Mechanical Relays: A Basic Overview

(Image source: Panasonic)

A very common phone call received here by our Applications Engineering department is from a customer that is totally overwhelmed reading the datasheet for a relay or trying to find one for their application. This blog will cover mechanical relays and their terminology. I will cover solid state relays in a later blog.

Let’s start with the basics. Mechanical relays consist of a coil (electromagnet) and contacts. The coil is activated when the proper voltage is applied and current is running through it. Once the coil is activated, it exerts a force on the common (COM) contact which moves it, connecting it with the normally open (NO) contact. For a video on how this works and a better visual of the internal pieces please watch:

The main purpose for using a relay is to remotely switch high current loads, control a high current load with a microcontroller or sensor, electrically isolate a low-voltage control circuit from the circuit it’s controlling, etc.

One of the most recurrent pieces of relays that individuals struggle with is the terminology. Below I’ve broken down some of the most typical terms and what piece of the relay they relate to.

First listed are all terms that refer to the coil or input.

Coil Voltage (‘Nominal Coil Voltage’ or ‘Rated Coil Voltage’): This is the rated voltage that the coil is designed to accept.

Coil Current (‘Nominal Coil Current’): This is the value of current that should flow through the coil when the proper coil voltage is applied.

Must Operate Voltage (‘Pick-Up Voltage’ or ‘Pull-In Voltage’): This is the minimum voltage required by the coil for all of the contacts to function.

Must Release Voltage (‘Drop-Out Voltage’ or ‘Release Voltage’): This is the maximum voltage threshold that requires all contacts to revert to their unoperated state.

Coil Type (‘Coil Designation’): There are two types of coils; latching and non-latching.

  • Non-latching will only activate when current is flowing, and will return to its resting position after current flow is removed. Ex. SPST-NO contacts will close when current is applied and open once it is removed.
  • Latching will change to the set position once current starts flowing and will remain activated even after current flow is removed. The only way to return back to the reset position is to reverse current flow.

(Image source: KEMET)

The figure above shows the non-latching relay first, which as you can see has no set or reset for the coil. The second item is a single coil latching relay that shows the polarity of positive on pin 1 and negative on pin 12 to set the coil and negative on pin 1 and positive on pin 12 for the coil reset. The third item is a dual coil latching relay which is similar to the single coil latching relay except the second coil is for the reset.

Maximum Applied Voltage: This is the maximum voltage determined by the manufacturer that can be applied for prolonged periods of time to the coil without causing any damage or failures. Some manufacturers will have a tolerance to allow for spikes in voltage within a certain duration.

The following terms refer to the contacts or output.

Contact Form: This refers to the poles and throws in the switching portion of the relay. The terminology is the same for switches. The pole is the input of the switch. The amount of poles determines how many individual circuits the switch can control. The throw is the output of a switch. SPST (single pole single throw) is the most simple contact form. One input, one output. The outputs can be NO (normally open) or NC (normally closed). (Ex. An SPST-NO relay will only activate the output and complete the circuit while the coil is energized otherwise it will remain open until activated.) SPDT (single pole double throw) has one input and two outputs. DPST (double pole single throw) is two SPSTs in one. There are plenty of combinations of poles and throws but the above should hopefully explain the concept.

(Image source: Digi-Key Electronics)

Switching Voltage: This can be listed as the maximum switching voltage (the highest voltage the contact can safely switch without damage) or nominal (the preferred voltage that the lifetime expectancy is based off of).

Contact Current Rating: This is the max current that can be switched by the contacts safely. It is better to choose a higher current rating, putting less stress on the contacts.

The following are electrical or mechanical terms.

Release Time (‘Reset Time’): The time it takes the contacts to return to their normal state once power is removed from the coil.

Contact Bounce: The time (ms) that lapses during switching due to collision of the metal parts.

Mechanical Life: This is the minimum number of times the relay is expected to actuate under normal conditions as defined by the manufacturer.

Mechanical relays are very simple in essence, but they can become overwhelming easily if you aren’t familiar with them. Hopefully this blog will prove beneficial the next time you need a relay for a project.

About this author

Image of Ashley Awalt

Ashley Awalt is a Technical Content Developer that has been with Digi-Key Electronics since 2011. She earned her Associate of Applied Science degree in Electronics Technology & Automated Systems from Northland Community & Technical College through the Digi-Key scholarship program. Her current role is to assist in creating unique technical projects, documenting the process and ultimately participating in the production of video media coverage for the projects. In her spare time, Ashley likes to – oh, wait, is there such a thing as spare time when you’re a mom?

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