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Zener diode configuration question. — Parallax Forums

Zener diode configuration question.

Could someone explain what the purpose of the two zener diodes labeled D1 & D2 in the attached circuit diagram?
3264 x 2448 - 2M

Comments

  • Those are probably there to stabilize the feedback voltage. This is an older op-amp, and it's output is not rail-to-rail, that is, with +/- 15 volt supplies, the output might swing +/-13 volts, but that is not a well controlled parameter and depends on operating conditions. The back-to-back series zeners clamp the feedback at +/- 7 volts, and the temperature coefficients in zener mode and forward mode tend to cancel out. The value of PWM depends on having a stable feedback voltage in relation to the control input.

    With a more modern rail to rail op-amp, the circuit might dispense with the zeners.
  • agfaagfa Posts: 295
    edited 2017-12-12 01:55
    Thank you Tracy for taking the time to share your expertise. You actually provided more insight than I expected.
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