1 LM317 to regulate both voltage and current , down to 0V

[dso310]

Hello.
I'm making a variable power supply, and using a PIC microcontroller to monitor the voltage the current, and display into an LCD.
Is there a way to regulate both voltage and current with ONE LM317?
(regulate and also know that limit, ex : Imax = 1.2/Rx)
I've found some schematics going with this design down to zero volt, but first, it uses a center taped transformer, second it uses two LM317 regulators...

Anyone to help with some ideas?

 

[williB]

Usually you just regulate the voltage
the current is limited by your load..
if your load is a motor your current is limited by the draw of the motor itself..
if your load is say 1ohm and your voltage is high , then your regulator should have built in current limit..

 

[SteveM]

You could check out
**broken link removed**

Its a dual rail power supply.

Cheers

Steve

 

[eeartist]

LM317 current limit

Hi,
This web sit may be of help to you.

http://members.shaw.ca/roma/

It may give you some good ideas.

 

[audioguru]

If you want both current and voltage to be regulated, you need an LM317 current regulator circuit followed by an LM317 voltage regulator circuit in series.

 

[Nigel Goodwin]

If you want both current and voltage to be regulated, you need an LM317 current regulator circuit followed by an LM317 voltage regulator circuit in series.

It's obvious that it's IMPOSSIBLE to regulate both voltage and current at the same time! - ohms law makes this quite clear!.

For a power supply you would normally regulate the voltage, and LIMIT the current - again, obviously, once current limiting is reached, voltage regulation is lost.

Or, you can regulate the current, using the power supply as a constant current source, the voltage limiting it automatic, limited by the maximum the power supply can reach.

 

[audioguru]

I think of a power supply current limiter as a fixed, crude, inaccurate protection device like the ones in 78xx and LM317 regulators.
However, when an accurate adjustable current limit is required, then a current regulator is required.
Usually a power supply operates as a voltage regulator, switching to cureent regulation (which reduces the output voltage) when a load tries to draw more current than is set by the current regulator.

I don't know of an application that requires a high-power current regulator. I have made many low-power ones then buffered or amplified their output instead. If you use a power supply having both current and voltage regulation as a current regulator then the voltage regulator becomes an accurate max voltage limiter.

 

[eeartist]

Look carefully at the data sheet and app notes provided by National Semiconductor for the LM317. The answers to the OP's questions are spelled out in that documentation.

Have a nice day.