# current limiter / voltage regulator question

• posted

Hello

I need to deliver a variable current between 0-2 Amps to a small load (15 Ohms).... At first I was thinking about building a current amplifier..... but after investigating that route again, i think, it's too difficult to build.

Then I heard about adjustable voltage regulators. And looking through the datasheet of an LM117, I noticed it had a curcuit example it called: "Precision Current Limiter" where the output current is controlled by a potentiometer and can be varied from 0 to 1.65 Amps.

I'm not sure if this is a variable constant current source, but it kinda sounds like it to me, is anyone familiar with this circuit?

In the "Precision Current Limiter" circuit the current is controlled by a potentiometer, but I need to control the current with a variable voltage (between 0 - 5 V), is there such a thing as a voltage controlled resistor? So that I could use that in this circuit?

any help is greatly appreciated, thank you Joshua

• posted

what would happen if you connected the 0 - 5V directly to your 15 ohm load ?

"all" you need to do is scale the 0-5V up to 0 - 30V the 15 ohm resistance will convert the voltage to 0-2 A

perhaps using a single-rail op-amp and a NPN power darlington in common emitter configuration.

Bye. Jasen

• posted

you only need a constant current source if you have a varying load such as a battery. with a fixed load a variable voltage supply will be fine .

• posted

Instead of using the LM117 "Precision Current Limiter", use the LM150 "Precision Current Limiter". It goes up to 3 amps. Yes it is a variable constant current source. You set whatever constant current you want, and as the load varies, it will maintain that current (provided the voltage supplying the constant current regulator is high enough).

• posted

What the last poster says is true, and recommendation of the LM150/350 is good, but in context of the original question, there are problems with this approach. (1) The OP stated that he needed to supply a fixed resistance with 0-2 amps. The "Precision Current Limiter will not go to zero amps. It can get close, but never to exactly zero amps. (2) The current limit pot that controls the current must be able to handle at least 2 amps... not an easy task for a common potentiometer. I suggest that the OP consider the circuit titled "Adjustable Current Regulator" in the LM150 datasheet for full realization of his requirements. The additional regulator and negative voltage source in that circuit lets the output current go down to zero amps, while still giving good control up to full output current.

• posted

Very good observations, Dave. When you don't go through the numbers, it is easy to overlook some of the details.

Item (1): That may or may not hurt the Op, based on what he is using it for.

Item (2). That would probably be a problem.

The "Adjustable Current Regulator" would be okay, if he has a negative supply. At least according to the drawing, he would need a -5 to -10 volt supply, too (even though a very low current negative supply). Or, am I reading it wrong?

I drew a diagram up, look at