Cherry Heaven TV has made some short on-line radio and tv programmes about making and working with kilns. This one is about replacing the relay on a Paragon SC2.

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A relay is a device which allows a low power circuit to switch a high power circuit. Most electronic programmers have a low-voltage supply, typically 5V or 12V. Although there are several ways of using a no-moving-parts low-voltage circuit to switch a high-current circuit, such as the kiln's heating elements, it's common to use a separate electro-mechanical device called a relay.

When the programmer decides to turn the elements on, it sends a low-voltage low-current to a relay. The current goes through a coil which becomes an electromagnet. The magnet part attracts a small strip of sprung steel which moves and allows two high-current contacts to touch. If the programmer decides to turn the elements off, it stops the current, the strip springs back, and the contacts separate.

High-voltage components aren't mounted on the low-voltage programmer circuit board: it's safer to keep the mains away from the user fascia in the event of a fault, and any heat away from the electronics. It's also easier to replace an inexpensive relay if it's not an integral part of an expensive circuit board.

Most relays are designed so that their natural state is off: an important safety feature. Prolonged spark corrosion of the contacts sometimes causes them to stick closed or to fail to turn on. You can buy relays in the on-line shop.