Crowbar Overvoltage Protection

Thread Starter

Endermaxximum

Joined Aug 20, 2022
6
Hello, I designed the Crowbar Overvoltage Protection circuit with the components following the picture.S__186843138.jpg

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
 

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crutschow

Joined Mar 14, 2008
34,697
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.
 
Last edited:

Thread Starter

Endermaxximum

Joined Aug 20, 2022
6
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.
 

MaxHeadRoom

Joined Jul 18, 2013
28,772
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.
 

crutschow

Joined Mar 14, 2008
34,697
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.
 
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