# Can a linear voltaage regulator work as fixed current limiter ?

#### Externet

Joined Nov 29, 2005
2,089
Hello.
This is not about current regulation nor constant current supply/control
Can a plain voltage linear regulator like 7812 be used to limit current ?

Is this image from the web correct ?

or refers to current regulation ?

What am after is an application where the current will never be higher than -say 1 Ampere- no matter Vin or Load R values. Feeding 5, 8,12V, whatever volts will not conduct over 1 A.
I may be getting entangled in semantics, sorry if that is the case...

#### bertus

Joined Apr 5, 2008
22,215
Hello,

Did you see this schematic in the LM338 datasheet:

Bertus

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#### crutschow

Joined Mar 14, 2008
32,852
Is this image from the web correct ?

or refers to current regulation ?
Current limiting and current regulation are basically the same thing, it mostly depends upon the application.
A constant-current circuit is typically called current limiting when you are just trying to limit the current to protect some load, and may not be particularly concerned about the accuracy or exact regulation of the limit.

Current regulation typically refers to a constant-current circuit with good accuracy and regulation (stiffness).

The LM117/317 circuit can be used for either purpose, as the current limit reference voltage is the LM117 internal reference, which is fairly stable, and the circuit has a high internal gain, to make the output stiff (little current limit change with load resistance or input voltage change).

The circuit posted by bertus uses the forward base-emitter bias voltage of the transistor to determine the current limit, so its accuracy and stability are subject to any change in that voltage, and the gain is low so it's not as stiff, thus it's more of a current-limiter than a current-regulator.

Cheers for the deciBel!

#### Jon Chandler

Joined Jun 12, 2008
732
The LM317 circuit is a constant current regulator. Understanding what it does will help you decide if it's what you're after.

In this circuit, the LM317 will vary its output voltage in an effort to maintain the current set by the resistor. Say the resistor is set for 100mA, and an almost dead short of 0.1 ohm is connected. The LM317 will adjust the output voltage to 10mV to maintain 100mA current flow.

If instead of a 0.1 ohm load, what happens if we replace it with a 1k load. To maintain a 100mA current flow, the LM317 will increase the output voltage – it will give it everything it's got to put out 100 volts to maintain the current flow. But of course it can't raise the voltage that much. The most it can do is its supply voltage minus some overhead which I don't recall.

So, in short, if the current fixed by R1 is more than that drawn by the load, the load will see (supply voltage – overhead). If the load draws more current then set by R1, it will see that current at the lower voltage needed to maintain that current.