Looking at controlling a HVDC motor like so -

  • Type DC781 (2) LSG
  • Operates on 6 - 220 VDC
  • 84 ohm terminal - terminal resistance
  • No load speed of 8000 RPM @ 220 VDC - 100 mA
  • Stall current at 220 VDC: 2.6 A

I would be using 120VAC input however


My questions are can I turn this on and off with a relay? What would be the best location for this relay, to the motor or from the AC connection? I imagine there may be issues with the inrush current.


Basically can I use this circuit replacing the "Power Switch" with a relay like this - https://www.digikey.com/en/products/detail/panasonic-electric-works/ALZ11B05W/645567

A rectifier (400V rating, 4A) - https://www.vishay.com/docs/88655/kbl005.pdf

Also what capacitor would I need or do I need it at all? Quick napkin calculation implies I'd need a huge cap, but I imagine a ripple doesn't really matter for a motor like this? Is this accurate that I can skip the cap entirely? And since 170VDC peak from rectifier averages back out to ~120VDC at 50-60Hz anyway? Correct?

And lastly selection of a thermistor to limit inrush current. I am unsure on what parameters to select here, I imagine just current.

  • \$\begingroup\$ Hi. May I know which software did you use to create the drawing? \$\endgroup\$ Commented Sep 5, 2022 at 12:57
  • \$\begingroup\$ Hi. The drawing was found online, but it looks similar to circuitlab.com \$\endgroup\$
    – nk13
    Commented Sep 6, 2022 at 14:12

1 Answer 1


Capacitive loads tend to cause relay contacts to weld when closing

unless torque-ripple is a problem you really only need a capacitor large enough to suppress the electrical noise of the motor. (100nF perhaps)

A spinning DC motor itself acts much like a capacitor - do some experiments at low voltage if you don't believe me.

Recitified 120V sine wave DC will act like about 160VDC when connected to the motor, but that's ok because your motor is good up-to 220V

Given the numbers presented that relay looks suitable, and you'll no doubt find it convenient that the coil terminals are distant from the switch terminals.

if you don't use a large capacitor you won't need to worry about inrush (unless the 2.4A stall current is a problem)

if you do use a large capacitor you'll probably need to upgrade the rectifier, and may want to consider adding a choke between the rectifier and the capacitor to help with power factor.

The best location for the relay is on the AC side of the rectifier.

if you don't like the relay option 2.4A is not much current so a solid-state solution should not be too expensive some sort of transotor or SCR controlling the DC supply.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.