current regulating 12v 0-3A

anyone know of a cheap way to do this? i have a 12v ps that goes up to 2.5A and i need to control the current and not affect voltage.. as a reward for any help anyone that provides a good solution ill gladly anodize parts in any color i have(black red yellow blue or clear). feel free to ask questions if you need more info.

nice to see you around man soo....... any news on those 12x sleds? :eg:

I still log in but mostly just to read up on stuff. I stay too busy now days to get on much.. every time I ask the supplier about 12x they bs me and tell me they just sold out. My other supplier has a fifteenth type of 12x but I've got to wait til my taxes get done before I make a large purchase.

ah cool any ways i suck at circuits and stuff so cant help you with the power supply

There are high current versions of the LM317 which can handle 3A, their designations escape me atm though. Still, you'd suffer a voltage drop out no matter what, typically 3v.

Cheap way? Define cheap!

There are a bunch of ways to do this. You can use a constant current switch mode power supply. I've built a few of these, and they've worked wondrously. They drop almost no voltage, and are generally the most efficient.

You could also use an LM317 with a series pass transistor. Basically the LM317 will regulate the output using the transistor as the current handler. Depending on the transistor used, you can current regulate a LOT of power. But, you need a pretty big sized heatsink to regulate higher currents/voltages. Something like this; http://talkingelectronics.com/projects/200TrCcts/images101-200/ConstantCurrent.gif



Or, you can go completely discrete, and use an op amp and power transistor too. Something like this; http://images.elektroda.net/16_1265738802.jpg

Also, Sig, I think the one you're thinking of is an LM350, which would work as well!

You can't hold voltage and current constant. It's either one or the other. What are you powering?

The lm350 would probably be the simplest. You'd be generating a lot of heat though, and you would only be able to operate things in the range of 0-8V. Try benm's circuit here if you want more scalability and a voltage operation between ~0-11V.

A buck driver would be much more efficient especially if you were driving at lower voltages. You can replace the current sense resistor on a $3 DX driver with a pot and get up to 1.8A if you're willing to live with that. Voltage range will be ~1-10V.

it will actually be the current control for the anodizing bath.. im starting to think it will just be easier to buy a suitable power supply b/c im not gonna have the time to buy components and build a circuit.. i was really hoping to find a simple way to do it..

Without dropping any voltage it will be very difficult to regulate the current for any design. Even buck supplies require some voltage overhead. Here are two ideas.
Buck: I have one of these at home to drive LEDs. If you are interested, I could do some measurements for you to find out how much voltage it drops at 12Vin. It has a sense resistor on the back of the PCB and if you attach some wires to it you can use it to "measure" the current with your DMM.
In case you change your mind and want to build something yourself, check this out. It is a simple linear LT3083 based design. It does not drop as much voltage as LM317 based current regulators and it is very stable, produces no current ripple like buck supplies and only requires a few extra components. You can use your DMM to measure the voltage drop across the 0.33 Ohm resistor in order to find the current.

You could also use an LM338T , same as the LM317T , but will handle upto 5A.