Paul,
Oh, the sweet ACOM. What an amp! I saw the ACOM in action and marvelled at
the silent and air-as-you-need-it-only variable speed
arrangement. It was that amp that got me onto this love of variable speed amp
fans.
But there is also a reasonable case to be made by the guys who suggest the MCU
et. al. modern goodies have a downside. In my case,
I am much less worried of a fan failure (as there are 2 fans providing
redundancy) - but the idea of RF getting into my control
circuitry and the fan controller not firing as a result - that would
undoubtedly happen in the heat of some latte night RTTY contest
and I would have crispy finals before I realized it.
The switch is simple, nearly impervious to failure, and will help me sleep at
night.
73, Jeff ACØC
www.ac0c.com
-----Original Message-----
From: Paul Christensen
Sent: Monday, November 08, 2010 8:10 AM
To: amps@contesting.com
Subject: Re: [Amps] time to death in the event of a blower failure
> "Here’s the idea...The cut-out mechanism would be one of these TO220 sized
> bimetal switches. They are available in a variety of trip points."
That's a concept I used back in the '70s to engage a boxer fan on the back
of an HW-101. A precision cylinder thermal switch was used to measure
temperature inside the 101's PA compartment. In that case, a switch with a
two-point hysteresis level was needed to keep the fan from toggling on/off
more than necessary when the temperature was at the sensor's threshold. In
your application, that wouldn't be a concern.
A companion idea involves monitoring the stack temperature inside the tube
chimney. The ACOM 2000A uses a LM35 (or its equivalent) on the Silicone
chimneys. The LM35 is a precision Celsius sensor, whereas the LM34 provides
output in Fahrenheit. There are no external components, although usual
bypass C practice is a good idea.
http://www.national.com/ds/LM/LM35.pdf
Both devices are available in a small 3-lead TO-92 package and are immune to
RF. They can be found in commercial temperature sensors used at high power
broadcasting facilities. There's no calibration either. The output
voltage is a linear function of the actual numeric temperature change.
Since the TO-92 is so small, a 1/8" hole can be perforated into the side of
the Silicone chimney (on ceramic VT amps), then use Silicone RTV to keep the
sensor in place, while keeping the sensor leads dressed adaquately away from
HV.
So, with minimal effort, one can have a low-cost temperature failsafe with
monitoring capabilities.
Paul, W9AC
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