ME Needs Advice: Current Capacity of Motor Controller

If you only need ON/OFF:

Look up MOSFET. If you need bidirectional motion, look up H-Bridge. You may even be able to buy motor drives of this capacity off the shelf. But a 12V signal can easily switch 20 amps through a MOSFET and use almost no input power. International Rectifier is a great source. Relays work well, too.

Of course - you'll still need 12V@20A to feed through the MOSFET to the motor.

If you need variable speed - that's a little more involved. If the variable output of the motor controller is pulse width modulation PWM - which it probably is, then a MOSFET still works. (But a relay won't) One thing to be concerned with is the switching frequency - if it's over a few kilohertz it may not work on your motor. If the variable output of the controller is a 0 to 12V analog signal, then you need a PWM chip, or better, an OTS motor controller that accepts an analog input.

Follow this link you should be able to adapt it to your circuit.

http://www.robotshop.ir/files/products/datasheets/md03.pdf

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can you just use a relay to turn the motor on and off??

You likely will need to look at more than just normal operating current. The in rush current when the motor starts could be 3 times your normal operating current.

There are a few more concerns here that need to be addressed before a suitable amplifier circuit design can be suggested. The controller you show can control 4 single direction DC motors or 2 bi-direction DC motors. Will the DC motor be bi-directional or not? Will this be used in a servo loop? The motor controller you specified, might be Pulse Width Modulated (PWM) and the earlier post about MOSFET usage might be capable but without knowing the pulse frequency and a few other pertinent facts, I'd steer away from MOSFETs. Right now I believe power Bi-polar Junction Transistor (BJT) will be the preferred method. I suspect that you will also need to do some dynamic braking to both quickly slow your motor system down and to protect the amplifier output circuit from back EMF. Just to state the obvious, you will also have to also provide a supply capable of producing all of the current to be simultaneously controlled.

Unidirectional control will likely be relatively easy. Bidirectional control will be more complicated. Both will produce considerable waste heat.

Thank you everyone for the replies so far!

I will be using two motors at 12V, 20A max and would like for both of them to be bi-directional.

The controller I am using outputs a PWM signal which is variable.

I will be using an automotive battery as my supply, which should be more than capable of producing the required current.

Anyone have further advice?

Sorry for the delay, the universe just could not get out of the way. Here's a nice simple schematic of some suitable International Rectifier MOSFET parts suitable for your motor control job.

You will require four of these circuits to operate two bidirectional DC motors. These will go between the output of your driver box and the motors themselves using the four channels described by the manual for driving two motors bidirectionally. I now notice that both the P and N MOSFETS are missing the S for the source lead. But you should be able to grasp that. To clarify my handwriting the transistors are IRF4105ZPbF and IRF5305PbF. There are many more power transistors capable of doing what you need but these were the first I located that seemed reasonable. My only immediate caveat is that this circuit configuration is designed to have a dead zone of about 3 to 9 volts. In other words, when V_in is between 3 and 9 volts referenced to RTN then neither transistor should be ON. This should allow your circuitry to not over draw power. Reducing R1 and R2 or increasing R3 and R4 will narrow this dead zone to the point that both transistors can be ON at the same time (not recommended to have both ON.) Reducing all four resistors will speed up turn ON/OFF time. You still may find that these transistors could get very hot. Heat-sinking maybe required. Be careful though, I believe that the case of these devices is the source lead. So bolting them to the grounded chassis will not be desirable.