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I want to choose a **gate current limiting resistor ** R2 for a crowbar circuit like this:

enter image description here

First of all I am not an electronic engineer and I have limited knowledge about circuits. I am a little baffled. I have chosen an SCR: BT152-600R.

Datasheet says:

I_gate = typically 3 mA and maximum 32 mA.

ok! This maximum 32 mA is actually the minimum guaranteed current that trigger the SCR. From what I know I should provide this 32 mA to the gate to be sure that it is going to trigger it.

But what baffles me is that there is another thing:

V_gate = typically 0.6 V and maximum 1.5 V.

Isn't the current enough? Is it important to have this 0.6V or 1.5V supplied to the gate?

In other words assume that I have LM7805 and a 5.1 V zener diode placed after that which pass 0.1 V to the line that goes to SCR. If that line provides enough current, is it going to trigger the SCR? If yes... so what is V_gate actually?

Now back to the main question.

If the Zener in the circuit below was 7V then in the over voltage condition it provides 2 V to the gate of SCR. The maximum current for triggering is 32 mA and V_gate is 1.5! So I think R2 should be maximum = R2 = (2-1.5)/0.032 = 15 ohm . Am I right?

The circuit below with 6.2V zener diode is going to provide 1.2 V to the gate of SCR during over-voltage condition. The maximum current for triggering is 32 mA and V_gate is 1.5!! How can I calculate it this way?! it yields: R2 = (1.2-1.5)/0.032 = -9.38 ohm!

Does it mean that I don't need a resistor? Does this mean that V_gate is irrelevant?

That's what baffles me.

I'd be thankful if you could clarify this for me. Thank you in advance.

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    \$\begingroup\$ IMO you don't even need a gate resistor if you put the fuse before zener diode. \$\endgroup\$ – Marko Buršič Aug 18 '15 at 14:23
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    \$\begingroup\$ I agree with the above and would even suggest to move the fuse even more to the left, between the transformer and the bridge rectifier. Then the fuse will protect the complete circuit. The 7805 will limit the current to 1A when it's working anyway. The 10 ohms gate resistor as in the schematic is fine. The value is not critical. \$\endgroup\$ – Bimpelrekkie Aug 18 '15 at 14:33
  • \$\begingroup\$ Yes, I have put the fuse before the LM7805. It is just a Schematic!! Actually I am using an LM7808 with a 9.1V Zener diode. I provided this picture to make the discussion easier... \$\endgroup\$ – arudino.tyro Aug 18 '15 at 14:38
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Isn't the current enough? Is it important to have this 0.6V or 1.5V supplied to the gate?

The SCR gate needs 1.5 volts to guarantee a 32 mA drive - the gate has a non-linear input impedance and, as far as I can tell you'll need to account for the 1.5V needed by the device's gate when the gate is drawing 32 mA.

If 32 mA is passing thru the 10 ohm resistor then this means that the input voltage to the left side of 10 ohm resistor is 1.5 volts + 32 mA * 10 ohm = 1.82 volts. If the zener is 7 volt rated then the circuit will crowbar at 8.82 volts.

As an aside - why are you putting a crowbar circuit on a 5V linear regulator like the 7805?

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Under overvoltage conditions and subsequent triggering of the crowbar some things are going to get stressed. The SCR has to turn on (which takes some time, microseconds or maybe tens of microseconds) and absorb the energy from the capacitors, and the absorb heating from the current required to blow the fuse. It might take a good fraction of a second to blow a 1A fuse with a 7809, which will only typically conduct a couple of amperes peak (2.2A is typical). Of course if the regulator was actually working there would be no need for the crowbar, so this only covers on particular situation (break in the GND pin connection of a functioning regulator causing overvoltage). The transformer will have an internal impedance too, which will limit the current and how fast the fuse will blow.

Make sure the fuse is low enough rating to protect the transformer. Putting it before the filter capacitor means that it has to absorb the current surge from charging the capacitor.

The SCR can also see a high gate current and the Zener diode can experience high dissipation, depending on how long the overvoltage persists before the SCR kills it. Definitely don't have the fuse where it is shown on the schematic- move it back.

Since usually sturdy SCRs with high Igm (amperes) are used for crowbar circuits, I would say the Zener is the most vulnerable part. A small gate resistor can protect the Zener diode from a pathological situation such as a high input voltage (say 20V) suddenly being shorted across the regulator. That will produce a very high current spike in the Zener diode and probably short it. The SCR will probably not turn on in time to save whatever is connected anyway. A small resistor that drops maybe a few hundred mV at the SCR trigger current is not a bad idea to make things a bit more bulletproof.

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