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Gate resistor value
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sureshot:
So i want to put a crowbar circuit together, i had a schematic but the SCR thyrister is obsolete.  I can source similar SCR thyristors but the gate trigger current is lower, but gate voltage is about the same. I'm looking to calculate the residence value for another SCR with a 0.020 mA trigger gate current instead of 0.040 mA of the obsolete device. My zener diode is 16 Volts 5 watts. My question is do i calculate the gate trigger resistor as follows 16 Volts - 1.5 V / 0.020 mA. Can anyone advise if that's how to calculate it. I'm not sure as the zener reverese voltage won't kick in until its above 16 Volts.
Thanks for reading, any help appreciated.
Zero999:
Just use the same value resistor. The SCR trigger current is a minimum value. Exceeding it will not do any harm. In this case, even if it does exceed the maximum rating, once the crowbar has kicked in, it will drop to a safe level, very quickly.

You might get more help, if you post the schematic and part numbers.
sureshot:
Thank you for your help. The obsolete SCR had a gate current of 40mA and the one I'm thinking of using has gate current of 15mA . The voltage for the gate is 1.5 Volts for both dvices. In the circuit diagram the gate current limiting resistor is 470 ohms. Will it not make a difference as the gate current is 25mA less than the obsolete device. Thanks again.
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