Battery connection and diodes?

General discussion of using Roboclaw motor controllers
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ChrisA
Posts: 23
Joined: Mon May 22, 2017 4:32 pm

Battery connection and diodes?

Post by ChrisA » Mon May 22, 2017 4:50 pm

I just read through the Roboclaw user manual. I see a number of wiring diagrams and several show a battery, fuse and switch wired in series with the controller. This is simple, but...

I see a diode in parallel with the fuse but no diode in parallel with the switch.

It seems to me you'd want a diode in both locations. If the circuit is open the back EMF does not care how. Or is the thinking here that the user would only open the switch after causing the motors to stop and never use the power switch as an "E-Stop".

The manual only says "high current" with no suggested value but I assume the max current the motor could produce is equal to the motor's stall current.

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Basicmicro Support
Posts: 1404
Joined: Thu Feb 26, 2015 9:45 pm

Re: Battery connection and diodes?

Post by Basicmicro Support » Wed May 24, 2017 10:20 am

I do recommend having the diode across both the switch and the fuse, however if you are not turning off the switch while the unit is running(eg it isnt being used as an emergency cutoff) then only the fuse requires the diode(since the fuse could open while the motors are running).

The flyback diode does not need to be spec'd to support the motors full current or even close really. It needs to be able to handle a the regen current for a fairly short time(10s of seconds or less, until the motors stop) up to the maximum current your motors could produce in regen(based on the application). I cant see any case where the regen current will be even close to the motor stall current. DC motors are not efficient generators for the most part.

In general 1a to 5a power diodes will work in most cases(with very few extreme exceptions). These diodes can usually(double check your specs of course) many times more current for a brief period of time. You may need to do some math based on watts disipated through the diode to determine the actual maximum current the diode can handle for the time it would be active.

Just so everyone understands, the flyback diode is only active when:
1. all other paths to the battery + are disconnected
2. the motors are in(or enter) regen when the disconnections happen.

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