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It is a LM2577 boost converter schematic. In page 20 of datasheet: VOUT = 1.23V (1 + R1/R2). Here R1 is the 100k potentiometer, R2 is the 2k resistor.

Why are 3 wires connected to the potentiometer? Why not just remove the wire in the red circle?

haiwuxing
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    Floating pins are friends of noise. – jay Nov 19 '21 at 01:37
  • Worse than noise ... if the wiper goes open circuit, you probably don't want the feedback loop to attempt to regulate the output voltage to infinity. –  Nov 19 '21 at 14:47

1 Answers1

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When a three wire potentiometer is connected as a variable resistor it is standard practice to connect the third terminal to the wiper so that if the wiper temporarily disconnects from the track the circuit does not go open. It just goes to the maximum value of the variable resistor.

In the circuit shown the pot value is larger than desirable. To achieve a 12v output the pot setting only needs to be 17.4k because the feedback point is 1.23v. A 20 or 25k pot would be a better choice that would still give 20% over adjustment without being excessive.

With a 20k pot the output would jump to about 13-14V if the pot goes open whereas with a 100k pot it will be tricky to adjust since the setting in the bottom 20% of the adjustment and the output will attempt to go to about 70V if it goes open. Obviously it will not do that and will be limited by the input voltage.

Kevin White
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