Understanding Relay Coil Voltage Specifications

Electromechanical Relays
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When learning about relays there are a couple coil voltage specifications that can be a little tricky to understand. There are typically 4 specification for coil voltage listed in the datasheet. For this example we will be looking at relay part Z2352-ND. This is a SPST non-latching relay with 1 Form A (normally open) set of contacts.

Coil Voltage or Nominal Voltage - 12VDC - this is going to be the nominal voltage to operate your coil at. While this is important to know it does not tell us a whole lot about the available voltage range.

Turn-Off Voltage (Min) - 1.2VDC - This voltage specification is telling you when the coil will no longer be energized. Any voltage less than this and the coil must not be energized and the contacts would be open.

Turn on Voltage (Max) - 9VDC - This specification is telling you when the coil must be energized. Any voltage about this the coil must be energized and the contacts closed.

Max Coil Voltage - 20.4 VDC - This is the max voltage you can safely operate your coil. There is often a temperature related. This part for instance is 20.4 VDC @ 23°C. There is a tolerance of 10%, however it is safest to operate as near to the nominal voltage as possible.

Looking at these numbers you will see an indeterminate range. In our part that range would be 1.2VDC to 9VDC. Generally in this range your relay will stay in its last condition, however it is possible to change conditions while in this range. For this reason it would not be recommended to operate in this range. In the diagram below you will see the ranges.

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As you can see these numbers determine when a relay Must be in an on or off state.

  • From 0-1.2VDC the relay must be in a deenergized (off) state. (Area in Red)
  • From 1.2VDC to 9VDC you are in an indeterminate range.(Area in Yellow)
  • From 9VDC to 20.4VDC the relay must be in an energized (on) state.(Area in Green)
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Thanks for the explanation of the coil voltage. I understand that’s what controls the NO/NC terminals, i.e., whether they are open or closed. So then is the “switching voltage” the voltage that actually goes through the relay, i.e., the power? In general can any relay be used for DC as well as AC power so long as max current limits are observed for each? Or are some for AC power only, some for DC power only, and some for both?

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yes, there is AC relays DC relays and some that do both so look parts in the selection it should tell you as well as the datasheet for that part series

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Hi jp1,
Yes, switching voltage is the voltage being switched on and what actually goes through the relay, where the coil voltage just turns the relay switching contacts on or off.

Mechanical relays can switch both AC and DC loads but they will have different voltage ratings for each, typically they will have a higher rated AC switching voltage since DC is direct current and produces higher power at the same rated voltage.

Solid State Relays (SSR’s) use semiconductor technology instead of a mechanical design and are much more fussy with the load voltage and load current, and will often switch only AC or DC, sometimes both. These usually have a minimum current requirement for the load as well to operate and are much more sensitive to noisy or inductive loads. As Robert had mentioned, the datasheet is a great resource when viewing the specifications for a specific relay, and the DigiKey website filters are a great way to get the selection narrowed down first based on your requirements.