I'm working on a project for my vehicle which will involve a few relays controlled by toggle switches.

I'm searching online for relays that fit the parameters I know I'm looking for (voltage, current rating on the contacts), and I came across a series of products which have some options I don't quite understand. (My electronics background is hobby-level; I'm no electrical engineer.)

The datasheet of the specific product I'm looking at is available here, though my question is somewhat general because I see this on other products (and it's a filterable field on DigiKey). The part I'm curious about is on the last page, where it shows the options with resistors or diodes across the coil: screenshot of the schematics showing optional resistor and diodes across the coil

Some searching online has mentioned a voltage spike when the power to the coil is cut off, and these components are to help mitigate it. This answer leads me to believe that a diode usually takes care of this. But if a diode is used to handle the voltage spike, why is there a resistor option?

How do I determine which is most appropriate to my application? Since I'm using a manual toggle switch (no electronic control for now), does this voltage spike matter to me at all? This question seems to indicate yes. But I'm still curious why the resistor option exists, and if I should consider that instead of the diode.

Edit: As requested in the comments, here's what I'm trying to accomplish:

My new (to me) car has a pushbutton start, whereas my last car had a regular key. When I get home, I often want to finish listening to my podcast before I go in, but I don't want to leave the engine idling. On my old car, I'd simply switch the key to the Accessory position, but my new car has no way to kill the engine without killing the radio (abruptly terminating the Bluetooth connection). On a similar note, I'd roll up the windows before heading in: a feature that was available in the Accessory position on my old car, but not on the new car without cycling the pushbutton from ON/RUN to OFF, then ACC, then ON/RUN again.

For both the radio and the power windows, they get their power from an ignition-switched-hot wire. I plan to insert a double-pole relay in these two switched-hot wires, and when the relay is activated, connect them directly to a (properly fused) always-hot connection. I'm not sure if I'm clearly describing this, so here's a rough schematic of what I'm thinking.

enter image description here

  • \$\begingroup\$ Consider spending more time taking about the specific details related to your application... \$\endgroup\$ Commented Aug 27, 2023 at 2:44
  • \$\begingroup\$ @periblepsis added those details \$\endgroup\$
    – maples
    Commented Aug 27, 2023 at 17:13
  • \$\begingroup\$ Relays were popular, practical and functional LONG BEFORE diodes were invented. The most common use case for the diode across the coil is to mitigate the high reverse voltage spike that develops when a semiconductor driver is turned off that poses a clear and present danger to many cost constrained semiconductor drivers. \$\endgroup\$
    – KalleMP
    Commented Aug 28, 2023 at 6:52

3 Answers 3


Quick answer is that the diode is harder on the relay contacts and easier on the driver (because the relay drops out more slowly).

The resistor is harder on the driver and easier on the relay contacts. The voltage will rise to the steady-state coil current multiplied by the resistor value, so the resistor value would be tied to the capability of the driver as well as the relay coil rating.

If you're not making mass production you'd likely best buy the relay without any internal suppression and add whatever is appropriate outside, be it diode, Zener diode + diode, resistor or whatever else you might deem appropriate.

For a switch, where you don't care about a bit of switch contact arcing and possible EMI resetting MCUs or whatever, leaving the suppression off entirely will make it even easier on the relay contacts (and a bit harder on the switch contacts).

  • \$\begingroup\$ A relay with no suppression is likely to fail with a coil breakdown. \$\endgroup\$
    – fraxinus
    Commented Aug 27, 2023 at 15:08

But if a diode is used to handle the voltage spike, why is there a resistor option?

There are several problems with using a diode as a relay coil snubber.

The first problem is that a diode snubber dissipates energy slowly. This means the the magnetic field from the coil collapses slowly. This means that the contacts open slowly, and this can lead to premature contact failure.

A second problem with using a diode snubber in an automotive environment is the danger of applying reverse polarity. People jump start cars, and occasionally they connect the jumper cables backwards. If a reversed polarity voltage is applied to the snubbered coil, large currents may flow through the diode destroying it. This might happen to multiple relays. The car may become unusable until these relays are replaced. Thus a simple error in jump starting a car can make it quite inoperable.

For those reasons, some relays come with an embedded resistor instead of a diode. The resistor will dissipate the energy in the relay coil faster, and will consequently cause the magnetic field to collapse faster, and the contacts to open faster, leading to longer contact life.

Also, a relay with an embedded resistor is immune to reverse polarity problems. The resistor has no polarity.

A drawback of simply using a resistor as a snubber is that the larger the resistor, the higher the spike voltage. Often a capacitor is added in parallel to the resistor (outside of the relay package) which counter-acts the high spike voltage, but permits the resistor to rapidly dissipate energy.

  • 2
    \$\begingroup\$ I think the usual connection for an RC snubber is with the components in series across the coil. This avoids the charging current spike when the switch is turned on. And it also can be used for AC coils. \$\endgroup\$
    – PStechPaul
    Commented Aug 27, 2023 at 6:01

In DC circuits (like yours), a diode is preffered because it doesn't allow the spike to reach a high voltage. That's the better way to protect the switches or the electronic around. In AC circuits, a simple diode can't be used because it will conducts the current half of the time. In this case, a simple way to limit the voltage spike is to use a varistor who is a voltage-dependant resistor. In your application, all relays will do the job, but I'd prefer the one with a diode. Be sure that the diode is in the correct direction.

  • \$\begingroup\$ All relays may do the job but the ones without spike suppression will degrade the switching elements with arcing and sparking. And diode is the worst solution for the contacts even if it is the easiest solution for the coil spike, better solutions exist. \$\endgroup\$
    – Justme
    Commented Aug 28, 2023 at 7:06

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