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Thermistors and Heater Chains

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The use of Thermistors

The resistance of a cold lamp or valve heater is much less than the resistance when hot. Thus on switch-on a current surge is applied to the heater and this can be many times the operating value. The surge weakens the heater wire by sending shock waves through it.

When heaters are used in series across the mains supply as is the case with Universal or AC/DC mains receivers without a mains transformer, the position becoms much worse owing to the much greater potential current surge. To prevent these destructive current surges a circuit element is included that has a large negative temperature co-efficient. This is the reverse of the heater situation. These devices now just called thermistors, but then also known as Varite or Brimistor devices, have a resistance at room temperature of, say, 5500 Ohms but when hot (250°C) this falls to about 40 Ohms.

The thermistor is soldered into the circuit in the same way as a rod resistor would be. The only additional care is to ensure that the very hot thermistor cannot damage sensitive components or affect the stability of tuned circuits.

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