I'm a mechanic!

So, please do not blame me for stupid questions. I did a lot of research using relays for switching DC motors. The following circuit is showing the current version I want to realize this to run a compressor with two motor circuits. One motor is driving one compressor unit.

The right side is powered by 110 AC voltage commutated to 110V DC. The high-voltage side is pretty much the same used in the original compressor.The motor is a 110V/0.9A DC motor. The low-voltage side is fed by a 12V DC power supply. The motor is a 12V/8A DC motor. I'm wondering if the shown low-voltage side is ok since I had to re-design it. Based on the research for switching DC motor it should be ok. The original device was using only one side at the same time. Since I would like to switch both side at the same time I'm not sure if the circuit is missing some components.

  1. is it ok to secure both relays with one diode (D6 = 1N4007) only?
  2. if the switch S1 is released is another diode on the low-voltage side needed to handle induced voltage by motor stop?
  3. the capacitor C1 is more or less copied from the original setup. What is that needed for?
  4. Is it ok to have the relay low-power side in the same power supply circuit as the relay high-power side for the 12 V DC motor?
  5. should I spend some components on both relays high-power sides to protect the relay switches from arcing (was not included in the original setup)?

Thank you very much for your support. I really want to make sure to not harm both DC motors. Any advice is appreciated.

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  • \$\begingroup\$ Hi JFS. 110vAC becomes about 150vDC (pulsed DC.) Not sure if that will hurt M1. S1 should close both relays if they have 12vDC coils. I would add 1N4007's directly across both motor terminals, with the striped end (cathode) to the positive connection. To reduce noise, 0.1uF capacitors could be placed across the motor leads as well. Those capacitors need to be rated for at least 2x their operating voltage (25v and 300v). Careful with 150vDC; a shock could be lethal. \$\endgroup\$
    – rdtsc
    Jan 3, 2020 at 0:36
  • \$\begingroup\$ @rdtsc, thank you very much for your feedback. The 110vAC motor should be ok with the voltage peak since it was originally built like this. What do you mean with "across both motor terminals"? Is it a 1N4007 and capacitor parallel to each motor? Thanks! \$\endgroup\$
    – JFS
    Jan 3, 2020 at 7:46
  • \$\begingroup\$ Exactly JFS. The windings in the motor are essentially an electromagnet, so they store considerable energy as a magnetic field. When the relays open, that magnetic field wants to collapse (instantly), creating a high reverse voltage. This voltage will increase to thousands of volts (and arc across the relay contacts) unless stopped by these diodes. The capacitors also help, since a capacitor resists instantaneous changes in voltage. The capacitors also "filter" some of the smaller spikes generated during normal operation. \$\endgroup\$
    – rdtsc
    Jan 3, 2020 at 12:46

1 Answer 1


Have you considered using solid state components like MosFet transistor in an half or full H-Birdge sturcture pattern ? According to me, it's not really usual to use relays for driving motors but maybe I am wrong. I hope more experienced contributor will explain why. But reliability and high voltage spikes may damage your relay fastly. Below a starting point. https://en.wikipedia.org/wiki/H_bridge

Sorry by advance if my answer doesn't help you much.

  • \$\begingroup\$ Thank you for the answer. I've read about using MosFet transistors for such kind of application. The relays are re-used from the original product. They are rated for high currents and high voltage. \$\endgroup\$
    – JFS
    Jan 3, 2020 at 7:51

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