I'm struggling to find clarity on some of the listed specs of a relay, in particular the difference between all the voltage terminologies used i.e. nominal voltage, coil voltage, contact voltage, switching voltage, control circuit voltage etc.

My understanding is as follows (please correct me if I'm wrong):

  1. Nominal voltage = coil voltage = supply voltage = input voltage
  2. Contact voltage = secondary voltage = switching voltage = control circuit voltage = output voltage
  3. The voltage listed in the name of a relay (like below) is the output voltage

If my understanding is correct then my confusion comes from when the relay such as the relay above is labelled a 24VDC relay, but then its maximum switching voltage is listed as 30VDC and its rated operational current is listed as 5A @ 30VDC as can be observed here

The reason for my concern is that I'm trying to power a 24VDC solenoid valve with this relay, but I don't want to be switching 30VDC to the solenoid valve as it's only rated at 24VDC. Therefore, if someone could help clear this up for me it would be much appreciated. Thanks


1 Answer 1


The relay contacts will switch whatever voltage you apply to them, but you should not use them to switch a higher voltage than their rating.

The contacts on the relay you show can be used in a 5 Volt DC circuit, or 12 Volt, or 24V, or anything else under 30 Volts.

A relay has two parts, each of which has its own voltage and current rating.

The relay coil ratings give the voltage and current required to move the relay switch contacts - this could be called the input voltage, coil voltage or control voltage.

The contact rating gives the maximum voltage and current that the switch contacts are designed to handle. I don't like the terms "secondary voltage" or "output voltage" for the contact voltage rating (and DEFINITELY NOT "control circuit voltage"!! - that would be the coil voltage). The relay switches a voltage you provide - it does not provide any voltage to the controlled circuit.

A relay can be thought of as an electrically-operated switch.

  • \$\begingroup\$ Does that mean that as the listed control circuit voltage is 24VDC, I would need to apply 24VDC to the coil to activate the "switch"? \$\endgroup\$
    – Seano
    Apr 21, 2017 at 5:04
  • \$\begingroup\$ Yes - the specs say the control circuit voltage is 24 volts DC \$\endgroup\$ Apr 21, 2017 at 5:13

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