Originally Posted by Rick Karlquist N6RK

The runaway oven problem isn't caused by too much transistor
current, rather it is caused by the transistor staying on too long.
Thus a current based system wouldn't work. Also, in the case
of the 5334A, the power supply is barely able to supply minimum
current for warmup, so the oscillator couldn't draw more than
normal warmup current even if it tried. In the 5334B, I improved
the power supply so that it warms up the oscillator in half the
time or less compared to the 34A.

Rick N6RK


"Dave Platt" wrote in message
...
In article ,
Fred McKenzie wrote:

Rick-

I've seen photos of these ovens on E-Bay, that had been stained by smoke
coming
out of the adjustment hole. I'd rather have some kind of protection.

I believe the oven uses proportional control, so the transistors' maximum
dissipation would occur when the heating element is half on. In a
"runaway"
mode, the transistors would be switched on with maximum current but
nearly zero
voltage. Also, one transistor failure mode is a short-circuit.

Seems like a self-resetting "Polyswitch" overcurrent limiter might be
a workable alternative. You'd probably want to pick one whose "will
pass" current is somewhat above the highest amount of current that the
oven would draw when it's quite cold. If the transistor shorts, and
the oven draws more current than that, the polyswitch would heat up
enough to go high-Z and chop off the current. Cutting power for a
minute or so would reset it.

--
Dave Platt AE6EO
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