I want to understand the below circuit which is supposed to detect voltage if it goes beyond a certain limit.

The Input voltage is between 15V-30V. The circuit means to detect if the voltage increases beyond 36V.

Datasheet of D1 36V Zener Diode - MMBZ5258

Datasheet of D2 6.2V Zener Diode - BZX84

Datasheet of Q1 PNP -BC807

I tried to simulate the above circuit, but I am getting some transient error which I was not able to solve.

The Anode of the 6.2V Zener diode is further connected to the Gate of P-MOS. P-MOS source connected Zener cathode and Drain connected to ground via 2x 47nF capacitors. (I ignored the MOSFET circuit because, I wanted only to understand this section of the circuit which proved to be confusing. If I could I understand this section, I would easily understand the MOSFET part.)


simulate this circuit – Schematic created using CircuitLab

My Questions, What I tried to understand, but I couldn't :

  1. Why do we have resistors connected to the anode and cathode of the Zener diode? What purpose do the top and bottom resistor serve.

  2. The datasheet sheet of D1 36V Zener diode has three columns . Izt1. Izt2 and Ir. I understand Zener diode will go to the breakdown region only when it crosses the breakdown voltage at its cathode and also it must have the required reverse current. But in the datasheet there are two test current column and one reverse current column. Why is that?

  3. In the same Zener datasheet. it is mentioned V=27V and Ir=0.1uA at Vr. What does it mean? Does the Zener clamp at 27V itself or does it start taking the reverse current of 0.1uA when the voltage at its cathode exceeds 27V?

  4. Can someone brief me the overall working of this circuit when the voltage is between 15V to 30V and when it exceeds more than 36V?

I tried to understand the circuit through simulations, but I am not able to get it clear.

My Efforts:

Tried below simulation tool too.

enter image description here

  • 1
    \$\begingroup\$ Hey! this is four broad questions in one. I'd recommend moving 2. and 3. to a separate question, but honestly, these are simply different operational conditions. \$\endgroup\$ Apr 17, 2020 at 12:45
  • \$\begingroup\$ Thank you! I understand those questions are a little off-track to my main question, which is actually Question 1 and Question 4. I had that thought too to split up my questions. But, since all my questions were related to the fundamental Questions, I thought not to split up the same topic into different questions which would be a burden for both myself and the people who can provide answers. And for people who actually face my same problem (having the same or similar doubts - All 4 questions), they could search and get an answer and also learn all within a single answer. \$\endgroup\$
    – user220456
    Apr 17, 2020 at 13:01
  • \$\begingroup\$ I really think it's better to split them off, but it's your question :) \$\endgroup\$ Apr 17, 2020 at 13:04

1 Answer 1


Why do we have resistors connected to the anode and cathode of the Zener diode? What purpose do the top and bottom resistor serve.

I recommend looking at basic Zener circuitrs to answer that, that'll clear up a lot of your other questions, too. Short answer:

Because without any serious resistor, the current through the diode would not be limited when the voltage exceeds the Zener voltage, and the diode will burn out.

  • \$\begingroup\$ Thank you for the answer. Yes, I am well aware of the series resistor that we place while using Zener diode as a voltage regulator. The current limiting resistor is usually placed before the zener diode (near the cathode terminal). But in my circuit, there is a resistor which is placed at the anode of the zener diode as well. I haven't seen that many circuits using zener diodes that have a resistor connected to its anode terminal. That's my query. \$\endgroup\$
    – user220456
    Apr 17, 2020 at 12:57
  • \$\begingroup\$ ah, the order of D1 and R2 doesn't matter, right? So, maybe it's easier if you place the resistor on top and the diode to ground: then you got a voltage divider between the zener diode and the supply voltage. \$\endgroup\$ Apr 17, 2020 at 13:05
  • \$\begingroup\$ Suppose I rearrange the bottom resistor and place it along with the top resistor, I would get a voltage divider right? But I don't want a voltage divider. Is there a specific reason to split up the two resistors in this fashion? And is there really a necessity to use two resistors instead of a single resistance having their equivalent resistances? Did they split into two to split the power dissipation in each resistor? Could you please provide an answer for my questions \$\endgroup\$
    – user220456
    Apr 17, 2020 at 13:15
  • \$\begingroup\$ you want a voltage divider, because Vcc-Vzener might not be the voltage you need to drive the base of the transistor \$\endgroup\$ Apr 17, 2020 at 13:18
  • 1
    \$\begingroup\$ both are both. I mean, you know how resistors work, what Kirchhoff's law is etc, otherwise you wouldn't be asking about transistor circuits. Don't forget to apply your basics knowledge. \$\endgroup\$ Apr 17, 2020 at 15:09

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