FET Constant Current Circuit Reqired

[UK Owl]

Hi All,

I am after a constant current circuit that I can use to provide a constant current e.g. 30 mA for a 3.4 volt LED from a 4.5 volt battery. I can make one using an npn transistor, but it will only operate down to 1.2 volts more than the LED voltage, so I thought a FET one that only requires a a couple of hundred millivolts more than the LED voltage would be ideal in this situation.

I read a post previously on CPF where a 'Magic Resistor' circuit using a FET is used, that will drop to below 1 ohm as the battery depletes, and I would like to try and do this.

I have a bunch of ST72A FET's at hand, but I am a little unsure of how they can be utilised.

Any help will be greatly appreciated.

 

[markus_i]

http://radiolocation.tripod.com/LEDdimmer/LEDlampDimmer.html

Haven't built it yet, and I'm going for a few modifications, but that one should fit the bill.

 

[Doug Owen]

There are other Low Drop Out regulators out there, one in particular I'd like to recommend for this use, the PNP transistor and LM334 based one.

I've built quite a number of these running at or near the 30 mA level. Efficiency is very high (with the right battery choice) and current regulation to under .1 Volt difference between Vf and battery are routine.

Total parts cost is under a dollar. A small transistor, a three lead IC the same size, 3 resistors and a small capacitor.

Doug Owen

 

[UK Owl]

Thanks guy's.

I will have a go at these this weekend when I have some time available.

 

[electronics4life]

On Dougs advice I have already purchased my components for the LM334 circuit. I Just have to build it now. Gotta dig up thoose older threads and see where the LEDs are actually suppose to go in the circuit.