MR16 with external mosfet for motor control

[theavguy99]

Hey Guys,

I'm using a mr16 to control my yard train for this year's show.

Any ideas on what I could use as a high current dc motor controller, that would be driven by one of the mr16 channels?

The motor I'm using is a power chair 24vdc brush motor, and has a stall current around 30 amps or so.

I was thinking of using something like the Crydom D1D60:

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but it's fairly expensive....

During testing of the mr16, I've been driving a small 12vdc relay from one of the channels, which I had planned on using this relay to then control a much larger solenoid that then would switch the motor on/off- but the problem is that running the motor at 24 volts, I think it will run too fast.  Not certain that I can run it at 12 volts (1./2 votlage) without overheating issues.  Therefore I need some method to reduce the motor speed.

I've also considered using a typical electric scooter cotnroller, but these have hall effect throttle controls, and I haven't figured out an easy method to interface the MR16 to the throttle control inputs.

 Any other ideas? 

[theavguy99]

Has anyone come up with any ideas on this one?  I posted this some time ago and haven't seen any replies yet.  Getting kind of desperate at this point...

Thanks!

[KeithTarpley]

Greetings,,,

Basically, I don't think any can jump in without having a good idea as to how it would work for sure.  You got two problems.  First, just the basic relay, if you go that route, then a speed controller.

The ESC, Electronic Speed Controller, may be what you want to look up.  Lots of them.

The cost may be more than what you want to spend right now.

Keith

[rm357]

I've never played with that much power, but here is some theoretical info to get you thinking. The TI chip on the MR16 is basically a Pulse Width Modulator (PWM - or constant current device if you like that terminology), which is the preferred method for speed control of DC motors. If I read the specs right, the grayscale clock is 30MHz with 4096 steps, which translates into a pulse frequency of about 7.3KHz (now that I'm curious, I'll have to dig out my frequency counter)... If you use a bank of mosfets or a solid state switch that can handle that frequency, then you can use the standard dimming values to control your speed.

The PWM works by being either 100% on or off (hence constant current) and using the "dimmer" value to control the ratio of on to off time. This gets around the "stall current" for DC motors and allows you to run slower than an analog voltage would allow.

Be sure to set up some kind of safety cutoff - the DMX protocol we use is not suitable for anthing safety related. If your train goes crazy or if someone/some thing gets in the way, you need to have a safety switch that automatically kills the power.

RM