I'm working on a project to run a 12 volt 12 amp DC gearmotor via PWM output from a controller. I am using a Crydom solid state relay to power and motor and trying to figure out what diode is correct for this application. I have read about using a zener diode for fast operation. Is this the proper type and how do you size the diode to this application?

Thank you

Edit: Motor: Leeson M1125273: .13 Hp, 12vDC, 12.0 A

SSR: Crydom DR2220D30U: Max Turn-On: 75 usec Max Turn-Off: 100 usec

PWM F is 100 Hz

  • 1
    \$\begingroup\$ You start by collecting all the specs of the involved components, i.e. the motor, its operational characteristics, what your relay tolerates and how fast it switches off, and you start drawing a schematic. \$\endgroup\$ Mar 15, 2019 at 19:38
  • \$\begingroup\$ What are the specs for motor L,DCR and SSR Ron and f_pwm and I start max \$\endgroup\$ Mar 15, 2019 at 20:01
  • \$\begingroup\$ got any specs yet? \$\endgroup\$ Mar 15, 2019 at 20:25
  • \$\begingroup\$ added specs to original post \$\endgroup\$
    – tmatt1066
    Mar 15, 2019 at 20:30
  • \$\begingroup\$ You would not use a zener diode. A zener diode is intended to operate in reverse breakdown. Flyback diodes operate in forward-biased mode. You want a fast switching diode like a schottky. Do you understand what a flyback diode is supposed to do? (Are zener diodes considered fast? TVS diodes are similar and are considered fast, but I personally never considered zener diodes to be fast). \$\endgroup\$
    – DKNguyen
    Mar 15, 2019 at 20:44

1 Answer 1


Your "catch" diode could have instantaneous current equal to your load current, so look for a fast switching diode with a current rating at least 12 amps. As @Sunnyskyguy points out, your solid state relay (SSR) is undersized for a 12A motor. Note that the SSR spec labeled "Load Current, DC Motor Starting UL508 FLA @ 40°C [ADC]" gives a value of 5.4 amps. This means that this SSR is rated to handle the starting current for a motor with FLA (Full Load Amperes) of only 5.4 amps, or less that half of your motor's 12A rating.

Good Luck!

  • \$\begingroup\$ Thanks for the explanation. I'm now looking at a Crydom DC60D20 SSR. The charts don't show the Motor starting amps like the other one. Does that mean that the max load current is for motors on this relay? \$\endgroup\$
    – tmatt1066
    Mar 15, 2019 at 21:47

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