Relay tutorial for beginners (Mechanical & Solid State Relays)- Electronics Crash Course #12

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Video Article-

Today we're going to learn about how mechanical and solid state relays relays work

Now a few videos ago, I talked about how transistors revolutionized electronics by their ability to a switch on and off high levels electricity with a small input current without any physical moving parts. Relays do the same job as a transistor but the way they do it is vastly different. A relay will isolate the current that is switching the electronics from the load while a transistor has to sink it's input current to the load while a relay doesn't. I'll explain this better further in the video.

Now 2 types of relays exist, mechanical relays that use electromagnets and solid state relays.

First we'll talk about mechanical relays and then cover solid state relays later in the video. They use they use electromagnet to physically move a conductive metal strip making turning the relay on or off-conductive or non conductive.


Let's tear inside this relay here to see exactly how it works
As you can see we have a flexible contact here with 2 fixedcontact points floating above it. As you can see when the electromagnet is activated, this armature pivots towards the electromagnet causing the flexible contact to break contact with the lower pin and touch the upper pin. Since the lower pin is in contact with out metal strip during without any power, it's called the low power pin. The pin that the contact touches when the electromagnet is activated is called the normally open pin since its normally open / non conductive without a electromagnetic force.

Since relays rely on this physical strip of metal to conduct electricity, they can conduct ,such higher voltages and currents then the typical transistor. Furthermore, they're a much cheaper alternative than high power transistors. However their mechanical nature means that they have a larger physical footprint than transistors and are slower to activate due to their moving parts and are therefore not suitable for fast switching signals that transistors are often used for.


Multiple different types of relays exist. Here are the common ones
SPST – Single Pole Single Throw
SPDT – Single Pole Double Throw
DPST – Double Pole Single Throw
DPDT – Double Pole Double Throw

The poles refer to the number of outputs and the throw refers to the number of positions (picture attached)

Now it's imperative to not run your relay above its rated voltage and current. The reason for this is that as you run higher voltages, they can arc across the contacts due to their short air gap and this can damage the contact points. Furthermore, too much power draw above the rated spec can fuse the contacts together making it a permanently on circuit.

A Electrical Relay Snubber Circuit can prevent against this by adding a capacitor in parallel to the load you're switching.


Next we must realize that the relays electromagnet creates a magnetic field. While this is fine for most normal applications, we should avoid relays I circuits that are sensitive to magnetic fields. Also do note that as the relay state is switched off, it will release a back end voltage sort of like a motor creates a reverse voltage when it freewheels while slowing down. This can potentially damage your controlling circuit and a fly back diodes can prevent this. Look at my diodes tutorial for more information


Solid state relays can mitigate most of these issues associated with mechanical relays. They use the contrling current to activate a led that illuminates a photoressisror and in turn activate a triac to switch your load on and off. You can watch my video on these light sensors here.


Now solid state relays might sound like transistors since they don't move but they're not. Unlike transistors both mechanical and AS relays isolate the activing circuit from the load unlike transistors where the activing current is sunk to the main load being driven.

This video is an introductory video on what relays can do and how to use them. For more detailed information, you can read the links posted in the description below.

I talked a lot about various different electronics used inside relays in this video such as ldrs, transistors, diodes, etc. If you want to learn more about them, click the i icon or the description links to watch my tutorials on them. If you liked the video and want to watch more electronics crash course videos like this, feel free to subscribe.