Help me...I'm trying to learn...

[Scotty Rienschield]

Forgive me for my ignorance...but I really am trying to learn. I'm trying to control the speed of a dc motor. One limiting factor that makes this tough is that I only have access to the positive side of the system. The motor is in a slot car...so the path goes + power supply...controller...+ track side...motor..- track side...- power supply. So basically I need to limit the power going through the controller. Typically done by resistance...but I was curious to dabble in pwm. I found a pwm dc motor controller Bakatronics MXA066. I knew this wouldn't work as built, but I thought I could modify it to work.

Attached is a schematic. I basically separated the pwm circuit from the motor + & - terminals. I powered the pwm circuit with a 9v battery...then connected the other side power supply +...motor - (which I connected to Source side of transistor)...Drain side/+ motor tab...+ motor...- motor...- power supply. When I turn power supply on, motor gets power even with dial turned down to 0%. The dial does change motor rpm with power supply at low voltage...but not noticeable at higher power.

Can someone explain what's going on. Is there any way to make this controller work? Maybe a different transistor? Thanks for any help!

 

[(*steve*)]

You really need a ground connection, or a battery to power the PWM part of the circuit.

It is also possible to grab power parasitically, but it would not be my preference.

If you only have access to +ve and ground is via the motor, the basic connection would be V+ from the car power supply to what is labelled as M- and the other connection (to ground via the motor) to the point labelled as G. You could then connect a 9 volt battery between the points labelled G and +12V.

The next thing to do is to connect a diode across the points where the leads to the car power supply and car go. This should be connected so that the end with the band is closest to the M- point. If you connect it the wrong way around, all that will happen is that the car will always run flat out (this will also happen if you connect the leads up the wrong way).

In practice, I would connect the motor to a point as close as possible to the source (S) terminals of MP1 and MP2 so as to minimise voltage spikes.

Once you have this working, you may be able to try removing the battery and shorting M- to M+. This will provide power to your circuit while the PWM has the motor turned off. Beware that this requires C4 to power the circuit while the motor is turned on, so running the car at or near full throttle may no longer be possible.

Best of luck.

 

[GPG]

What are the part nos of MF1,2

 

[Scotty Rienschield]

@(*steve*)

With your explanation, is that with the circuit set up as provided from the factory? Or modified to separate 12v/G from M+/M-? I'm guessing as setup from factory, but just wanted to check.

@GPG

I'll list the numbers in a few..

 

[Scotty Rienschield]

@GPG

See attached pic

 

[(*steve*)]

With your explanation, is that with the circuit set up as provided from the factory?

Yes it is.

 

[Scotty Rienschield]

@(*steve*)

Thanks so much for your help..it worked exactly as you described! I have not tried to eliminate the battery yet, but will attempt that step next.