# Crowbar Circuit To protect Sensor for overvoltage

#### IgnacioMorac

Joined May 30, 2021
39
Hello partners
I come to you to help me determine what resistance values or what other components I need to be able to design a crowbar circuit that protects my sensor against overvoltages.

My intention is to take care of my sensor for voltages above 40V. So I started to design this crowbar from scratch. Well what did I do?
First look at the components page of my nearby store to see if I will have the components for your construction.

So I chose the 15-volt zener diode, and a 90-volt resistor that I sized using the current from the zenner, this current was obtained by dividing the power by the voltage and thus I obtained that the value is 0.0666. On the other hand, the current that the sensor will consume is around 29.4mA, but I have not yet put the microcontroller, the rtc, a datalogger and an Xbee. Therefore it should consume around 343mW.

But hey anyway, I just want my circuit to protect me from surges greater than 40 volts.
When I do this test, the simulation doesn't work and it's frustrating, I don't know what I'm doing wrong.
It is assumed that the gate voltage of the SRC thyristor is at 6 Volts therefore in the resistance of 90 6 volts have to fall, when the current is 66.66 mA. This causes the SCR to activate and conduct shorting and preventing the sensor from burning out.

#### Ian0

Joined Aug 7, 2020
10,277
It would appear that V2 is the voltage that you are trying to measure. Now you have introduced R3 with about 30mA going through it. That will introduce an error of 6V on the voltage you are measuring.
Also, you need a component rated at 10W for R3.
You should consider the source of the possible over-voltage.
If it results from pickup of short (<1us) spikes on the supply, then the crowbar is probably not the best way of eliminating them. Every time one comes along, the circuit will have to be reset manually which will become tiresome and customers will complain.
Short spikes are more easily removed by LC filtering, even perhaps just a ferrite bead.
If the over-voltages last longer, you should consider how often they are likely to occur, and whether a manual reset will be too inconvenient. How will the rest of the circuit continue to work after the crowbar is activated?

#### LowQCab

Joined Nov 6, 2012
4,314
Power-Zener..........
Faster and/or more precise set-ups are easy to do also.

#### DickCappels

Joined Aug 21, 2008
10,247
The above circuit is not a crowbar but a voltage clamp. It has the advantage of not needing to be reset. For very high fault currents, this can only handle short periods of excessive voltage (terms are relative).

What is the nature of the source of the excessive voltage you expect your circuit to be exposed to?

#### click_here

Joined Sep 22, 2020
548
I've only ever seen a crowbar circuit that pops a fuse on over voltage (like attached)

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#### IgnacioMorac

Joined May 30, 2021
39
It would appear that V2 is the voltage that you are trying to measure. Now you have introduced R3 with about 30mA going through it. That will introduce an error of 6V on the voltage you are measuring.
Also, you need a component rated at 10W for R3.
You should consider the source of the possible over-voltage.
If it results from pickup of short (<1us) spikes on the supply, then the crowbar is probably not the best way of eliminating them. Every time one comes along, the circuit will have to be reset manually which will become tiresome and customers will complain.
Short spikes are more easily removed by LC filtering, even perhaps just a ferrite bead.
If the over-voltages last longer, you should consider how often they are likely to occur, and whether a manual reset will be too inconvenient. How will the rest of the circuit continue to work after the crowbar is activated?
Ian0 thanks for explaining.
I don't know if I will find a ferrite bar here in Costa Rica.

"How will the rest of the circuit continue to work after the crowbar is activated?"
That is what I would like to get to, to have a discussion about it. While it is true that the crowbar circuit can protect the sensor, a deficiency is that you have to turn off the source (which will not be possible) or disconnect it, and that makes the system is very slow and full of manual work.

If it is not the circuit, what else can I use. Something basic. I also depend on whether the two electronic component stores that are here in Costa Rica have those components. And not to affect the design of the PCB circuit in terms of dimensions.

#### IgnacioMorac

Joined May 30, 2021
39
The above circuit is not a crowbar but a voltage clamp. It has the advantage of not needing to be reset. For very high fault currents, this can only handle short periods of excessive voltage (terms are relative).

What is the nature of the source of the excessive voltage you expect your circuit to be exposed to?
Hi DickCappels the source is a Solar Panel.

#### IgnacioMorac

Joined May 30, 2021
39

#### Ian0

Joined Aug 7, 2020
10,277
Hi DickCappels the source is a Solar Panel.
Well, a solar panel looks like a current source in parallel with a big and rather poor zener diode. If you take the value for open circuit voltage and extrapolate it for the lowest temperature you are likely to achieve then you pretty much know what the highest sustained voltage is.
If there are inductive loads being switched on and off, then you may encounter spikes, but if the loads are resistive then the voltage is very unlikely to exceed the open circuit voltage.
@LowQCab 's circuit in post #5 would be perfect to deal with anything that came along that exceeded that value, as a solar panel doesn't object to being shorted out. The SCR version will be effective, but annoying.

#### IgnacioMorac

Joined May 30, 2021
39
Well, a solar panel looks like a current source in parallel with a big and rather poor zener diode. If you take the value for open circuit voltage and extrapolate it for the lowest temperature you are likely to achieve then you pretty much know what the highest sustained voltage is.
If there are inductive loads being switched on and off, then you may encounter spikes, but if the loads are resistive then the voltage is very unlikely to exceed the open circuit voltage.
@LowQCab 's circuit in post #5 would be perfect to deal with anything that came along that exceeded that value, as a solar panel doesn't object to being shorted out. The SCR version will be effective, but annoying.
interesting, yes thank you very much. I will take into account the voltage at a lower temperature. In fact I feel that it is 40Volts since the teacher provided me with this data.

#### DickCappels

Joined Aug 21, 2008
10,247
Make sure you have sufficient heatsinking for cold, sunny days.

#### IgnacioMorac

Joined May 30, 2021
39
Make sure you have sufficient heatsinking for cold, sunny days.
Thank you DickCappels
I will keep it in mind.

#### Ian0

Joined Aug 7, 2020
10,277
Voc = 39.9V @ 25°C is a common value for a 60-cell panel. Temperature coefficient is about 0.3%/°C so here we would probably calculate the maximum voltage at -10°C, and that would add 10% so about 44V, which is your absolute maximum for the op-amp. My suggestion, if it were in use here, would be to clamp at 47V (43V zener plus Vgs of the MOSFET), and redesign the op-amp stage to suite, but if you're stuck with the op-amp stage you might have to clamp at a lower voltage and dissipate some power on all those frosty mornings you get in Costa Rica!

#### DickCappels

Joined Aug 21, 2008
10,247
I'm in Thailand, and except for the highest mountain in the country it doesn't get below about 10°C. I can imagine Costa Rica is much milder than that.

#### IgnacioMorac

Joined May 30, 2021
39
snowy mornings you get in Costa Rica!
HAHA nice one.
Wish I have a 43V zener, the only one with hight voltage is

#### Ian0

Joined Aug 7, 2020
10,277
I'm in Thailand, and except for the highest mountain in the country it doesn't get below about 10°C. I can imagine Costa Rica is much milder than that.
I'm just looking for some climate data - usually there's a graph on Wikipedia, but not for Costa Rica.
But 10°C would give only 5% increase, so a clamp at 42V (39V zener + MOSFET) would probably never activate.

#### IgnacioMorac

Joined May 30, 2021
39
I'm in Thailand, and except for the highest mountain in the country it doesn't get below about 10°C. I can imagine Costa Rica is much milder than that.
The solar plant is in a place where the lowest temperature can rarely reach 14°C, but during the year it oscillates between 16°C and 25°C.

#### DickCappels

Joined Aug 21, 2008
10,247
Very pleasant! I am jealous.