# Current regulation before or after voltage regulation?

I'm making a +12V/-12V power supply (2.5A max. output current each) with voltage regulation and current limiter (+ option to go lower on voltage than 1.25V). In the datasheets, I've came across, that in the Datasheet, page 8, FIgure 20 that there's a current limiter BEFORE the voltage regulation, which didn't made much sense to me. Isn't is supposed to be the voltage regulation first in a "row" here? Because if the current will be limited too low, there's not even necessarily that I could get 1.25V on the input of the "voltage LM-317"?

In the second photo, there's my schematic for the power supply with voltage segment before the current limiter. Is this the more correct one or is the one from the datasheet the one, which I should use?

I marked with red the voltage segment, while the current limiter is red.

there's a current limiter BEFORE the voltage regulation, which didn't made much sense to me.

You can put current limiting after the voltage regulator only if the current limiter causes no excessive volt drops up until the point when it needs to regulate / limit the current.

Isn't is supposed to be the voltage regulation first in a "row" here?

Or, you can put a current limiter before the regulator and the odd volt or so of volt drop that it might incur (in normal operation) can be mitigated against by having a slightly higher feed voltage into the circuit.

Alternatively, you can put the current limiter after the voltage regulator's pass-transistor but, the voltage regulator's sensing feedback system should be after the current limiter thus the voltage regulator still effectively regulates and "lives with" the added volt drop of the current limiter in series with the output of the voltage regulator's pass transistor.

Because if the current will be limited too low, there's not even necessarily that I could get 1.25V on the input of the "voltage LM-317"?

And that is the whole point - if you are limiting current the voltage regulator must reduce its output else how can the current be limited. You cannot have simultaneously over-current limiting AND voltage regulation occuring together.

• "You can put current limiting after the voltage regulator only if the current limiter causes no excessive volt drops up until the point when it needs to regulate / limit the current." Is this ment such as if I don't mind the additional 1.25V drop? "Or, you can put a current limiter before the regulator and the odd volt or so of volt drop that it might incur (in normal operation) can be mitigated against by having a slightly higher feed voltage into the circuit." Well, I already have a 12V toroid which I intend to use, so I won't be buying a new one for this occasion. – Jakey May 28 '18 at 11:37

The current limiter will have a voltage drop across it. If you put it second then you will have an output that will not equal the voltage setting in the first section.

When you put the current limiter first then any voltage drop across it is not seen at the output as long as the input voltage is high enough and the voltage control does not adjust to too high of a voltage at the output. In this case the output section will need a minimal amount of current to bias the LM317 and its feedback network. But in any case the first stage will limit the total current that the output stage can pass through before its voltage output begins to sag down.