Question ---> Alternate Method To Control The Speed of a Treadmill DC Motor

I have a MC-70 treadmill controller. This is similar to the MC-60.
I intend to use this motor to replace the AC motor on my drill press so that I can do some milling operations.

When I first picked up this power supply/controler and the motor, it did not work. The circuit board had two out of three zeners bad and one of the high wattage resistors open. I replaced the zeners and resistors. This did not bring the board back to life. Although this seems to be the common problems with this particular model. The high voltage side of the power supply works properly. Without any means to control the speed the motor runs at or near max for the supplied voltage. This was confirm bu shorting the source and drain on the MOSFET just before the motor output.
There is some sort of an issue with the signal that controls the FET's gate

I have exhausted my capabilities of troubleshooting this PWM logic side of the controller board.
As with all these treadmill control boards there are many things going on besides providing a PWM signal to regulate motor speed.

Now for the reason I've started this thread....

Since the high voltage side of the power supply functions it seem to me that all I need to do is to provide a means to control the original
MOSFET's gate terminal.

I've looked into several possibilities. One would be to use an Arduino programmed to send PWM to the MOSFET. Not the best choice in my mind. Seem like an over-kill.

Another possibility would be to build a simple PWM circuit using a common 555 timer IC. Although a practical approach, I have an idea that I would like to run past whom ever happens to read this thread.

I have a very simple pre-biult DC motor controller that is PWM and uses the 555 timer. The circuit uses the motor supply voltage, through a 7809
which operates the 555 PWM logic. The output of this PWM ckt. controls the gate of the MOSFET which drains to the motor. These pre-built DC motor controls are very cheap and found everywhere on the internet. The one I have, if I remember correctly, will control a motor anywhere from 12 to 40 volts DC.

Now here is what I'd like to do.....

I can get 15 VDC from the MC-70. If I connect this 15 volts to the small motor controller to operate the PWM circuit. that should result in crating the PWM that will control a MOSFET. If I disconnect the MOSFET from the small controller and run just the single wire for the PWM output to the Gate of the original treadmill MOSFET Will this work??? Oh, forgot to mention the positive for both the high and low current sections of the original control are common. So in my mind, the treadmill MOSFET should be controlled my the small motor control circuit.

This would simplfiy the speed controller and do away with such useless original features as the 'Soft Start" that so many complain about.

Any thought???

 

If I disconnect the MOSFET from the small controller and run just the single wire for the PWM output to the Gate of the original treadmill MOSFET Will this work?

Unless you can keep the connecting wire very short you may find that its inductance, combined with the gate's capacitance, causes spurious oscillation which could result in the FET overheating. Care would also need to be taken with linking the grounds of the controller and the FET's circuit board so that heavy current through the motor and FET doesn't disrupt the controller operation.
Welcome to AAC!

 

Good to know that you think my idea is workable. Taking into consideration your precautions
I would be keeping the length of wire going to the Treadmill FET gate short as I would be placing the small controller on top of the MC-70 board. over where the unused low voltage ckt is now.

Your other caution regarding how the "heavy current through the motor and FET doesn't disrupt the controller operation".
Are you saying that the power to the PWM controller my fluctuate with the current requirement to the motor and thus create differences in the PWM output to the Treadmill FET?

 

If you don't use a 'star ground' system there is a risk that voltage drops produced, in any ground rail shared by the motor and the PWM controller, by motor current surges will alter the potential of the PWM controller's ground and hence affect the controller's behaviour in an unpredictable way. In a star ground system, signal (low current) grounds and power (high current) grounds are brought together only at a single point.

 

Thank you again for your reply to my question.

Not sure if there will be a problem of not having a single point for the ground rail.
Basically what I will be doing is cutting out the original low current/logic circuit traces from the PCB and replacing that logic circuit with another.

Allbeit, it wont be on the same PCB but will be using the same power traces on the PCB as the original logic ckt. In a nutshell, on the power supply board there is a single AC input. That input is rectified at a single point. That DC output from that point is split. Full voltage goes through filter caps to the motor via a FET. From that same AC rectifier is a regulated low voltage circuit (using zener diodes and filter caps) to operate the logic circuit that controls the motor through the FET.

Unless you see a problem here, I'm willing to try out this concept. I'm only into this a few dollars, so it won't hurt to try it out to see how it operates. Although, it won't be for a few days now. Since I screwed up and killed the MOSFET which left a short across the source and drain.

Let me know
,

 

Give it a try. Good luck.

 

You may want to exercise care on components used, use control pot with nylon shaft etc, the power common can be at mains potential.
Max.

 

the power common can be at mains potential.
Max.

Good point. If that's the case, then perhaps the word 'common' should be substituted for 'ground' in my posts above.

 

Point Taken !!!

I appreciate your concerns.

I personally lost a friend who was accidentally electrocuted with 220 VAC.
When something like that happens so close to home, one develops a greater sense for those things that can jump out and hurt you. Especially those things that you can not see, hear, smell or taste.
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