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What is a starter PTC?

A PTC (Positive Temperature Coefficient) relay is a starting device for refrigeration compressors. It is responsible for feeding the starter winding for a short time to help start the refrigerator compressor motor. If the fridge cannot start, the PTC relay is likely to be defective. This article shows how to test them to determine if they need replacement. Fortunately, these common parts of the refrigerator are relatively inexpensive and easy to replace.


  • Circuit protection current limiting devices as fuse replacement. The current through the device causes a small amount of resistive heating. If the current is large enough to generate more heat than the device may lose in its surroundings, the device heats up, causing it to increase its strength. This creates a self-hardening effect that drives the resistance upward, thus limiting the current.
  • As timers in the degaussing winding circuit of most CRT displays. When the display unit is initially turned on, the current goes through the thermistor and the degaussing coil. The coil and the thermistor are deliberately dimensioned so that the current will warm the thermistor to the point where the degaussing coil will stop under a second. For efficient release, the magnitude of the alternating magnetic field produced by the degaussing coil must decrease slightly and continuously, rather than suddenly stop or decrease in steps; the PTC thermistor does this naturally as it heats up. A degaussing circuit using a PTC thermistor is simple, reliable (for its simplicity) and inexpensive.
  • As heating in the automotive industry to provide additional heat inside the diesel engine cabin or to heat the diesel in cold climatic conditions before the engine is injected.
  • In voltage controlled oscillators with temperature compensated synthesizer.
  • In lithium battery protection circuits.
  • In an electrically driven wax motor to provide the heat needed to expand the wax. Many power motors and dry-type power transformers include PTC thermistors in their coil.

When used with a monitoring relay, they provide protection against over temperatures to prevent damage to the insulation. The equipment manufacturer selects a thermistor with a highly nonlinear response curve, where resistance increases dramatically to the maximum allowed coil temperature, causing the relay to function.

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