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I am building a relatively simple circuit which essentially consists of two lead acid batteries rated at 12V and two motors rated at 12V. My original plan is to wire them all in series, and control them with a motor driver of some sort using a pwm signal. However, all of the motor drivers I can find that can handle this are rather expensive, and it seems as though it would be just as acceptable to do it with one or two mosfets. I am not sure of what quality/type of mosfet to get, and where to place them within the circuit. I am thinking it would be easiest to do just one high quality one before both motors, but would I be able to control the speed of the motors using a pwm signal that way though? Any advice is appreciated.

EDIT: The mosfets will be controlled by a simple microcontroller such as a pic or avr.
Here are three N-FETs I have found that may work. 87A 100A 104A

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Copied from a comment - extra data should be in question for easy reference.

The motors rated power is 337 Watts (at 2655 rpm, 172 oz-in, and 68 amps). The load is present at all times, and should stall minimally, but I have a current sensing ic in place to monitor the current and reduce the voltage if it starts getting too high. I don't care too much about bidirectional control, and the current measuring circuit is a 180 A rated attopilot chip.

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  • \$\begingroup\$ Controlling a large load is not a walk in the park. You need to consider the danger of such high currents. What is the power demand from the motors? Do you need to consider fault conditions. E.g. what if the motor is stalled, you might set fire to the wires or even worse explode the battery. \$\endgroup\$
    – Jay M
    Sep 25, 2012 at 11:55
  • \$\begingroup\$ I've designed a motor controller that will run at 12-24V at 150A (probably higher, but limited by PCB traces) for an electric go-cart for my (autistic) great nephew. Naturally it includes several safety features. I was going to make two, but making a third will cost little more if you are interested in one? \$\endgroup\$
    – Jay M
    Sep 25, 2012 at 12:05
  • \$\begingroup\$ I am more interested in building this myself as a project to improve my ability, so information about how you went about it would be appreciated more. Thanks. \$\endgroup\$
    – a sandwhich
    Sep 25, 2012 at 13:41
  • \$\begingroup\$ You left out much important and relevant information. Are these DC brushed motors, and therefore only have two connections each? What current will the motors require during expected operation? What is their worst case stall current at 12 V? Do you only want to control their drive level in one direction, or must the motors be reversible? \$\endgroup\$
    – Olin Lathrop
    Sep 25, 2012 at 13:46
  • \$\begingroup\$ Sorry about that. The current ratings are listed in a difficult to read fashion in the link and they are brushed motors. I also only listed in a below comment that I don't care about bidirectional. Peaks at a stall around 110A with two connections each. \$\endgroup\$
    – a sandwhich
    Sep 25, 2012 at 16:22

3 Answers 3

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Do not put the whole shebang in series. Instead deploy your circuit with each battery individually connected to a motor and then through a N-FET to GND. PWM Drive on the N-FET gates will allow you to control the speed of the motors. Use two PWM signals to permit individual control of each motor's speed.

Selection of an N-FET type will take some careful work. Since your motor can draw 27A at normal load (133A at stall) you will need some beefy parts. If you design the circuit around nominal conditions plus a guard band you may choose a 70 or 80A N-FET. If so then you will have to design the circuit with some intrinsic current limiting or add some current overload sensing that couples back into the FET gates to shut them down when the current starts to get to high. Further details regarding FET selection are stymied by lack data regarding what you want to control the gates of the FET with.

Keep in mind that this configuration causes you to only be able to control the motor in one direction of rotation. If you require bi-directional control then you will need an H-Bridge for each motor that consists of four power FETs per bridge.

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  • \$\begingroup\$ The pwm will hopefully be controlled by a simple microcontroller or two, and I have a current sensing ic in between the motor and the battery that can measure the current and cause the pwm to drop if needed. \$\endgroup\$
    – a sandwhich
    Sep 25, 2012 at 4:05
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All 3 MOSFETs are potentially OK. All have low Rdson, which is a very important factor.

Key questions include:

  • What is the motor rated power. This allows the steady state current to be easily calculated and gives some idea of the startup currents.

  • Whjat are you driving? ie is the load present during startup.

  • Do you want bidirectional control.

With 2 x 12V batteries and 12V motor you can get bidirectional control with two FEts per motor by joining the +ve on battery A to -ve on Battery B and using the joining point as ground. You now have +/-0 12V and can drive a motor either way by applying either PWMd +12V or PWMd -12V. Applying both at once is a very bad idea :-).

More answers if more information comes ...

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  • \$\begingroup\$ The motors rated power is 337 Watts (at 2655 rpm, 172 oz-in, and 68 amps). The load is present at all times, and should stall minimally, but I have a current sensing ic in place to monitor the current and reduce the voltage if it starts getting too high. I don't care too much about bidirectional control, and the current measuring circuit is a 180 A rated attopilot chip. \$\endgroup\$
    – a sandwhich
    Sep 25, 2012 at 12:38
  • \$\begingroup\$ The current sensor is the below link. Do you think it is a viable setup? sparkfun.com/products/10644 \$\endgroup\$
    – a sandwhich
    Sep 25, 2012 at 13:39
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Think about what will happen in a fault condition, should the motor stall or fail. Simply connected a 12V car battery through a couple fets to a 100A motor is not very sensible.

There are a couple home-build projects you can take a look at:

Take a look at OSMC or (less mature) OpenReVolt.

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