As the name implies, the push-to-start switch utilizes a spring loaded set of switch contacts that will provide a path for current flow when held closed. When the switch is released, the return spring will open the switch contacts thus opening the circuit to the motor, but this handy switch has a way to keep the dryer running, even when you take your finger off the button. Lets get into this little switch and see how it works.
Theory of Operation
The process begins with pressing the start button and closing the start switch contacts, which allows current to flow to the dryer motor and starts the drying operation. Current will also flow through a coil of wire within the switch housing which produces a magnetic field that keeps the start switch contacts closed even when you release the start button. This magnetic field will continue to be active as long as the timer is on a drying cycle, and the door remains closed. Any disruption of the circuit will result in the magnetic field collapsing, and the switch contacts opening. Below is a sample wiring diagram with only the start and motor circuits drawn, to show the active circuit during operation.
Testing the Switch
Push-to-start switches will have three wires connected to them that are used as part of the motor circuit, but additional wires may be found depending on the type of switch. If you have the tech sheet, compare the wire colors to verify which two wire combinations are the switch, and which two are used for the coil. But if you don't have a wiring diagram, you can simply do a resistance check with your meter between any two wire combinations while pushing the switch closed. When making this check, you should find an open circuit on one pair (no connection), a short circuit on one pair (this is the switch), and a small resistance on the final pair combination (the coil). If you find an open circuit on more than one wire pair, the switch either has an open coil, or damaged switch contacts and needs to be replaced.
Common Switch Failures
The switch assembly is a robust unit that really only has one moving part, and most of them will last for years. There are, however, a few failures that could be seen from time to time.
1. The switch contacts arcing shut resulting in a short circuit to the motor. The symptom will be the motor starts running as soon as the timer is turned to start a cycle, and the door is closed.
2. The switch contacts become burnt preventing current flow to the motor. The symptom will be the motor will not start. Easily checked by doing a voltage check into the switch and then out of the switch.
3. The wire coil has become broken causing an open circuit and no magnetic field. The symptom will be the motor will start, but will only run as long as the start button is pressed.
These switch assemblies really are a nice compact component that can be found on many makes and models of residential driers. And once you understand there operation, diagnosing a failure becomes quite easy.