On Thu, 29 May 2008 18:57:47 -0400, Dave M
wrote:

Remember that the purpose of the crowbar is to protect the load from power
supply overvoltage, not to protect the power supply. The crowbar should be
connected to the regulator output, not the input. If connected to the input,
the reservoir capacitor(s) on the output are still charged when the crowbar
fires, and the overvoltage condition is applied to the load until the capacitors
drain. The load is unprotected.

Just put a reverse biased diode across the series pass transistors.
When the crowbar at the main capacitor fires, the output capacitor
will be discharged through that diode and crowbar, protecting the
load.


Remember also that most of the Astron supplies have current foldback limiting.
That means that on a shorted output condition, the regulator should reduce the
available current into the short to a significantly lower level, protecting the
pass elements and regulator.

Foldback limiting requires that the differential amplifier (in the
723), driver and series pass transistors are working properly, thus,
the crowbar+foldback limiting works properly only if the 723 reference
voltage is erratic. In all other failure modes, such as dead 723,
shorts in driver or series pass transistors, it is quite unlikely that
the foldback would work as expected, if the crowbar fires due to
output overvoltage.

Paul OH3LWR