Crowbar Circuit Component Selection for 25-40V Sensor Overvoltage Protection

I am building, this sensor will be capable of measuring voltages between 25-40 volts through the ADC of a microcontroller and at the same time feeding on these voltages.

A fundamental part of every sensor is to have an overvoltage protection circuit, to take care of the microcontroller and any extra components from any overvoltage that happens in the future.

When I started looking for protection circuits I found the Crowbar circuit, I have looked everywhere, to know how and what component values to use so that this crowbar circuit protects my sensor from voltages above 40 volts.

I tried to do the simulation as you can see in the following image but something is wrong and it does not allow my circuit to work at 39 - 40 volts, it just burns.

I would be very grateful if you help me with this problem, it has really been a headache.

Crow bar circuits are typically used to shut down power supplies, and normally use a series fuse to facilitate disconnection. Some are self resetting, not using a fuse, but in any case they usually rely on much higher currents to operate, signal currents would often be many orders of magnitude lower.
If you need to protect your op amp inputs, then I believe a clamping circuit is more appropriate, in a fashion similar to the one you have already used on U3A. These may use shunt diodes (there are some already on most input pins of an op amp, but there can be issues using them) or Zener's, and rely on a series resistor to limit the current. The resistor value is chosen on the basis of the limiting current, op amp offset, and noise. See the two articles below regarding trade offs.
https://www.analog.com/en/analog-dialogue/articles/op-amp-input-overvoltage-protection.html
https://www.ednasia.com/provide-robust-input-...rotection-for-amplifier-analog-input-modules/
They often incorporate shunt capacitors as well to limit the bandwidth of the circuit.
If I am missing something here, please give a few more details,
cheers,
Richard

Whilst I am sure I would use the technique pointed at by Richard or perhaps a Transient Voltage Suppressor, I think I should point out that there are other electronic techniques that approximate a crowbar and will even current limit or disconnect the protected circuit. 40V limits your choices, but the first place to check is this URL
https://www.analog.com/en/parametricsearch/11394#/

There are several that will meet the 40V requirement.

I am sure you will find several other analog ic manufacturers who make similar products. Try Texas Instruments.

Richard,
The first step here is to reset your expectations. You wish to measure between 25 V and 40 V at the input. How far above 40 V are you willing to tolerate? Here is a clue, 40.2 V as a clamp or trip voltage is not achievable! In your circuit you have used a 15 V zener (for some unknown reason).

A zener typically has a ±5% selection tolerance to start with. At 40 V, ±5% corresponds to ±2 V.

Obviously you want to measure the 25 V to 40 V range accurately so you would like to scale the 40 V to say 5 V for the microcontroller. So you need to scale the ADC input more heavily so that maybe 45 V corresponds to full scale in order to not blow up the ADC input.

A crowbar is a very unpleasant limit circuit. It is called a crowbar by analogy with open power lines where you can imagine throwing a crowbar across the power lines and shorting them out (not recommended!) This blows the fuse and protects the ‘downstream circuitry. In your circuit you would have to short circuit R2 and put a resistance in series with V2. D3 needs to draw enough current to pull down the resistive output from V2 (+new series resistor). If you need a low source impedance from V2 this new series resistor is a problem.

A crowbar has another problem, transients. Any fast transient will trigger the thyristor (D3) and short the power rail causing unexpected circuit failure. Not helpful.

U3A is a disaster as wired. The input goes above the power supply! That particular opamp may survive it, but you can’t expect it to follow the input correctly. Automotive circuits have horrific transients to deal with. It is not clear what your overvoltage limits can be or where they are coming from. I would guess at 50 V or more but that obviously needs some detail to be fleshed out. Without a spec you can’t even start the design aspect.

As for protection there are fundamentally only two methods:
(1) clamp the input
(2) disconnect the input

The detail of the design follows from the approach and the specifications (and the allowed cost, and the production quantity). Have a think about what you can tolerate in terms of over-voltage limit, and how high that could go if not limited.

More details = better advice.

My idea is to measure the voltage of the photovoltaic panel.
So I thought about making a simple sensor, that can measure the voltage of the panel and that at the same time this sensor is fed by the voltage of the cell to work.

This image is the block diagram representation of the sensor.
The first stage is an overvoltage protection circuit, which I thought about using a Power Zener, made up of a zener and a transformer, but it didn't work for me, so I think I'm going to discard it.

The second stage consists of two circuits, with the first circuit I seek to regulate the voltage to be able to have 5 volts at the output and to power the ATMega328 microcontroller, the RTC DS1307, the MicroSD catalex module and the XBee S2C 2.4 digimesh T.h.
All of these are powered by 5 volts.

The second circuit of the second stage is the circuit that reduces the voltage to a measurable range by the ADC of the microcontroller, this would be a range from 0 to reference voltage, in this almost a voltage of 5 volts.

The third stage is the microcontroller and the RTC module and the micro SD module, this stage is to keep a record of the date and time at which the measurement was made. In turn, the microcontroller will form a frame that will send through its UART ports to the XBee.

The fourth and last stage would be the XBee, this module will be in charge of sending the data frame to a computer.

I already did the programming of the ATMega328 and already configured the network so that they can communicate. Unfortunately it did not work since I had that power problem. I would be very grateful if you would help me.

Jose,
Concept sketch attached. Values depend on load current drawn from 5V regulator and exact ADC input impedance requirements. The zener acts as a crude pre-regulator for the 78L05, which does not like 60 V input.

I came across this article in the June/July 2021 edition of Electronic Design. I thought it might be of some interest to you.

How to Choose the Right Protection for Your Circuit

https://www.electronicdesign.com/power-manage...151H2756901C1M&oly_enc_id=8151H2756901C1M