nuckolls.bob(at)cox.net
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 Posted: Thu May 15, 2008 5:20 am Post subject: PMA-6000 Intercom - 28v install |
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At 08:23 PM 5/7/2008 -0700, you wrote:
| Quote: | PS Engineering s PMA-6000 intercom has the following note:
To reduce the amount of heat dissipated in the audio selector panel, when
used in a 28
Volt aircraft, it is required that the 15-Ohm, 15-Watt dropping resistor
(p/n 701-015-1501) be in-
stalled in series with the power input.
The install manual goes on to say failure to do so will generate
unnecessary heat inside the unit and may void PS Engineering s warranty
. Does anyone have experience installing one of these units in a 28V
environment? Is the dropping resistor really required? The only reason I
ask is the local radio shops don t seem to know much about it which makes
me wonder if they ever used them. They all say they can order one but
nobody has one in stock.
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It is risky to disregard the words of the manufacturer's
installation manuals. It's also disappointing that the
manufacturers of this device did not craft a user-friendly
design.
The simplest thing to do is comply with the instructions.
The simplest way to do this is acquire a component like
this:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=FVTS10-15-ND
Note that it can be mounted to some handy, out of the
way surface on a long screw. The data sheet at:
http://www.heiresistors.com/PDF/FVT_FST20%20spec.pdf
tells us the hole through the middle of this device is
0.19" in diameter . . .
The drill versus threaded sizes chart at:
http://www.phy.mtu.edu/~suits/tapsizes.html
says this is too small for a 10-32 screw but will
be just fine for an 8-32. The resistor data sheet suggests
we need one with 2" or so of over-all length. You'll
be cranking a hard force down against a ceramic tube.
Put a soft washer (some kind of rubber, plastic or even
two layers of paper from the box that comes with a tube
of toothpaste) between the end of the resistor and the
mounting surface. Tighten the screw down to finger
tight + 1 turn and spot the threads with some runny
super-glue to keep it from drifting off that pressure setting.
If your resistor comes with wires, clip them off.
Attach lead wires for intercom power to the resistor's
solder-tabs. Put double layer of heat-shrink over these
joints for vibration support.
This process has been used thousands of designs in
various vehicular products including airplanes. Simple,
lean, works as the manufacturer wishes and lasts a long
time.
BTW . . . the 15W rating is super conservative. The
intercom may draw 1A peak current but the AVERAGE
current will be much lower unless you're particularly
fond of playing Wagner at 100dB in the headphones.
Now, if you're REALLY wanting to make this more
complicated, you can purchase one of B&C's DIM3-28
dimmers at:
http://www.bandc.biz/cgi-bin/ez-catalog/cat_display.cgi?10X358218#dim3-28
and instead of installing the dimmer potentiometer,
you put the appropriate resistor between pins 1 and 3
to achieve a constant 14-volt output from the "dimmer".
Bob . . .
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