Another mystery item

[elanman99]

I removed this (over 40 years ago) from a linear low voltage PSU I was modifying for some long forgotten reason, it was wired into the low voltage output.

The part in the picture is 40mm x 40mm x 12mm high and is dense encapsulating resin in a nice shade of turquoise.

I have never seen another one, this one has no makings whatsoever (other than my felt tipped ones).

Ian

[turretslug]

A crowbar module with threshold set by the resistor? A crowbar circuit was sometimes offered as an option.

[elanman99]

Spot on!

I presume the PSU manufacturer (Weir I think) must have made the crowbar device in house.

Internally I suppose it act as a sort of Zener diode on steroids but if it just shorts out the PSU something is likely to get quite hot.

Ian

[duncanlowe]

Originally Posted by elanman99

Spot on!

I presume the PSU manufacturer (Weir I think) must have made the crowbar device in house.

Internally I suppose it act as a sort of Zener diode on steroids but if it just shorts out the PSU something is likely to get quite hot.

Ian

It kind of depends on the output limitation. Although these days we tend to have current limit (so maintain a constant, limited current) it used to be common to have foldback limiting. That is, the current gets reduced down to near zero. Works great with crowbars, and saved me a lot of digging once. I happened to choose the only PSU in the lab that had foldback. The device I was testing was blowing fuses and PCB tracks, even though it had passed EOL testing at the supplier. The foldback supply showed the surge at power on, which their test bench supply didn't.