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I designed an overvoltage protection circuit. My input voltage is 90V, and I need overvoltage protection at 90V. However, I used a 91V zener diode in my circuit. I have a problem calculating the current limiting resistor for the zener diode because the formula for calculating the resistor value is [𝑉supply−𝑉zener]/ zenercurrent(Iz). ​ In this case, if I calculate my values[ Vsupply is 90V and Vzener is 91V], it will become negative value. What should I do?

Note: I will send the reference design if you need it.

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  • \$\begingroup\$ To perform a current limit function with only one resistor you need to have substantial excess voltage overhead dedicated to the resistor. But I don't know anything about your circuit. So perhaps start there and provide it. Many questions may be resolved then. \$\endgroup\$
    – periblepsis
    Commented May 8 at 10:28
  • \$\begingroup\$ A very warm welcome to the site. Stepping back from your calc, I'm afraid this seems based on a quite common mistake with Zener diodes: seeing them as 'magic voltage drops' that crisply go full-on or full-off at an exact voltage. It's a passive component with strengths and weaknesses, like any other, and can't just be dropped in like you imagine. Need a different circuit. Also, you don't say the 90V supply's tolerance, where it's from (line regulation), load behaviour (load regulation) etc. Please edit your question to add all this info and much more (don't post new info in comments). Thanks. \$\endgroup\$
    – TonyM
    Commented Oct 16 at 10:15

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Your circuit is an over-voltage protection circuit. You need to figure out what the potential overvoltage could be. Let's say you want to clamp Vgs at 90 V as you mention and you are "guessing" an overvoltage scenario at 200 V this could mean that your Vsupply = 200 V

200 V - 91 V = 109 V 109 / Zener_C

Remember that you need to ensure your system Zener can dissipate 109 V * Zener_C watts.

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