This is a very useful device for those who are frequently on the move. It will keep your tea, coffee or food warm while consuming little power. The circuit is simple. The ubiquitous timer 555 is used as a free-running astable multivibrator. Diodes 1N4148 are connected in reverse direction to facilitate maximum variation of the duty cycles
Power transistor T1 is Darlington type with 5A capacity and output of more than 60 watts. The chosen discrete components assure fixed frequency of 1 Hz (approximately) at pin 3 of timer IC1 (555). Resister R1 and potmeter VR1 (1-mega-ohm) allow adjustment of the duty cycle. The higher the duty cycle, the higher the output of the heater.
You can connect up to five 10W heating elements in parallel, totaling 50 watts. The consumption of current will be significantly less if fewer coil elements are connected in parallel through toggle switches S2 through S4. Each of these switches has a 6A rating.
Assemble the circuit on a general-purpose PCB. Mount power transistor TIP120 on a thick heat-sink. Isolate the circuit from the heating elements using only two wire connections. Use wires that can carry more than 6A current. Fix the coil elements below an aluminium or steel rectangular plate which is at least 1mm thick. Do not forget to insulate the heating plate from the elements. Use the car battery for the power supply with a proper current-carrying-capacity wire.
