PWM 12V to Constant 12V

pwm 12v

Hi,
I'm trying to convert a pulsed 12V signal into a constant 12V signal.
The circuit pulses several times a second so very quick.

Can you tell me what sized capacitors, resistors i'd need to do the job?

I was going to run the cap/resistor in parallel to flick a relay coil. to switch 12v line from a 12v battery.

ciruict >
pulse to cap to resistor to coil
cap and resistor, coil grounded on other end

looking at all the voltages/wattages on maplins website and none of it makes sence.
Please help.

If it truly is PWM, what's the range of duty cycle: 0% to 100%?

Do you want the constant 12VDC output whenever there is any width PWM pulse, and 0VDC when the PWM is 0%?

What will the load on the constant 12v be?

Do-able, but the devil's in the details

Ken

I would like constant 12v on the first pulse. Stop when the pulse is removed for more than 2 seconds, kill the circuit.

Its flicking a relay so there's no load on the circuit.

The pulse is about 10 times a second, or at least thats the sound of the relay at the moment.

I was thinking a 1uf capacitor in series might do it?

If I have this right, the PWM is driving a relay coil, with one end of the coil to ground? You would need a capacitor across the relay coil and diode between the PWM source and the +12v end of the coil/capacitor. The delay would depend on the size of the capacitor, the resistance of the coil, and the drop-out voltage of the relay. If the 12v relay drops out at about 4V, and the relay coil resistance was 200Ω you would need 10,000uF/16v capacitor, for a 2 second delay after the last pulse. Fully charging that large a capacitor and turning on the relay with the first pulse would depend on the drive capability of the PWM source.

This could also be accoumplished electronically with a "retriggerable monostable", sometimes called a "missing-pulse detector". More electronic components, but much smaller component footprint.

Ken