Here is a simple heater circuit that can be operated from a 12V battery. The first part of the circuit is an a stable multivibrator build around the two transistors Q1 and Q2 .The ON time of transistor Q2 is set to 0.5 S. The ON time of transistor Q1 can be varied by using the POT R2.The output pulses at the collector of Q2 is used to drive the Darlington power transistor Q3(TIP122).The transistor Q3 drives the heating elements L1 to L3.The net heat produced can be varied by selecting the desired combination of heating elements at the output circuit sing switches S1 and S2.The net heat can be also varied by varying the duty cycle of the triggering pulse using POT R2.
Notes.
* The circuit can be assembled on a general purpose PCB.
* The transistor Q3 must be fitted with a heat sink.
* The elements L1 to L3 can be 10W heating coils.
* The switches S1 and S2 must be able to withstand at least 5A.
* The circuit can be powered from a 12V battery.
* The LED D1 gives a visual indication of the duty cycle of the circuit.
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