Simple current variable circuit

[harp]

At electronic point of view, what circuit is best in performance to control shutdown pin of adjustable voltage regulator, and why?

[stj]

those make no sense,
using a transistor to bypass the ajustment resistors and ground the feedback pin should force the regulator to go full-on.

[sam_sam_sam]

The LM317 does not have shut down pin you need to look at something like XL4015 or something like this but if you buy the premade module they do not use this pin for nothing so you would have to solder a wire to that pin so you could control it that way

If you want to use a LM317 you can buy a kit with all the parts or you can get it assembled module and they are not that expensive and you could add a 5 amp relay with a push button for the on and off switching and if you want to get a little fancy you could add LED light indicator so you would know if the output is on or off will do the same thing but my suggestion is to put the positive and negative output wires to the relay for best results this would be the way I would do it if I was building one

[stj]

you could power the regulator input through a fet

[harp]

When adj pin is connect to ground, vreg act that lower output to 1.2v.
I see I am not be clear in question... regardles to arrangement (orientation, configuration, position) of "sensing transistor", what circuit can best match proper lab power supply?

This ask because I build something like in picture 3, and it work, but not work like expected, ie, cant setup a small current limmit like 10mA (minimum is 190mA) and affect voltage control.

[stj]

none of those circuits monitor the output current.
you cant monitor the input because it will be effected by losses in the regulator.

[redwire]

There's two problems with those circuits - sensing the current, and then dialing down the output voltage, to get lower current.

Best they can do is take the ADJ pin to ground, giving 1.25V output. So you will still get full current below 1.25V !
A short-circuit or charging 1.2V battery... there is no current limiting at all. Unless you add a -ve voltage rail like -1.3V

Once I designed a bench power supply and the lowest output voltage it could go to was 0.05V and I set the current-limit to 100mA it was working fine. Tried shorting the output and got over 5A! Took me a while to figure out what was wrong. The current-limit circuit could only dial down output voltage to 0.05V but this was lots of current into shorted leads.

To sense current using a transistor E-B junction, you need a 0.7V drop across the sense-resistor. This is very lossy and makes heat. It's only a good cheap scheme for outright overload protection. Terrible for variable current-limiting.

[sam_sam_sam]

When adj pin is connect to ground, vreg act that lower output to 1.2v.
I see I am not be clear in question... regardles to arrangement (orientation, configuration, position) of "sensing transistor", what circuit can best match proper lab power supply?

This ask because I build something like in picture 3, and it work, but not work like expected, ie, cant setup a small current limmit like 10mA (minimum is 190mA) and affect voltage control.

It would help to know exactly what you want to do with this will depend on what circuit you are wanting to do

I may have the same need as you but I looking for a solution for a 5 volt but the lowest milliampere is 50 but I using for charging very small battery cells that have a low current output but I might just use a LM317 setup in current mode and a on / off switch for the output

I could use my battery testing machine to charge them for testing but I really do not want to use it for just charging a battery pack this way In the past i have used LM317 regulator in the current mode now for very low current you do not have to use a heat sink

But if you are planning on using it as a bench power supply I would highly recommend you use a heat sink now if you want to have high current and low current use use a switch to go between high and low because if you do high current you need to use a wire wound pot control if you use a carbon based pot control you will burn it but trust me I have done it before

For a very long time back in the day before 18650 and 21700 came on the market and the only rechargeable battery cells your only choice was nicad battery cells I had a LM317 setup in the current mode and a resistor and pot control the reason for the resistor was to control the maximum current that the pot control would give you and had another LM317 in voltage mode when I wanted to do fast charging I also made a charger that had voltage and current control but I dismantle this many years ago when 18650 came on the market

[harp]

Thank you all for information!
So, it will work if control current circuit be moved as-is and put between output and divider of LM350 vreg, while feed negative -1.3v on adj pin?

Sam, I know what I want, but dont know how to do it, or if possible on simply way. My goal is to add current limmiting circuit on a clasical simple LM350 voltage regulator, seamlessly without complicated circuit, and that work more-less like classic lab psu.
The power is not important, max may be 3A for that vreg or less, but the ability to adjust wanted current is crucial, especialy near zero.
When I have finished circuit, fine adjustment can be make, like proper spanning section of transistor or so.

[sam_sam_sam]

I have try those circuits with the bypass transistor but I was not very impressed with the results that I got from them either because I did use the exactly same parts that were listed or just because I did not have add the fine control pot I am not sure if the LM350 are still available because I have not seen any advertisements for any of them in many years now if you find some please post where you got them from the popular transistor used for the bypass circuit was the 2n305 but I am not sure if they are still available or not either because the last time I saw some available was several years ago

I just did a eBay search for a bench power supply and all I see is switching power supply really there are certain things that you should not try to test with a switching power supplies like motors and things like that

I have bought a switching bench power supply and I was not really impressed with it because after using it a year or so the voltage was not as stable as it was new and even after recapping it was not much better so I have not bought another one a good thing I have several bench power supples that have real power transformers in them and to me the way I use mine a switching bench power supply would not last

I did a google search and found a linear bench power supply but they have really gone up in price since the last time I bought one

https://www.parts-express.com/Bencht...904?quantity=1

i can see the reason why you want to build one

If you are looking for a transformer you can still find them at MPJA electronics store here in Florida or at least the last time I looked which was not that long ago