Fri, 9 Feb 2001 19:55:52 -0500
> in the use of a shorting or non-shorting switch in the pi-network
> output of an amp?? Are there any real advantages one way or the
> other?? always wondered about it!! carl / kz5ca
These switches are called the wrong names all the time.
A "shorting switch" picks up the next contact before releasing the
one it was just on. It has less contact-to-contact holdoff voltage,
and is a bad choice for this application.
A shorting switch is normally used when switching isolated loads,
where the moving contact (rotor) always has to be connected to
something while rotating to prevent a failure of some type.
A "progressively-shorting" switch, or "pick-up-and-hold" switch
progressively picks up contacts when rotated, and is a good switch
They can be ordered with either shorting or non-shorting type
A "non-shorting" switch rotor releases the stator contact fully
before hitting the next stator contact. They generally have much
higher contact-to-contact voltage ratings.
They are used where the rotor has to fully drop a contact before
connecting to the next one. They are often used to switch multiple
power sources that can not be cross-connected.
The best switch for amplifier tanks is a non-shorting contact
configuration using a pick-up-and-hold rotor.
The reason you need the pick-up-and-hold rotor is because on
higher bands the tank can have a self-resonance where it looks like
two or four back-to-back L networks. If the voltage at the center of
the L-networks formed by the stray capacitance and unshorted
areas of inductor becomes high enough, the switch or other
components will arc over and fail.
The same problem occurs with *some* roller inductors, when a
wide frequency range is covered, unless they have some form of
additional shorting other than a single moving tap.
73, Tom W8JI
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