Precision current limiter LM317

How exactly are these circuits meant to work?

I have made a bench top power supply with a small salvaged transformer. This has 2 sets of secondary windings, one of which is center tapped. In both cases I have made a rectifier to give me +-32V and +-24V


I have then used an LM317 to make a variable voltage regulator with a pot that I can plug into the above outlets so that I can vary the voltage from about 22V to 1.5V.

But when I tack circuit below on to the output I can only vary the voltage between 21V and 17V or something like that. My idea was to protect the secondary windings of the transformer from being inadvertently fried.

Any other suggestions how I could accomplish over current protection without affecting my variable voltage regulator?

But when I tack circuit below on to the output I can only vary the voltage between 21V and 17V or something like that

Click to expand...

How? Show your circuit.

The presented current limiter will however expose a minimal voltage drop o 2 to 3V. It might change the behaviour of the voltage regulator.

Speaking more generally, a current limiter should be designed into the voltage regulator circuit. Commercially available lab supplies are achieving this perfectly.

FvM said:

How? Show your circuit.

The presented current limiter will however expose a minimal voltage drop o 2 to 3V. It might change the behaviour of the voltage regulator.

Speaking more generally, a current limiter should be designed into the voltage regulator circuit. Commercially available lab supplies are achieving this perfectly.

Click to expand...

Well my LM337 and LM317 circuits are straight out of the datasheets - adjustable voltage supply with improved ripple protection and protection diodes. Stock standard I would think.

I simply series added current limiting circuits, using LM337 and LM317 respectively, to the output of my LM337 and LM317 voltage regulator circuits.

E.G. The first LM317 regulated the voltage and the second LM317 limited the current coming from the former. No idea of whether you can do this but I thought I would just try it.

But actually come to think of it the voltage regulators themselves have current limiting built in so I am probably wasting my time and the extra LM317 and LM337..

I see that you didn't yet think about the effect of the current limiter voltage drop and how it affects the voltage regulator. You should do.

Exchanging the order (current limiter in front of voltage regulator) makes more sense. It limits the available output voltages, but doesn't affect the regulation below a certain value.

boylesg, I think it would be useful if you looked at how the current is limited in this LM317 circuit so you understand why it isn't suitable when placed after the voltage stabilization.

The LM317 family are actually voltage regulators and the intention is to maintain constant current by dropping a regulated voltage across a fixed resistance. It works on the principle that I = V/R and if 'V' is regulated and 'R' is a fixed value, so must 'I' be a fixed current. What the circuit is doing is treating the resistor as the entire load for the regulator (imagine the ADJ pin is grounded) and to is trying to stabilize the voltage across it to 1.25V, the reference voltage inside the LM317. You can do the same thing with 780x fixed voltage regulators if you choose appropriate series resistors. No matter how you look at it, there will be voltage dropped in the LM317 itself plus up to 1.25V dropped across the resistor.

Brian.