Debugging LM 317 Regulator

amy_pur · Nov 16, 2003

Hi,

I am trying to build a variable powersupply using LM 317 IC. The input DC voltage (after rectification) is about 26 volts. However, at the output I am getting voltage from 1.2 V to 6.2 V only. What could be the reason?

Thanks and Regards,
amy_pur

crust · Nov 16, 2003

Post your circuit -- but you might not have the right values for your resistor and potentiometer.

The equation is V=1.25(1 + R2/R1)

R2 goes from the adjust pin to ground and R1 goes from adjust to the output. R2 should be about 200R1 or thereabouts.

panic mode · Nov 16, 2003

Try replacing potentiometer for bigger one, it sounds like you have 1kOhm only. Use 5kOhm.

amy_pur · Nov 17, 2003

panic mode said:
Try replacing potentiometer for bigger one, it sounds like you have 1kOhm only. Use 5kOhm.

Thanks. Yes, I have used a 1 KOhm potentiometer. I will change as suggested and check.

amy_pur · Nov 17, 2003

crust said:
Post your circuit -- but you might not have the right values for your resistor and potentiometer.

The equation is V=1.25(1 + R2/R1)

R2 goes from the adjust pin to ground and R1 goes from adjust to the output. R2 should be about 200R1 or thereabouts.

Thanks Crust. You are right, the current ratio is (R2)1000 Ohms : (R1)220 Ohms. As suggested by another friend, I will change the POT value to 5000 Ohms. This should solve the problem.

Debugging LM 317 Regulator

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