On Tue, 2009-03-31 at 20:17 -0500, Stuart Rohre wrote:
> Curt,
>
> Crow bar circuits used to be sold after market for supplies made by the
> open frame power supply companies like Power Designs. You could do
> that, or you could place a shunt zener diode on the output of the supply
> rated at say 16 volts, for a 12 volt supply. Fuse the supply up stream
> of the zener diode, so that if it goes over voltage, the zener shunts
> full current or tries to, and the fuse blows.
>
> 73,
> Stuart
> K5KVH
>
Many ARRL Handbook power supply projects have included a crowbar. Typically
its a large SCR connected so when triggered it shorts the supply, sometimes
before
the regulator. I see in the 2002 ARRL Handbook page 11.38 in figure
11.51 there is such a circuit. This one is to protect a 28 volt supply
so it uses a 33 volt zener. For a 13.8 volt supply, 15 or 16 volts would
be more appropriate. The problem with most crowbars is that they will
trip for short transient over voltages or RF getting into the power
supply which can be a serious nuisance. But putting 22 volts into a
radio designed for 14 tends to be bad for the radio. I have a different
solution in my ham shack, but using MOSFET regulators I can regulate
with a much lower voltage drop than for a supply using a gaggle of
paralleled bipolar transistors and my unregulated supply is only 16
volts at no load. For that I don't need a crowbar, but its not a
commercial product.
73, Jerry, K0CQ
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