I am currently using an 80A automotive relay to control a motor. The relay seems to work great to control the motor, but on disconnect a visible spark can be seen through the clear relay housing. I understand that the spark could damage the relay and I need to prevent it.

Should I use a diode or a capacitor? Also what size/ ratings are required?

Thank you


2 Answers 2


Assuming it's a 12V DC PM motor (and assuming you're not reversing current to the motor, or allowing the motor to be driven backward from the shaft) you can put a diode across the motor.

Diode PIV rating has to be sufficient to handle any transients on the 12V line. Current rating needs to be high enough to absorb the magnetic field energy in your particular motor under all conditions (including stall).

  • \$\begingroup\$ Sorry for not being specific, the motor does reverse the current. Each motor is controlled by two relays wired like this: bpesolutions.com/gadgets.ws/gprojects/svrelay1.gif \$\endgroup\$
    – brad123664
    Commented May 6, 2014 at 17:48
  • \$\begingroup\$ You can use four diodes- one pair on each common, to +12 and 0V. \$\endgroup\$ Commented May 6, 2014 at 17:54

This method is offered as an alternative to using diodes (Spehro's answer).

You could use a non-polarized capacitor in parallel across the motor - it will limit the voltage seen when the relay(s) open but, you have to choose the correct value and, it can be a little hit and miss unless you know the leakage inductance of the motor. Assuming it is 1 mH and the peak current is 10A, this is a stored energy of: -

E = \$\dfrac{L\cdot I^2}{2}\$ = 0.05 joules.

Using the energy storage formula for a capacitor (\$E = \dfrac{C\cdot V^2}{2}\$) you can equate the two and calculate the capacitance: -

0.05 = \$\dfrac{C\cdot V^2}{2}\$ therefore C = \$\dfrac{0.1}{V^2}\$.

Let's say the charge voltage on the capacitor is allowed to rise to 100V to protect the relay contact. This means C = \$\dfrac{0.1}{10,000}\$ = 10uF.

If you need the motor to stop more quickly, a capacitor can have a benefit; the motor current (energy) sloshes back and forth between motor leakage inductance and capacitor and deteriorates faster - you can even make the capacitor into a conventional snubber by using (say ) a 10 ohm resistor in series - this will increase the initial peak voltage from 100V to 200V when first open circuiting the motor (current was 10 A I assumed originally) but the energy is dissipated faster. If you can live with a higher voltage than what a diode would permit then maybe using a capacitor should be considered.

With diodes, the energy does take longer to dissipate and on speedy applications where the motor has to be reversed (or stopped) quickly a capacitor or snubber is preferred and, realistically you need to understand the motor's leakage inductance to choose a diode so maybe, if you have the system already built, you can try a 1uF, 200V non-polarized capacitor and see if that appears good enough.


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