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