Hello, I designed the Crowbar Overvoltage Protection circuit with the components following the picture.

The question is I'm using a Zener diode (Zener Voltage (Nom) 33V DC) but in the simulation, I increased the voltage to 100V and measured the voltage I still got 98.7V instead of < 1V. I'm unsure if this circuit is correct or if I'm missing something.
Thank you

 

I increased the voltage to 100V and measured the voltage I still got 98.7V instead of < 1V. I'm unsure if this circuit is correct or if I'm missing something.

You are missing that there is little to limit the current through the SCR when it triggers.
(How did you think it would work?)
The simulation has an ideal, zero resistance voltage source through a MOSFET to the SCR so it's only the on-resistance of the MOSFET and its forward-biased substrate diode limiting the SCR current.
If you look at that current you will see it is very high (likely hundreds of amps in the simulation).

Normally a crowbar circuit has a fuse or circuit breaker in series with the supply circuit, so when it triggers, the current then goes high enough to open the fuse/breaker, removing the supply voltage from the circuit.

You should also add a resistor in series with the SCR gate to limit its current.

 

At 9745 Amps, something will be getting rather warm.

 

At 9745 Amps, something will be getting rather warm.

Only for a short period of time.

 

You are missing that there is little to limit the current through the SCR when it triggers.
(How did you think it would work?)
The simulation has an ideal, zero resistance voltage source through a MOSFET to the SCR so it's only the on-resistance of the MOSFET and its forward-biased substrate diode limiting the SCR current.
If you look at that current you will see it is very high (likely hundreds of amps in the simulation).

Normally a crowbar circuit has a fuse or circuit breaker in series with the supply circuit, so when it triggers, the current then goes high enough to open the fuse/breaker, removing the supply voltage from the circuit.

You should also add a resistor in series with the SCR gate to limit its current.

Thank you for your advice.
Could you please help me, amend my circuit to the best version.

 

Could you please help me, amend my circuit to the best version.

You just need to add a fuse or circuit breaker at the output of the power supply.

For the simulation, add a resistor in series with the power to limit the current to about 10 amps.

 

You just need to add a fuse or circuit breaker at the output of the power supply.

Thank you

 

You just need to add a fuse or circuit breaker at the output of the power supply.

For the simulation, add a resistor in series with the power to limit the current to about 10 amps.

Is this better?

 

Is this better?

Yes, that should work.

 

In the real world, the fuse or circuit breaker comes BEFORE the crowbar SCR, so that the circuit is disconnected when the SCR is triggered. Fusing the output will not do that.

 

In the real world, the fuse or circuit breaker comes BEFORE the crowbar SCR, so that the circuit is disconnected when the SCR is triggered. Fusing the output will not do that.

Thank you for your advice

 

In the real world, the fuse or circuit breaker comes BEFORE the crowbar SCR,

It does.
The power input is on the right of the schematic.

 

It does.
The power input is on the right of the schematic.

OK, they got me on that one. A reversed drawing can be confusing in the very early morning hours.. There is a reason for consistency of flow in electrical drawings. Power or signal, the direction we write in.

 

OK, they got me on that one. A reversed drawing can be confusing in the very early morning hours.. There is a reason for consistency of flow in electrical drawings. Power or signal, the direction we write in.

Completely agree.

 

In countries that read from right to left, which way does the signal flow in circuit diagrams?

 

In countries that read from right to left, which way does the signal flow in circuit diagrams?

My reading skills are largely limited to languages that flow from left to right. Also mostly limited to the 26 character alphabet, and electrical symbols.

 

In countries that read from right to left, which way does the signal flow in circuit diagrams?

Then you have the question of top-down method!

 

Then you have the question of top-down method!

View attachment 303688

In this circuit drawing the functionality flows to the right. The first rung of the ladder, which has fallenover to the left, contains the first action components The second rung is an indicator (or something) and the third rung includes a higher number component.
Clearly this is set up as a trick question.

 

Hammond used to make a series of regulated linear power supplies, some of which had crow-bar protection, these did not blow a fuse etc, they just had a fold back circuit, active as long as the over-voltage was present.

 

some of which had crow-bar protection, these did not blow a fuse etc, they just had a fold back circuit, active as long as the over-voltage was present.

Technically that's not a crowbar circuit, which drops a short across the output when an overvoltage is detected (hence the name crowbar).
A crowbar circuit is a totally separate circuit, and thus independent of any failures in the regulating circuit.