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Thermal flasher and relay latch (start/stop)

Transistors and thyristors · Beginner · about 10 minutes · part of Electronics Technician

Left: a car indicator. Lamp current heats a bimetal strip in the flasher; it bends, opens the circuit, cools and closes again, about 1.5 times a second. Right: a motor starter. START energises the contactor, its own auxiliary contact (13–14) keeps the coil fed after you let go, and STOP breaks that hold.

Thermal flasher and relay latch (start/stop): the circuit on the simulator's bench

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How it works

With two 21 W lamps (3.59 A) the bimetal heats to its opening point in 296 ms and cools back in 330 ms: 1.6 Hz.

Remove one lamp (a blown bulb): the current halves and the strip never gets hot enough to open: it stays lit, the classic warning.

Try it on the bench

  1. Delete one lamp: the flasher stops flashing.
  2. Press START briefly: the motor lamp stays on after you release it. Press STOP to drop it out.

Parts on the bench

In the course

Electronics Technician, module “Transistor circuits”.

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