 470ohm 10W precharge resistor
  3. 120A/60V Roboclaw controller
  4. 48V DC motor.

I also need bypass diode in parallel on positive wire between battery and contactor. I was looking for 1N5400 series diodes, and I'm in doubt. Should I go with 1N5400 or 1N5408 diode? Or something different? Do I need one diode on the contactor also?

This is wiring diagram, bypass diode is labeled D1, F1 is fuse, R1 is precharge resistor on the contactor.

wiring diagram, bypass diode is labeled D1

  • \$\begingroup\$ The 1N5408 can withstand 1000 V reverse voltage while the 1N5400 can only withstand 50 V. For the rest these diodes are identical. Unless the 1N5408 is considerably more expensive, I'd always choose the 1N5408. I'm not making this an answer as I have no idea if a diode is suitable for what you want to do, to be able to judge that I would need to see a schematic. \$\endgroup\$ Oct 31, 2020 at 22:07
  • \$\begingroup\$ Thx for answer, I edited question with schematics. \$\endgroup\$
    – littlerock
    Oct 31, 2020 at 23:01
  • \$\begingroup\$ I think this is what I need. \$\endgroup\$
    – littlerock
    Oct 31, 2020 at 23:49
  • 1
    \$\begingroup\$ Diode location across fuse is fatally wrong. This bypasses the fuse and acts as a second fuse. It will probably go open circuit if the fuse blows but this is not certain. It MUST NOT be in that location \$\endgroup\$
    – Russell McMahon
    Nov 1, 2020 at 4:55
  • \$\begingroup\$ Like the schematic is drawn now the diode doesn't do anything until the fuse blows. Hmm, when does a fuse blow? When things are not going as they're supposed to go, fuses are protection devices that are supposed to disconnect things so no currents can flow. In your schematic, after the fuse blows, there's still a diode that can conduct current. Poor diode, the fuse has blown (because something went wrong) only then it can "do something", chances are, it will blow up as the fuse didn't blow for nothing. You need to revise your schrmatic!!! \$\endgroup\$ Nov 1, 2020 at 12:50

1 Answer 1

  1. Fuses

Fuses are used as a last resort measure to prevent fires. They should be able do completely disconnect the circuit.

Pay some respect to fuses and Don't shunt them with anything.

Your design should not rely on the fuse blowing at some point in order to work correctly.

  1. The relay (contactor)

I think you should not expect the contactor to disconnect in case of a regenerative braking. Except if you have a complex and profoundly broken logic that controls it. So no diode needed.

  1. The diode.

If you still want some diode to protect you from currents generated from regenerative braking, you may want a diode that handles the full power of the motor. With some safety margin. 1N54xx diodes are rated for 3A. They are also rated for 200A, 8ms impulse. Is your regenerative braking that short? What about running downhill? You may want a way bigger diode.

  1. The regenerative braking voltage

A permanent magnet DC motor can generate more or less the same voltage that is used to run it to the same speed. Your controller may or may not be able to boost the generated voltage to a value higher than its own voltage rating. I would check if the controller needs protection in the first place.

  • \$\begingroup\$ Fuse are protection from pulling current from battery, that controller can't handle. That is why is the same size as controller max current. Contactor with precharge resistor is used as safetly switch, powering the whole system, preventing onboard capacitors to blow due inrush current. \$\endgroup\$
    – littlerock
    Oct 31, 2020 at 23:44
  • \$\begingroup\$ I just need proper diode size. \$\endgroup\$
    – littlerock
    Oct 31, 2020 at 23:47
  • \$\begingroup\$ No, fuse cannot protect neither the controller nor the battery. It is way too slow, maybe 100 times slower than one needs to protect semiconductor elements. Fuses prevent fires that can happen when something else breaks. The controller can be protected from overcurrents only by its own control logic. \$\endgroup\$
    – fraxinus
    Nov 1, 2020 at 0:08
  • \$\begingroup\$ p.s. precharge resistor does not protect the capacitors, it protects the BMS from the inrush current. And yes, using a Li-Ion battery as a voltage clamp comes with its own issues. And no, you cannot improve anything by adding a diode. \$\endgroup\$
    – fraxinus
    Nov 1, 2020 at 0:13
  • \$\begingroup\$ precharge resistors... controllers feature a bank of onboard capacitors. When power is first applied to a system, the capacitors rapidly charge, which causes a large amount of current to flow in to the controller, this can cause damage. To prevent this from happening a suitably sized resistor is wired in parallel across a switch on the positive lead of the power supply. The resistor allows the board to slowly charge the capacitors before the main power is applied so that a large amount of current does not flow when power is applied to the board. And the main power is applied by contactor. \$\endgroup\$
    – littlerock
    Nov 1, 2020 at 13:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.