Protection Circuit Design for Sensor

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Thread Starter

IgnacioMorac

Joined May 30, 2021
35
Hello partners

The design that you are going to see next belongs to a sensor that 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.

1622352519244.png
Crowbar Circuit
1622352703374.png
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.
1622352556564.png

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

DickCappels

Joined Aug 21, 2008
7,426
The first thing I spotted is that the non-inverting input o U3A can go to 40V but the power supply voltage to the amplifier is limited to 18 volts. From the schematic representing the circuit, the amplifier goes open loop when the inputs approach the positive power supply voltage.
 

Thread Starter

IgnacioMorac

Joined May 30, 2021
35
The first thing I spotted is that the non-inverting input o U3A can go to 40V but the power supply voltage to the amplifier is limited to 18 volts. From the schematic representing the circuit, the amplifier goes open loop when the inputs approach the positive power supply voltage.
This is to regulate the voltage, since the source voltage can vary between 20 to 40 but at the output I always want to have 5V to power my microcontroller.
 

Ian0

Joined Aug 7, 2020
2,192
http://ww1.microchip.com/downloads/en/AppNotes/AN-D66.pdf
Use the circuit in figure 9 to derive the power supply for the op-amp and MCU.
Then use a voltage divider to scale your 40V input to 3.3V (or 5V) for the MCU's A/D
A divider of 160k/13k will give 3.3V output for 43.92V input (accurate within 0.2%, given suitable resistor tolerance) and meets the input impedance requirements for most MCU's A/D converters (which generally like <20k source impedance).
If the input exceeds 40V, the 160k resistor will limit the current sufficiently for the MCU's input protection to deal with it.
No need for any crowbar circuit, and with 173k input resistance, no need for any op-amp either.
 

Thread Starter

IgnacioMorac

Joined May 30, 2021
35

I just found the place in the datasheet where it says this kind of operation is permitted.
The vast world of electronics. But still thanks for the observation. Now, since we know that the operation is allowed, it is possible to make a Crowbar circuit that protects me from overvoltages above 40 volts.
 

Thread Starter

IgnacioMorac

Joined May 30, 2021
35
http://ww1.microchip.com/downloads/en/AppNotes/AN-D66.pdf
Use the circuit in figure 9 to derive the power supply for the op-amp and MCU.
Then use a voltage divider to scale your 40V input to 3.3V (or 5V) for the MCU's A/D
A divider of 160k/13k will give 3.3V output for 43.92V input (accurate within 0.2%, given suitable resistor tolerance) and meets the input impedance requirements for most MCU's A/D converters (which generally like <20k source impedance).
If the input exceeds 40V, the 160k resistor will limit the current sufficiently for the MCU's input protection to deal with it.
No need for any crowbar circuit, and with 173k input resistance, no need for any op-amp either.
Thanks You Ian0.
The only thing is that the circuit is already advanced, the components have already been purchased and the assembly has already been tested, 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.
 
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