# Over voltage protection Crowbar Circuit design issue

#### Vijana Maparathna

Joined Feb 2, 2016
3
I'm designing a Crowbar over voltage protection circuit with following specifications,

Maximum input voltage - 30V

Maximum input current - 5A

Output voltage - 5V

Output current - 5A

So I have chosen zener diode (1n4733A -> Vz = 5.1V,Iz = 49mA) and (2n5060 -> Vgt = 0.8) components for my circuit.

According to the calculation I choose R = 16.3265ohms resistor. But my simulation doesn't work as I expected.

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#### crutschow

Joined Mar 14, 2008
27,243
It's rather difficult to help you with you problem when you haven't stated what the problem is, other than you have one.

Note that no standard resistors come in the value you show for R2.
Pick the closet standard value.

#### dl324

Joined Mar 30, 2015
12,873
Welcome to AAC!

You don't need to operate the zener at Izt and 0.8V is the maximum turn on voltage. You should include a fuse in series with R1 unless your intent is to have the power supply deal with a short.

#### crutschow

Joined Mar 14, 2008
27,243
As dl324 noted, you need a fuse in the circuit.
As you show the circuit, the theoretical supply current will be around 12,000 amps when he SCR fires, as limited by R1.

In real life your SCR would have been zapped long before that level of current was reached, of course.

#### Vijana Maparathna

Joined Feb 2, 2016
3
Hi,

I replace R1 resistor with fuse and set R2 = 16.4 Ohms. Still my circuit is not working

#### crutschow

Joined Mar 14, 2008
27,243
Well, I still don't know "how" it's not working, but here's the LTspice simulation of a similar circuit, which does.

Note that 5.1V Zeners have a fairly sharp knee characteristic, so you don't need to run them at the rated current to get a breakdown voltage close to their nominal value, thus I used a 100Ω gate-ground resistor instead of the low 16.4Ω you used.

R2 is just to limit the current for simulation purposes since my simulated "fuse" doesn't actually blow.