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so I tried to make a power supply using Lm317 and it works fine when i used a 12 v transformer but when I used a 9-0-9 transformer like in the picture,when i turn potentiometer, it sparks and then the output become 23v and burnout .

here is the circuit that i used .

Lm317 Schematic

transformer 9-0-9

I don't know where the probem and I hope you can help me

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  • \$\begingroup\$ How are you connecting the three secondary wires of the transformer to the two input terminals of the circuit? \$\endgroup\$
    – Finbarr
    Commented May 5, 2017 at 15:53
  • \$\begingroup\$ You're going to have to elaborate on how your circuit is connected to the transformer. \$\endgroup\$
    – Hearth
    Commented May 5, 2017 at 15:53
  • \$\begingroup\$ The wiper current rating of your potentiometer is not that great - can it take 50 mA through the wiper? \$\endgroup\$
    – Andy aka
    Commented May 5, 2017 at 15:54
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    \$\begingroup\$ @Andyaka The wiper doesn't seem to even be connected, though that's a different problem. \$\endgroup\$
    – Hearth
    Commented May 5, 2017 at 15:56
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    \$\begingroup\$ @Felthry I think that's just a pin-out issue, Pin2 is normally the wiper. \$\endgroup\$
    – Trevor_G
    Commented May 5, 2017 at 16:03

1 Answer 1

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If your pot is of poor quality, any wiper break will cause a sudden high resistance that will have unfortunate consequences to the constant current driver in the regulator. Voltage on the wiper will spike to full input voltage and the output voltage will also spike at full on.

Use a better pot and add a capacitor across it as indicated in the spec sheet.

enter image description here

Even then, I would be tempted to add a fixed resistor across the pot so you know what the worst case will be if the wiper loses contact, or as Peter mentioned, wire the pot as shown below so the body of the pot is always presenting a resistance when the wiper breaks contact.

enter image description here

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    \$\begingroup\$ I would connect the pot so the full resistance element is always in-circuit, with the wiper connected to one end of the pot - that way, no extra components are required, and an open wiper connection will not leave the Adjust terminal floating. \$\endgroup\$
    – Peter Bennett
    Commented May 5, 2017 at 17:10
  • \$\begingroup\$ Good add @PeterBennett \$\endgroup\$
    – Trevor_G
    Commented May 5, 2017 at 17:55

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