Pre-Charge and Bypass Diode

General discussion of using Roboclaw motor controllers
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Pre-Charge and Bypass Diode

Post by seurban » Tue Nov 03, 2020 9:27 am

Hi, I have a few questions about the pre-charge resistor and bypass diode. I'm using your RoboClaw 2x60A Motor Controller with an ElectroCraft M-26 motor (max current 80 A) and another smaller motor (2-4 A max). Input voltage is 24 V.

PreCharge Resistor
  1. What is the minimum time for pre-charge? Would .25 seconds still protect the controller?
  2. How much current would be flowing through the circuit for pre-charging? I'm putting a switch to let me turn off the pre-charge and want to make sure it's rated for enough current.
  3. If power is removed from the board briefly (maybe .1 seconds) while switching from the pre-charge circuit to full power, would this damage the board?
  4. I think I read somewhere that the damage to the controller happens over time, so failing to use the pre-charge one or a couple times shouldn't totally kill the board, but doing so regularly would. Is this correct?
Bypass Diode
  1. In the image above it looks like the bypass diode is letting regenerative voltage go around the fuse and into the battery. Is this correct?
  2. If so, how does the fuse protect the battery from a regenerative voltage spike? Or is that not the fuse's job? Sorry if this is obvious, I am still a relative novice with electronics.
Thanks in advance for your help.

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Basicmicro Support
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Re: Pre-Charge and Bypass Diode

Post by Basicmicro Support » Tue Nov 17, 2020 6:54 pm

1. Precharge time can be a few dozen milliseconds to a second or longer. Depends how much current you want to pass and how long you want it to take. The objective is to keep Di/Dt low so you dont get a large ground bounce which can cause a negative spike on sensitive electronics. It will also reduce the positive spike but that is usually less of an issue than the negative since it is so much closer(-5v will damage almost anything on any Roboclaw).

2. I = V/R, Ohms law is your friend. First determine the time you want to prechagre. 100ms for example. Charge time is 5 * R * C. Capacitance is the all the big caps on a Roboclaw added up(400 microFarads on the 2x60A). R = .1/(.000400*5) = 50 ohms. Current would then be 24/50 ~= .5 amps.

3. Caps wont discharge fast after charging. I manually precharge caps all the time by physically touching a lead with X resistance to my board before connecting the main power, also manually.

4. At 24v it could go either way. At higher voltages I wouldnt recommend ever powering on without a pre-charge.

Your second set of questions:

1. Yes. And you should have it or another diode also bypass the power switch.
2. The fuses job is to protect YOU from FIRE. That's its only job. Which means it doesn't care about regen energy. It is there to cut out the battery if the motor controller fails and shorts out the battery. Simple as that. The only way regen is getting high enough to out pace the battery is if you have a significant external force providing that energy. In that case you need to start looking at voltage clamps and very large resistors.

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