Crow bar with SCR trigger current

[Faddei]

Hi,

I want to make a crow bar protection circuit with a zener diode and an SCR. I found NXP SCR BT148W-600R. In the datasheet gate trigger voltage and gate trigger current are specified. How do i ensure the trigger current condition?

Thanks.

 

[FvM]

How do i ensure the trigger current condition?

What do you mean? The cicruit will trigger in any case above a certain input voltage.

 

[Faddei]

What do you mean? The cicruit will trigger in any case above a certain input voltage.

Why is the gate current trigger then specified in the datasheet?
Igt=50uA at Vd=12V

 

[FvM]

The recent datasheet says typical trigger current 15 µA, maximum 200 µA. As said, the circuit doesn't have problems to provide the trigger current. What's your problem?

 

[Faddei]

The recent datasheet says typical trigger current 15 µA, maximum 200 µA. As said, the circuit doesn't have problems to provide the trigger current. What's your problem?

How did you calculate that the circuit doesn't have problem with providing the current? Is it because the trigger current is so low. If this is true why did they specify this anyway? Is this independent of the zener specs?

 

[godfreyl]

The Zener can provide plenty of current.

 

[FvM]

You'll usually select a resistor value that makes the circuit operation almost independent of the actual trigger current, e.g. 200 to 1000 ohm.

Vgt can be assumed around 0.7 V, The zener diode should have a good specified zener voltage at the respective current. In so far zener diode specification matters.

 

[crutschow]

A zener has a very low impedance beyond its breakdown voltage. Thus as the output voltage increases slightly above this breakdown voltage plus the SCR gate trigger voltage, the current will rapidly increase through the zener and the SCR gate until the SCR fires.