The Circuit Relay

The Circuit Relay is one of the most common wearing parts in modern kiln controllers.  They come in many different shapes, sizes, and voltages, and are used in both computer controlled kilns and some manual kilns.  But what is a relay, and what does it actually do?  Why do they wear out, and how do I know when I need to change mine?  This month’s Tech Tip is all about the Circuit Relay.

A Gate for Electricity

The best way to understand what a Circuit Relay does in your kiln is to imagine the faucet in your kitchen sink.  When you need water, you turn on the faucet, and water flows through it.  When you don’t need water you turn it off, and the water stops.  The Circuit Relay does much the same thing, but with electricity:

Screen Shot 2014-02-15 at 12.02.25 PM

This diagram shows a very simple kiln circuit.  The yellow arrow is electricity waiting at the relay for the kiln controller to let it through to the elements.  When the computer calls for heat, it sends a signal that turns on a small electro-magnet in the circuit relay.  With a familiar -CLICK-, the magnet pulls the open contacts down (or up) where they complete the circuit to the elements, and allow electricity to flow.  When the computer program decides that enough heat has been produced, it stops sending the signal to the relay’s magnet.  The contacts, which are spring-loaded, pop back into the “open” position; making another -CLICK- sound.


The computer will cycle the relay from the “open” to “closed” position and back again more frequently at the beginning of a firing, and gradually change the frequency during the firing in order to match the desired temperature increase of the program.


Its NOT just computer controllers that run relays, though.  On some kiln models, kiln Infinite (or stepless) control switches do the same thing.  In this case, the switch itself is cycling the relay on and off, but you have to turn the switch up and down to change the frequency of the cycle.


So how can a relay go bad?

Relays can go bad in a number of ways.  Most commonly, either the magnet that opens and closes the circuit goes bad, or the actual contacts inside the relay can burn out.  This can be caused by electrical spikes, or can simply happen over time through use.  When they fail, there is one of two results:  Fail OFF, or Fail ON.



Usually, a relay fails in the OFF position.  This means that even though the computer is telling the relay to let power through, no power can get to the elements.  If you suddenly see an entire ring of your kiln go dark, I’d start by looking at the relay first.


Relays can, however, fail in the ON position.  This is known as a runaway or stuck relay.  In this case, the contacts within the relay fuse together, and the circuit cannot open when the computer (or switch) tells it to.  If you have a ring that stays running, even when the program is stopped, or the switch is turned off, then you have a stuck relay.  This is one of the MANY good reasons to make sure you do not leave your kiln unattended…and that you make sure the kiln has actually turned off when the program is complete.


How often should I replace the relays in my kiln?

Relays should last quite a long time, but there are a great number of factors that can cause them to fail.  I typically recommend changing relays every other time you change your elements, but you should also have one new relay in your spare parts (along with a thermocouple) in case of a relay failure.  That way you can minimize the amount of time your kiln is down.  The part isn’t very expensive, and you may always call or email us here at Bracker’s Good Earth Clays.  We will be more than happy to help you determine the right relay for your spare parts kit.


Are there different kinds of circuit relays?

You bet, and they are not always interchangeable.  There are even solid-state relays and mercury relays that have no moving parts, so you need to make sure you have the right one for your kiln.  Kiln relays are also made from materials that are designed to withstand the heat inside the control box.  Not all relays are rated for this heat, even though they may look identical.  Bracker‘s Good Earth Clays stocks the correct relays for most kilns, or can order one sent directly to you from the manufacturer.


Can I change my own relays?

Certainly, though you really should learn about your specific kiln model, and have a wiring diagram in front of you when you do it.  As with ANY repair, you should make sure you understand what you are doing, and how to replace the part before you start.  When in doubt, STOP and ask questions first.  Feel free to call or send me an email.   I am always happy to teach you how to change your own relays when I am working on your kiln so you can do it the next time.


Next month, I will be going over routine maintenance for your kiln, and what information you need to know when you call for help.

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