# Low voltage regulator

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#### BOOJAN

##### New Member
I need one low voltage regulator, which can supply voltages less than 1V (<1V)….so can anybody help me??

#### Hero999

##### Banned
Why?

How much current does it need to provide?

What's the input voltage?

#### tcmtech

##### Banned
how much less than one volt?
and whats is it being used for?

#### BOOJAN

##### New Member
I have psu with lm723, but it only have voltage control, and I want to modify it so I can control the current. My idea is to put a regulator that can sum the voltage of the sense resistor and the regulator.....I don’t know if you can understand me, but I don’t know how to explain this anymore....

#### Hero999

##### Banned
You still haven't answered our questions.

I hope you realise that you can't control both the current and voltage simultaneously. You can design a regulator that will limit the current and effectively become a constant current source once a certain current limit is exceeded.

Given a low current negative supply most regulators can work down to 0V.

Here's a project I designed a while ago which uses the LM317 and a low current negative supply to give 0V to 13.8V.

0 to 13.8V LM317 Power Supply

#### BOOJAN

##### New Member
1-is the voltage regulator(the PSU)
2-the sense resistor is 0.22R/5W
3-summing circuit
4-the low voltage regulator(the one i was asking for)

What I want to do: at 1A the drop down voltage of the sense resistor is 0.22V. So to cut down the output at 1A between pins 2 and 3 there needs to be 0.63V. I want with the other voltage regulator and the sumator the voltage between those two pins to make it to be 0.63V so lm723 can break the output and the other circuit that I am testing can’t burn out....do you get it now??

btw the other part of the circuit isn’t drawn because there was no need to do that..and that 1A current was an example...

#### be80be

##### Well-Known Member
he wanting a fuse A resettable
break the output and the other circuit that I am testing can’t burn

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#### Hero999

##### Banned
Why not simply put a potential divider across the reference?

Another option is to simply increase the value of the sense resistor.

#### BOOJAN

##### New Member
Why not simply put a potential divider across the reference?

Another option is to simply increase the value of the sense resistor.
i want to use all 2,7A that the psu can give at his outputs, and i was thinking of that but constant current regulation is better...

#### be80be

##### Well-Known Member
Your talking about a computer psu some new one's can put out 30 amps to the 5+ volt side they make good lab supply's but you got to watch them

#### BOOJAN

##### New Member
no no...homemade psu....i need regulated output, i was thinking to do that, but from computer psu you can get 3 diferent voltages, and thats sucks

##### Banned
So toss a variable linear regulator on the 12 volt line of the ATX supply. Easier than designing one from scratch. Constant current can be done pretty easily with a Mosfet and a current sense resistor.

#### be80be

##### Well-Known Member
So toss a variable linear regulator on the 12 volt line of the ATX supply. Easier than designing one from scratch. Constant current can be done pretty easily with a Mosfet and a current sense resistor.
He right I use 1 every day I just use a resistor to limit the current . And as a plus I have 5+ to 5- and 12+ to 12 - and just the go old 5+ to gnd and 12 + to gnd. You may want to check this out.http://www.electro-tech-online.com/custompdfs/2009/03/AN10322_1.pdf

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##### Banned
The negative rails are usually very low current just to warn ya.

#### be80be

##### Well-Known Member
Yes it pay's to read the supply and see what it can do

#### Hero999

##### Banned
i want to use all 2,7A that the psu can give at his outputs, and i was thinking of that but constant current regulation is better...
I'm confused.

A voltage reference only needs to supply a tiny current into an error amplifier which will have a high input impedance.

Constant current regulation isn't better or worse, it depends on what you're doing: if you want to power a microcontroller then you want voltage regulation and if you want to power an LED you want current regulation but most of the time you'll need voltage regulation.

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