DIY Appliance Repair: Infinite Switches
An Infinite Switch is a electromechanical switch that operates quite similarly to the thermostat that controls the heating and cooling of your home, both employ a bimetallic strip of metal to control the opening and closing of the switching contacts. The Infinite Switch differs from the thermostat in that where the thermostat depends on the ambient temperature to affect the movement of the bimetallic strip or coil, the Infinite Switch depends on the electrical current flowing the the bimetallic strip heater coil to effect its movement. The flow of electrical current through the bimetallic strip actually generates the heat required to effect the bimetallic strip's movement. The position of the Infinite Switch's shaft determines the tension being placed on the strip and therefore the temperature the strip must reach before it moves enough to open the switches contacts.
What is a Bimetallic Strip?
A bimetallic strip, is a strip composed of two different metals, usually copper and steel. At time brass may be substituted for the copper.
The two different strips are formed into a sandwich by riveting, welding, or brazing them together. Since both of these metals have different coefficients of thermal expansion, they bend into a curve when heated. This curvature is effected by placing the metal with the higher coefficient of thermal expansion on the outside/top of the bimetallic strip as shown in the above drawing.
Tools and test equipment needed.
Digital Multimeter (DMM) or an analog Volt-Ohm-Miliammeter (VOM)
A Word of Warning.
Keep in mind that when working on an electric range or cook-top, you are working with 240-VAC and it could kill you. In the case of a free-standing electric range,unplug it from the wall or turn off the circuit breaker at the service panel. In the case of a hard wired range or cook-top, always turn the branch circuit breaker off on the service panel before beginning to troubleshoot. There will be times when you will have to troubleshoot with the power on, but this is not one of them. All the tests that you will be performing are continuity and resistance tests which require the power to be disconnected or turned at the panel.
Make a sketch.
Unless you have a schematic of the electric range or cook-top that you are working on, it would behoove you to make a sketch showing how the wires connect before removing them from the Infinite Switch's terminals. Depending on the make and model appliance you have, the wires may be connected by screw terminals or by push-on terminals. This drawing is a typical layout for the terminals on an Infinite Switch.
This is actually a Chromalox CH Infinite Switch and the one I prefer using. Chromalox switches may not be the most inexpensive switches on the market but they are the industry standard and the most reliable.
How to test the Infinite Switch.
Using 1/2—inch masking tape and a fine-point marker, label each of the wires as you remove them from the terminals on the back of the Infinite Switch.
Switch your DMM to the Ohms Range if it is an auto-ranging meter or to its R X 1 Range if it is a manual ranging meter, and refer to the above diagram as you make the following test.
Turn the Infinite Switch fully clockwise, to its maximum setting, and check for continuity between the “L2” and “H2” terminals. No continuity, indicated by an “O.L.” on a DMM LCD, means the Infinite Switch is defective.
With the Infinite Switch still set at maximum, check for continuity between the “P” Terminal and “H1”. If there is no continuity between “P” and “H1”, the pilot light circuit is defective. The switch will still control the burner but should be replaced if the pilot light function is a necessity.
Check for continuity between “L1” and “H1”. No continuity indicates a defective Infinite Switch.
If the switch checked good, replace the wires as marked. If the switch is bad, remove the switch and install a new one wiring it as indicated on the diagram.
In most cases, the hardest part to this project will be gaining access to the switches. Since that will vary from one make and model of range or cook-top, I will not go into that here.