A little out of my depths here...

Need a DC circuit which when a 0-5v input (<200mA) signal is applied gives a voltage out of 0 - 12V (0-3A), is fairly stable and reasonably linear.

Thanks for looking

Matt

 

should specify;

a variable input giving a variable output. Will be going to electromagnets.

 

You could use a rail-rail op amp with a gain of 2.4 driving an NPN darlington power transistor connected as an emitter follower powered from a 14V supply. The transistor will need a good heat sink since it will be dissipating a maximum of about 42W.

 

Such a device is commonly called an amplifier.

 

Thanks for that, do I need to use a capacitor in series on the load resistor, and will pullling the - input on the op amp to ground and one of the diferential inputs be ok. Most show as +/- and ground.

Are there any standard parts (transistors/opamp chips etc..) which I should look for?

 

Thanks for that, do I need to use a capacitor in series on the load resistor, and will pullling the - input on the op amp to ground and one of the diferential inputs be ok. Most show as +/- and ground.

Are there any standard parts (transistors/opamp chips etc..) which I should look for?

From what you've said so far, you can use a single supply with no "-" side, just "+" and ground. As mentioned, you'll want an op-amp that can sense all the way to the ground rail. This will allow an input down to 0V to still give a controlled output voltage. I believe you could use an op-amp such as LM358 that does NOT include the positive rail, though. It's an old and slow op-amp, but cheap and easy to find.

I have no idea what you're talking about with the capacitor and resistor. I suggest you post a schematic.

You'll need a power supply for all of this. You haven't mentioned what you plan to use. An old computer PSU would do it.

 

......................... I believe you could use an op-amp such as LM358 that does NOT include the positive rail, though. It's an old and slow op-amp, but cheap and easy to find.

...............................

He wants high current, thus an LM358 would not be a good choice, since, to minimize power dissipation, you want the op amp output to go as close to the supply rail as possible. An op amp with both input and output rail-to-rail operation would be a better choice with an emitter-follower driver.

 

An op amp with both input and output rail-to-rail operation would be a better choice with an emitter-follower driver.

I was missing that part of the plan. You're saying that, by using an op-amp that goes to the rail, he can achieve 12V from a supply with only 14V, whereas if it goes to rail-1.5 like the LM358, he'd need a 15.5V supply and therefore would be burning off that much more power all the time. Makes sense.

 

and now im lost... could one of you possible post a little scematic for me to copy?
Cheers

 

i think this device is known as Amplifier.

 

and now im lost... could one of you possible post a little scematic for me to copy?
Cheers

Go back to post #3 as your guide. Use the references on this site or Google op-amp circuits. You want a non-inverting amplifier configuration. Then learn what an emitter-follower transistor configuration is. You'll use your op-amp to drive the emitter-follower.

Draw up your findings and post them here. You'll get plenty of help.