[Boatanchors] HT-32

[Rob Atkinson]

Thermistors can work well under the right circumstances but two
conditions apply:  1.  They are positioned so they can dissipate heat
safely because they run hot.  2.  They have time to cool down before
being used again.  In other words, if an AM rig is switched in and out
of transmit by switching the primary of the PA plate supply
transformer, then the thermistor in series with the transformer
primary line needs some off-air time to cool down before being
energized again.  The protection they offer goes away if they stay hot
as in for example a number of brief closely spaced transmissions.  If
the rig is operated in a net or roundtable where there are 15 or more
minutes off-air then they're fine.

I speculate that if the rig has a 10H choke input filter and 10 or
less mfd capacitor following the choke, then in-rush may not be so bad
compared to a 30 or 40 mfd capacitor filter.  Then the transformer
failure mode of concern may be a short from the secondary winding to
the core and that can be guarded against by insulating the transformer
from the chassis.

73
Rob
K5UJ

> I have used the CL-60 inrush limiter (and others) in several Johnson 500 transmitter rebuilds and the limiters work great.