REGARDING LM317

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CHAITANYA_KUMAR_REDDY

Joined Jan 8, 2017
1
Hi every one,
I want to design a new variable voltage supply(-7 to +7vollts ) using lm317. what is the relation between the input and output voltage. i want to use digipot. is there any other way to do this. please reply me
 

WBahn

Joined Mar 31, 2012
26,398
First, read the data sheet for the LM317. Not only does it explain how the device operated, but also there are usually several reference circuits. Wanting a bipolar output makes things more complicated, but there are a few ways of dealing with that.
 

crutschow

Joined Mar 14, 2008
26,321
The LM317 is for positive voltages only.
Do you actually need adjustment from -7V to +7V or could you use two supplies with one being a 0 to -7V supply and the other being a 0 to +7V supply instead?

If you really need -7V to +7V you could use an op amp driving a complementary transistor output stage powered from plus and minus unregulated voltages.
 

WBahn

Joined Mar 31, 2012
26,398
The LM317 is for positive voltages only.
Do you actually need adjustment from -7V to +7V or could you use two supplies with one being a 0 to -7V supply and the other being a 0 to +7V supply instead?

If you really need -7V to +7V you could use an op amp driving a complementary transistor output stage powered from plus and minus unregulated voltages.
That's the method I would (and have) used and recommend. But it is possible to use a single LM317, you just need to refer it to the negative rail instead of the 0 V reference.
 

hp1729

Joined Nov 23, 2015
2,304
Hi every one,
I want to design a new variable voltage supply(-7 to +7vollts ) using lm317. what is the relation between the input and output voltage. i want to use digipot. is there any other way to do this. please reply me
How much current do you need?
Yes, LM317 will not go negative.
Simpler circuit, Pot (digital pot is good), op amp, buffer transistors. +V and -V still needed.
What kind of resolution did you need? Cheaper digital pots what give mush resolution.

14 V range (+7 to -7) a 256 step pot will only give you about 50 mV per step. Zero will not likely be 0 V.
 

crutschow

Joined Mar 14, 2008
26,321
That's the method I would (and have) used and recommend. But it is possible to use a single LM317, you just need to refer it to the negative rail instead of the 0 V reference.
But how will that source +7V or sink -7V from one LM317 (which appears to be what the OP wants)? :confused:
 

WBahn

Joined Mar 31, 2012
26,398
But how will that source +7V or sink -7V from one LM317 (which appears to be what the OP wants)? :confused:
What if the OP had said that he had power rails of 0V and 20 V and wanted to use an LM317 to provide an output from +3 V to +17 V? Would that have cause any heartache?

However, the point about needing to sink current is an issue. The LM317 is not rated to sink current and, in fact, has a minimum output current that has to be maintained. So this approach won't work (the basic concept is valid and it does work with opamps that can both source and sink current, which is what I was using as a basis).
 

hp1729

Joined Nov 23, 2015
2,304
Just a starting point ... AdjustasbleView attachment 118431
HI EVERY ONE,
I WANT TO DESIGN A NEW VARIABLE VOLTAGE SUPPLY(-7 TO +7VOLLTS ) USING LM317. WHAT IS THE RELATION BETWEEN THE INPUT AND OUTPUT VOLTAGE. I WANT TO USE DIGIPOT. IS THERE ANY OTHER WAY TO DO THIS. PLEASE REPLY ME
Just a starting point ... adjustable +7V to - 7V, 1 Amp or so, as simple as possible.

(Error schematic was deleted. Go on a few more posts.)
 
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WBahn

Joined Mar 31, 2012
26,398
One problem you are going to have with that is that it is going to be hard, if not impossible, to get an output of +/- 7 V. To get 7 V at the output, you need about 8.4 V at the output of the opamp. But a 741 can generally only get to within 2 V of the rail and most are only spec'ed to get within 3 V, so you can't count on it getting anything above about 7 V, meaning that your output might not be able to go any higher than about 5.4 V.

One trick I came up with (though I now know that I was far from the first) was to put a resistor between the output of the opamp (I used the LM384 for my application) and the circuit output. This allowed the first bit of current to be sourced by the opamp and made the transition region between push and pull much gentler.
 
Last edited:

hp1729

Joined Nov 23, 2015
2,304
One problem you are going to have with that is that it is going to be hard, if not impossible, to get an output of +/- 7 V. To get 7 V at the output, you need about 8.4 V at the output of the opamp. But a 741 can generally only get to within 2 V of the rail and most are only spec'ed to get within 3 V, so you can't count on it getting anything above about 7 V, meaning that your output might not be able to go any higher than about 5.4 V.

One trick I came up with (though I now know that I was far from the first) was to put a resistor between the output of the opamp (I used the LM384 for my application) and the circuit output. This allowed the first bit of current to be sourced by the opamp and made the transition region between push and pull much gentler.
Good comments.
 
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