The venerable three terminal fixed and adjustable regulators have
been around for decades.

You can breadboard a precision adjustable current source with a
317 and a bunch of discrete components. Many companies have
improved upon the three terminal regulator by adding additional
pins. I remember a datasheet that showed a four terminal regulator.
The forth pin allowed for easy current limiting without the use of
current limiting resistors, etc. You simply applied a control voltage
to the forth pin to limit the output current.

There are so many variations on the original three terminal linear
regulator, I could search for many hours without finding the IC
I'm looking for. Can someone point my browser in the right direction?

 

Hi Oakton,

you won't need an IC with four pins. Just use an adjustable voltage regulator and drive it as a constant current source.

The LM317L can deliver max. 100mA. Tie a current limiting resistor from the output pin to the adjust pin. E.g. To obtain a constant current of 20mA you calculate the resistor like Uref(1.25V)/Iout, R=1.25/0.02 = 62.5Ohm. Use a precision trim pot (25 turns) for accurate adjustment.

The output current is pretty much independant of the input voltage because the IC always tries to maintain a voltage difference of its reference voltage of 1.25V between the adjust pin and the output pin.

If this current isn't high enough use an LM317T which is capable of 1.5A. (Some datasheets say 1A)

Boncuk

 

I have a simple constant current setup good for a couple amps using an IRF510 and an LM386 opamp. The fet is driven from the opamp which gets it's feedback from a current sense resistor, dirt simple. I think the OP might be looking for a voltage regulator with an adjustable current limit.

__________________

 

An LM386 is not an opamp. It is a power amp with built-in negative feedback for a gain of 20.

An opamp has a gain of 200,000. Ten thousand times more.

__________________
Uncle $crooge

 

I appreciate your replies. Thanks guys!

The 317 and I go way back. I even submitted a couple of
"Ideas for Design" to the trade magazine Electronic Design.
Now defunct, I think. My ideas were published.

You guys like current limiting or sense resistors? I don't.
Unless you can get expensive high wattage resistors with
very tight tolerances. If you work for a big company you
can call up Dale, or some other company that makes 0.1%
resistors, and they'll send over a few without cost.

Its incredibly easy to calculate a resistor value, the problem
is most common (cheap) resistors don't even come close
to their nominal value. Heat is the number one problem.
As soon as current starts to flow through a resistor they
begin to drift, at least the cheap ones do. How much they
drift depends on how hot they become.

You can use a MOSFET as a high current variable resistor.
The only problem is controlling the gate voltage. If its not
controlled with great precision you're in trouble.

 

hi oakton,
What range of currents are you considering.?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/