[SOLVED] Variable current source design issue : ~1.5A

Hi guys, I am a new member here. I have been lurking for a while though and really gaining a lot from reading the forums as a non-member. I now have a design issue of my own that I can't resolve and so I figured I'd join and see if yall can help :grin:

I am designing a variable current source that can deliver between 0 and 1.5A (does not have to be absolutely 0, but the lower the better). I designed a circuit in PSPice that simulates the exact results I am looking for, however my actual circuit is not outputting more than 200mA. Any help would be much appreciated! I will post schematic and specs below.

(I suspect the transistor may be a bottleneck for current flow since it is getting VERY hot)

Specs:
- uA741 opamp
- 12V 1.5A rated wallwart power supply
- A42B331 High voltage transistor
- 1/4 watt resistors (I know they are under-rated for this power level, but I dont mind frying a few while I use them as a test load)

Schematic:


You can ignore the two switches and transistor on the far left of the circuit. They are just there for triggering the circuit when I put it into my larger design. The output current was the same with them just tied to ground.

For anyone wondering, this is going to be a laser diode driver for a 1 Watt diode.

Thanks very much!
John

A 2N2222 is too small. You need to use a power transistor e.g. 2N3055, mounted on a heatsink.

Next problem is the 741 doesn't have enough current to drive the power transistor, so you need to use a Darlington arrangement e.g. 2n2222 + 2n3055.

Something like this:

ok that makes perfect sense! I had an idea the transistor was the weak link. I will have to make a run to radio shack to pick up a power transistor tomorrow. Thanks so much!

A couple more thoughts.....

• I would use a 1 Ohm resistor instead of 5 Ohms for the current sensing, to reduce the voltage loss and power dissipation.
• IIRC, a 741's inputs can't go all the way down to the negative supply rail, so it's probably a good idea to use a different opamp, or change the biasing.

thanks again! I will adjust the biasing and pick up a shunt resistor while I am there :grin:

That worked like a charm! I went and picked up a TP120 darlington transistor from radio shack, and I used a diode to raise the voltage bias at R-sense, and now Im able to draw over an Amp of current! Thanks a million!