How to choose a “gate current limiting resistor” for an SCR in a crowbar circuit using gate voltage and gate current from SCR’s Datasheet

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

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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.

Best Answer

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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