29-12-2017, 01:18 PM
Power dissipation in a crowbar zener is a red herring - it's all over so very quickly.
To answer the question about R1, we need to think about the zener diode. Specifically, look at the graph of voltage vs current: https://www.digikey.com/en/maker/blogs/z...c61e2640f3
Notice particularly the "knee" that you get as the zener approaches its operating voltage.
So as the voltage across a zener depends on the current applied, you'd choose the operating current accordingly - certainly, an operating current that is sufficiently around the "knee" is good. Say, at least 5mA, for example. Some might say 10mA. All diodes are different.
Remembering that we need 0.6V across R1 when the SCR is to trigger, and we'd like 5mA to be flowing at that point, that tells us that R1 should be 120 ohms.
But this is very approximate. As well as the basic voltage tolerance, the minimum recommended current for zener diodes varies a lot from diode to diode within the same family (as there are actually 2 distinct mechanisms used in zener diodes, according to their rated voltage), so look at the datasheets to get a feel for this (e.g. page 5 of this: https://www.vishay.com/docs/85607/bzx85.pdf)
Also, that ignores the current taken by the gate of the SCR. You'd need to check the chosen SCR's datasheet to decide if that's low enough to ignore...
In short, experiment and see what happens
To answer the question about R1, we need to think about the zener diode. Specifically, look at the graph of voltage vs current: https://www.digikey.com/en/maker/blogs/z...c61e2640f3
Notice particularly the "knee" that you get as the zener approaches its operating voltage.
So as the voltage across a zener depends on the current applied, you'd choose the operating current accordingly - certainly, an operating current that is sufficiently around the "knee" is good. Say, at least 5mA, for example. Some might say 10mA. All diodes are different.
Remembering that we need 0.6V across R1 when the SCR is to trigger, and we'd like 5mA to be flowing at that point, that tells us that R1 should be 120 ohms.
But this is very approximate. As well as the basic voltage tolerance, the minimum recommended current for zener diodes varies a lot from diode to diode within the same family (as there are actually 2 distinct mechanisms used in zener diodes, according to their rated voltage), so look at the datasheets to get a feel for this (e.g. page 5 of this: https://www.vishay.com/docs/85607/bzx85.pdf)
Also, that ignores the current taken by the gate of the SCR. You'd need to check the chosen SCR's datasheet to decide if that's low enough to ignore...
In short, experiment and see what happens
