Electronics 2000

Hi Everyone..

I produced a pwm circuit comprising of a 555 timer astable at a frequency of 10Hz which switches a Darlington transistor (tip120) { collector connected to another supply different from that of the 555 ic} {base connected through a 10k resistor to the output pin of the 555} the emitter is connected to a Lm7806 to give a stable output voltage of 6v... the idea is to distribute a voltage of approximately 7V at the input of the voltage regulator .... and then regulate the noisy voltage supply to a clearer one.

thus an efficient supply of 6v is taken from say 18Volts supply...
unfortunately this doesn't work...can anyone help me.Thanks in advance.

Hi

Firstly 7V input to the LM7806 is too low to give a regulated 6V output - the input needs to be at least 2V higher than the output. If you can't raise the input voltage you could use an LDO (low drop-out) regulator.

Check which bits of the circuit work - do you have the expected output from the 555 into the transistor? If not disconnect the transistor and check the 555 output on it's own. Beyond this a circuit diagram would help explain things better.

If I understand correctly and the purpose of the circuit is to step 18V down to 6V without wasting too much power in the regulator you could use a switching regulator. I think switching equivalents for the conventional LMxx linear regulators exist, although I don't have part numbers to hand.