On Wed, 2009-04-01 at 08:22 -0500, Hulett, Russell wrote:
> Evidently several years ago, someone suggested installing a 1.5KE15A
> transient suppression diode with a fuse at the PS output. This seems like a
> simple solution to me compared to SCRs, added circuits etc. One question,
> since these diodes will clamp, thus protecting the load ( my Triton IV ) why
> won't the 4A slow blow on the transformer input do the job.
>
> I'm thinking of building the zener and an in-line fuseholder into the cable
> between the 262G and the 540. That's to avoid modifying either the power
> supply or the radio. Wonder if anyone else has used that approach, and
> whether I'm missing something crucial.
>
> 73, Curt KB5JO
The problems with the big zener on the output are that they don't have
zero impedance once they are drawing current. So if the available
current is big, the voltage drop is big. Then they achieve dissipate
lots of power as do the remaining power supply parts. The SCR dissipates
little power because once triggered into conduction its voltage drop is
small. A good crowbar addition to a power supply is connected to sense
the output voltage, but shorts the output of the unregulated supply
right after an added fuse so the resistance of that fuse isn't
destroying the power supply's output regulation.
For a decent circuit (needs maybe a dozen common parts) look in recent
ARRL Handbooks in the power supply chapter where there's a 28 volt
supply with optional crowbar. That scheme works. The 1 uf cap is crucial
to minimizing nuisance trips from transients and RF.
73, Jerry, K0CQ
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