RE: QRP+ Polarity Reversal

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From: cebik@UTKVX.UTCC.UTK.EDU
Date: Thu Dec 07 1995 - 07:08:45 EST


On the resistor in the ATU: It is useful only for slower charge
build-ups, where slower is a relative term. For a truly fast spike, the
combination of L and C in the ATU and even in the RX may be enough to
slow the time constant down so that a very high spike can do its damage.
With solid state, it does not matter for a 1 cycle spike whether the
circuit is complete (cuz it won't know that til its been there) or
whether you work from + down or circuit ground up: a high voltage
differential between an input (transistor base, etc.) and a plane
(whether the ground or supply voltage line) may be enough to be
destructive in a blink of an electron's eye--and since Ls and Cs are
energy storage units, they may in certain configurations make the problem
worse. In some configurations, they can turn protective circuits into
dangerous ones, relative to a sensative input structure. Cure? Perhaps
a sturdier input device. Vacuum tube, anyone? MMIC? In the end, safety
procedures may be the safest procedures, like the total disconnect
recommended for the QRP+ not in use, power cable connection before the
supply (completely discharged) is plugged in to the wall, added line
switch (DPST) in a battery supply, etc. Even if Timex made all the QRP
rigs (takes a lickin and keeps on tickin), I'd still cling to that old
saying that haste makes waste, do not hurry, have patience, etc. (Pick
your favorite version--better yet, pick the one that irritates you most,
since you are less likely to forget that one.)

-73-
LB, W4RNL


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