27-12-2017, 06:28 PM
Have a look here to learn about SCRs: https://en.wikipedia.org/wiki/Silicon_co..._rectifier
As shown, you can make one from two "normal" transistors. I've done that a fair few times over the years - it's occasionally a useful building block. I find it useful to look at the transistor circuit to understand the operation.
Between the gate and the cathode, you have a PN junction. If you put about 0.6V across it then, as you'd expect for any forward-biased PN junction, current flows. This fires the SCR and latches. However, the last thing you want from a crowbar is "nuisance triggering", which is why R1 and C2 are there to hold the gate close to the cathode. Otherwise, noise can cause the SCR can trigger...
R1 can't limit the current seen by the zener. As you try to increase the voltage across a PN junction, it simply passes more current, doing its best to hold the voltage at 0.6-0.7V. It's just a diode, effectively...
So as you exceed the nominal 7.4V given by the combination of the zener and gate-cathode junction, the current will rise and rise... But as Ref suggests, the SCR should trigger and short the whole circuit before any harm can come to the zener. So if you've damaged zeners, there's something amiss somewhere. Perhaps your chosen SCR is a slow one, perhaps you've got a mistake in the construction - who knows? But if you're worried, you could put 100 ohms or so in series with the zener. It will raise the threshold slightly, but that shouldn't be an issue during development...
As part of a PSU course I run, I get folks to build up one of these. But they connect an incandescent light bulb in series with the crowbar (in place of the fuse), then slowly advance the output of a bench PSU until the crowbar fires - which simply lights up the lamp with no drama - and then they can record the voltage from the PSU that caused the crowbar to fire. During the debugging of this, I'd recommend a similar approach.
As shown, you can make one from two "normal" transistors. I've done that a fair few times over the years - it's occasionally a useful building block. I find it useful to look at the transistor circuit to understand the operation.
Between the gate and the cathode, you have a PN junction. If you put about 0.6V across it then, as you'd expect for any forward-biased PN junction, current flows. This fires the SCR and latches. However, the last thing you want from a crowbar is "nuisance triggering", which is why R1 and C2 are there to hold the gate close to the cathode. Otherwise, noise can cause the SCR can trigger...
R1 can't limit the current seen by the zener. As you try to increase the voltage across a PN junction, it simply passes more current, doing its best to hold the voltage at 0.6-0.7V. It's just a diode, effectively...
So as you exceed the nominal 7.4V given by the combination of the zener and gate-cathode junction, the current will rise and rise... But as Ref suggests, the SCR should trigger and short the whole circuit before any harm can come to the zener. So if you've damaged zeners, there's something amiss somewhere. Perhaps your chosen SCR is a slow one, perhaps you've got a mistake in the construction - who knows? But if you're worried, you could put 100 ohms or so in series with the zener. It will raise the threshold slightly, but that shouldn't be an issue during development...
As part of a PSU course I run, I get folks to build up one of these. But they connect an incandescent light bulb in series with the crowbar (in place of the fuse), then slowly advance the output of a bench PSU until the crowbar fires - which simply lights up the lamp with no drama - and then they can record the voltage from the PSU that caused the crowbar to fire. During the debugging of this, I'd recommend a similar approach.
