Relay Hysteresis Demonstration

Added: 2026-05-05

[00:00:06]Did you know that relays have hysteresis?

[00:00:10]In order to illustrate that phenomenon, I have built this simple circuit. We have this relay and it is connected to the power supply in order to activate the coil. It is a 12-V relay and this LED which is powered by two AA batteries.

[00:00:35]The LED is connected to the normally open contacts of the relay.

[00:00:41]Therefore, if the relay is not powered, the LED is off. When we apply power to the coil, the normally open contacts close and the LED turns on.

[00:01:00]In order to illustrate the hysteresis behavior, I will start with applying 2 V to the relay. Of course, the voltage is very low and the coil is not activated with only 2 V, but let's start to increase slowly the voltage to see at what voltage the coil activates the relay.

[00:01:37]There it is.

[00:01:39]The relay works with 8.3 V.

[00:01:43]It is a 12-V relay, but it can work with only 8.3 V. Now, we can think that if we decrease a little bit the voltage again, the relay will turn off. But, that is not the case. Let's see.

[00:02:03]We decrease.

[00:02:05]7 V, the relay is still on.

[00:02:10]6 V.

[00:02:14]5.

[00:02:17]Still on.

[00:02:22]We need to decrease up to 3.2 V in order for the coil to deactivate.

[00:02:29]And now, if we go up again, we will see the same numbers.

[00:02:40]8.2 activation and 3.2 deactivation of the coil.

[00:02:58]It is this difference in voltages of activation and deactivation that we call hysteresis in the relay. And hysteresis is present in a lot of different physical systems.

[00:03:14]We can think that it is just a funny behavior. However, we can take advantage of the hysteresis in the relay in order to build interesting circuits. In a forthcoming video, we will make a push-on push-off system using a relay and the hysteresis behavior.

[00:03:36]Thanks for your visit.