On 9/3/2010 6:53 AM, Hulett, Russell wrote:
I owned a TT PS for awhile, but it had a single pass transistor
without the OVP module ( crowbar ), so fear of putting 25 VDC into
the rig from a 30-year old transistor failing caused me to never use
the thing. For awhile I contemplated building a crowbar, but ended
up selling the PS, now use a switching supply with meters and just
keep the current levels below the danger point.
So you have abandoned the current limiting of the Tentec PS which
protected the PA from mismatched tuning the caused high collector
current. Out of the frying pan (as yet not heated because the single
pass transistor is sturdy enough) into the the fire. I doubt the
unregulated supply with only one pass transistor is as high as 25 volts
because part of the reason for a high unregulated voltage is that to
effective divide the current between bipolar pass transistors in
parallel requires dropping 2 or 3 volts in the emitter resistors that
force the even current sharing. Linear Astron supplies tend to
progressively blow pass transistors open because they don't drop enough
in the emitter resistors to force good current sharing.
Using the switching supply you NEED the approved Tentec fast acting
circuit breaker, though for my Corsair II on my large station supply, I
use a 15 or 16 amp fast blow automotive fuse from a reputable
manufacturer (Buss), not an import. So far while manually tuning the
tuner, I've blown a couple fuses and no PA transistors.
When did Ten-Tec start incorporating OVP ( and/or dual pass
transistors )into their power supplies? Which models are "safe" from
the destroying the powered device because of single-component
failure? So far, I've avoided buying any TT power supplies because
am never sure which of them are "safe".
When there are multiple pass transistors in parallel, the failure
shorted of ANY of them causes the over voltage problem, so the
reliability of those supplies worried about that failure mode is poorer,
there are more possibilities to fail.
There have been numerous crowbar or over voltage protection device
circuits in the handbooks, so its not hard to work out. Usually it
involves a sturdy SCR (Silicon Controlled Rectifier) a zener diode to
set the voltage limit, a couple resistors and a capacitor at the SCR
gate to keep out RF. Working out the zener voltage and the resistor
values is a balance between reliable triggering and minimizing triggers
from short transients. Then you want to add a fast blowing fuse at the
input of the power supply regulator so the SCR doesn't have to fry the
power supply to shut it down.
73, Curt KB5JO _______________________________________________
73, Jerry, K0CQ
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