# Cant Understand Current Adjustment Function in Lab power supply???

#### message4guru

Joined Jun 20, 2017
15
Hi to all!!!!

I cant understand the function of LM117 present accross the output. Why it should be connected to negative supply voltage to the output terminal.what is the function of 150 ohm variable resistor.

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#### Dodgydave

Joined Jun 22, 2012
10,961
The Negative supply is used to make the output go to Zero volts. The second chip is to provide a constant current source for the 150 ohms preset.

#### message4guru

Joined Jun 20, 2017
15
The Negative supply is used to make the output go to Zero volts. The second chip is to provide a constant current source for the 150 ohms preset.
Sir ,
why negative supply voltage has to be given to Vout terminal of IC. What will happen to IC. How to Analyse?

i dont know how to ask my doubt.

1)Please explain the secong LM117 function?
2)why negative supply is given to vout terminal of second LM117?
normally 1.2V will be present between adj pin and Vout pin.
3) what is the function of 150 ohm resistor?

Please explain things with schematics. thank you sir.

Last edited:

#### Dodgydave

Joined Jun 22, 2012
10,961
Because the Lm series of regulators will only go down to 1.2V, so they need a Negative supply to take it to Zero volts.

#### message4guru

Joined Jun 20, 2017
15
Because the Lm series of regulators will only go down to 1.2V, so they need a Negative supply to take it to Zero volts.
Sir,
Between adjustment pin and vout pin there will be 1.2V. now we are applying negative 10V at inverting terminal of opamp which is present internal to LM117. vout and inverting terminal are at same potential. Now how to analyse the circuit/

what is the purpose of 150 ohm variable resistor?

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