I have been working on the design of a circuit that can measure a voltage within the range of 25-40 volts.

But I wanted to design a protection stage, since I will use a microcontroller for the ADC and other components.

My idea is to design a good crowbar circuit to activate after 41 ~ 42V.

When I did the research on the crowbar website, I noticed that it is a simple circuit to make and simulate.

I understand that the circuit "activates" when it exceeds the voltage that we have given it. My question is the following: What criteria or what formulas should I use to size the resistors?

I have simulated the circuit and it has not worked. I read in a forum that if I put a switch and activated it simulating the overvoltage input, it worked but nothing so far.

[...] the circuit on the right is working well and does its job. Please don't see that circuit. The only thing I want is to make a protection circuit for overvoltages. For that I searched the Internet and there were several options, some very basic and others more complex, in the end I chose to use CROWBAR. The circuits where the amplifiers that are used as voltage followers are, please do not see them. Those circuits work fine. The only thing I want is a protection circuit, that when it exceeds 40 volts, it makes a short and protects the circuit from overvoltages.

I am going to explain a little so that you understand the schematic, since I uploaded a screenshot of the entire schematic. This schematic is a design of a circuit that I designed to be able to power a microcontroller and at the same time measure the voltage, with the same voltage that is taken from the solar panel. This circuit works very well and gives me the values ​​I need both at the output of the first voltage follower (the one above) with a voltage regulator for the power supply and at the output of the other voltage follower (the one below) to be able to measure with ADC.

I want to protect my circuit using a crowbar. My problem is that I don't know if I'm sizing it right. For this I use a 15 volt zenner diode. So if I want my system to be protected for voltages greater than 40V, what I have to do is say ok 40 Vdc input voltage is equal to the voltage of the zenner diode Vz and the voltage of the resistor Vr. But how do I size the resistance value? Then I solve for the value of the resistance from the equation Vr = 40-15 Vr = 25 .

Now I look for the reverse current of the diode to be able to obtain the ohms of the resistor. The reverse current is 5microAmps. And it gives me that the resistance is 5 Mohm.

@Math Keeps Me Busy Thaks for your Advise. But The circuit is already advanced, the components have already been purchased and the assembly has already been tested, the design of the PCB it is alost finished I only occupied a protection circuit against overvoltages. I appreciate your time in sharing this information with me and opening my eyes to another new solution.

But like I said earlier. I'm just looking for something, a simple circuit that can protect my circuit from overvoltages.

Circuit Design of my sensor and the crowbar section

Figure 1: Circuit Design of my sensor and the crowbar section

Datasheet of the zener diode, I use 15V zener diode

Figure 2: Datasheet of the zener diode, I use 15V zener diode

I added an RC filter, but it doesn't work too.

RC filter

Figure 3: RC filter

  • 1
    \$\begingroup\$ For an analog input, a clamp circuit would be more customary. Crowbar is normally used for power rails. Where is your circuit? \$\endgroup\$
    – Kartman
    May 28, 2021 at 13:59
  • \$\begingroup\$ In Solar Panels, it is to measure the voltage. So we are talking with DC problem. \$\endgroup\$ May 28, 2021 at 14:42
  • \$\begingroup\$ I have no idea what happened, my comments disappeared and this simulation appeared. Take a look at the schematics for a few multimeters to get a good idea of how to protect your system. I would use a resistor and a diiode. \$\endgroup\$
    – Gil
    May 28, 2021 at 14:53
  • \$\begingroup\$ I notice that you have the supply voltage tied to the + input of your op-amp. I'm not sure what you are trying to do. I think a better way to get the voltage you want is with a resistive voltage divider. If necessary, this can be followed by an op-amp voltage follower. The resistive voltage divider will automatically provide a great deal of protection to your microprocessor etc. \$\endgroup\$ May 28, 2021 at 14:54

1 Answer 1


The following circuit will reduce a 40 V input to 5 V, and will limit the maximum output to about 5.17 V


simulate this circuit – Schematic created using CircuitLab

A drawback of this circuit is that 7.35 k\$\Omega\$ is not a standard 1% resistor. However, it can be made by connecting two resistors in parallel or in series.

Here is a curve showing the output.

enter image description here

If needed, the output can be fed into a op-amp voltage follower. The input/output characteristics can be modified by changing resistor values.

If the output voltage needs to be level-shifted and/or amplified, an op-amp may be used. Depending upon the op-amp circuit's input impedance, one or more resistor values may need to be modified.


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