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If an inductive DC load is controlled via a relay, would that relay need or benefit from a flyback diode for protection from the loads off switching voltage spikes?

Note that this is not about a diode protection for a transistor, but a mechanical switch = relay.

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  • \$\begingroup\$ Aren't the contacts in many relays already protected like that? \$\endgroup\$
    – Solar Mike
    Oct 4, 2023 at 6:01
  • \$\begingroup\$ I have no idea. It's decades since I open up a relay. Don't remember seeing a diode inside. \$\endgroup\$
    – MiNiMe
    Oct 4, 2023 at 6:05
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    \$\begingroup\$ some relays might come with a diode across the solenoid ... they usually have both AC and DC rating, so no diodes across the contacts \$\endgroup\$
    – jsotola
    Oct 4, 2023 at 6:17
  • \$\begingroup\$ Surely this is a FAQ with plenty of duplicates? \$\endgroup\$
    – Lundin
    Oct 4, 2023 at 9:54
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    \$\begingroup\$ Well no. I wanted to know if a diode is needed to protect a relay when that relay in turn controls an inductive load. \$\endgroup\$
    – MiNiMe
    Oct 4, 2023 at 10:14

3 Answers 3

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Yes, a diode across an inductive DC load can extend the life of the relay contacts, perhaps greatly so.

There are a few downsides:

  • The current through the inductive load will take longer to fall off. For example, a solenoid will take longer to drop out.

  • You have to get the polarity of the diode correct or it will be
    destroyed and the high current may well damage the relay contacts.

  • It's an additional cost.

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  • \$\begingroup\$ In addition to contact life, EMI is greatly reduced. EFT (electrical fast transients) is a typical consequence of mechanical contact opening on inductive loads, and a frequent cause of unexpected MCU resets or other spooky behavior in amateur and prototype circuits using relays. \$\endgroup\$ Oct 4, 2023 at 15:27
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I am unable to add a comment as i am yet to gain more reputation 😅, hence i am using this section to add my thoughts. The internal circuit of a relay can be seen in the datasheet provided by OEM. If you have the model/make of the relay you can share here.

I have used flyback diode for inductive loads like a brushed DC motor operating at 30V. The flyback diode was connected to the VOUT of motor driver. It definitely protected the motor driver from back EMF which is generated whenever the DC motor is suddenly stopped.

The concept of using a flyback diode is that, it provides a path for the current to flow back into the coils of DC motor. The return current gradually decreases as it flows though the inductive load and hence sudden surge current(spikes) are not impacting any other components.

If the relay you are using does not have any internal flyback diode, then you can consider connecting a Schottky Diode having respective current and voltage rating to that of your inductive load.

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  • \$\begingroup\$ +1 It should not have been a comment and is a good answer so this is the best way to do it :-) \$\endgroup\$
    – Russell McMahon
    Oct 16, 2023 at 10:34
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If an inductive DC load is controlled via a relay, would that relay need or benefit from a flyback diode for protection from the loads off switching voltage spikes?

Generally yes. Without any form of back-emf protection, when the contact opens, it will form an arc and burn the energy stored in the inductive load. That arc can degrade the contact surfaces and cause premature failure. In an AC circuit you can't use a diode (for obvious reasons) but, in a DC circuit you can. In an AC circuit, the protection is usually called a snubber (a resistor and capacitor in series) and, this can also be used on DC.

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  • \$\begingroup\$ Ac a side note (since you mentioned it) the arc when switching AC is extinguished a lot faster than during the same but DC condition. Circuit breakers for example, there's a not scientific at all but effectful YT video by RODALCO demonstrating how bad an AC breaker is abused by DC. \$\endgroup\$
    – MiNiMe
    Oct 4, 2023 at 9:45

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