I have a 90V PMDC motor with an triac/diac speed controller. Although, I am not sure, the power output is around 90V, but it may be higher. The torque is fairly constant, but it hiccups at the lowest setting unless you put some load on the motor. Here's a schematic:

enter image description here

I purchased the following PWM controller online:

6V-90V 15A DC Motor Pump Speed

Unfortunately, I don't have a 90V DC power supply and didn't want to fry my new board, so I connected the original speed controller output to the new PWM controller power input. As soon as I turned it on, POOF! The rectifier and triac were destroyed on the original board, but the PWM was unscathed. Can't figure out what happened, but my theory is that the low speed hiccup was probably caused by the voltage dropping below zero and the PWM controller sent it back whence it came from (latchup?).

I am able to get the motor to work, but only on low voltages, which easily stalls with finger pressure.

Now, I don't have a 90V power supply anymore, so I will have to make one. Can I just use a voltage divider using high power resistors and a full wave rectifier?


Let's say you wanted to make a voltage divider that created 45V @ 15 amps output. Your resistor would have to handle the other 45V @ 15 amps. That's 675 Watts that a single resistor would have to handle. A kilowatt resistor runs somewhere around a couple hundred bucks. That's only for a single power setting. You'd want a variable kilowatt resistor. I'm not sure that even exists. If it does, it'd cost you even more.

In summary, NO. It's a bad idea. You need a smarter power supply so that you're not having to dissipate so much energy directly as heat.

  • \$\begingroup\$ how about capacitive divide? \$\endgroup\$
    – user148298
    Jan 20 '15 at 15:01
  • \$\begingroup\$ @user148298 15 amps at 60hz, you're looking at two huge capacitors which would likely be just as costly if not more so than the resistor. \$\endgroup\$
    – horta
    Jan 20 '15 at 15:27

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