[GreenKeys] Loop supply rheostats Watt Rating?

[hwhall at compuserve.com]

If the point of all this is some concern over whether the rheostats are adequately rated for power in a TTY loop...The original manufacturer printed values: 0.1 amp max thru all 2500 ohms = 25 Watts max.  We're going to be running very close to 60mA loops.60mA running thru the whole 2500 ohms dissipates only 9 Watts.60mA running thru half the windings, 1250 ohms, dissipates only 4.5 Watts.That looks very comfortable.
Wayne
WB4OGM
-----Original Message-----
From: Jeffrey Angus <jdangus at att.net>
To: greenkeys <greenkeys at mailman.qth.net>
Sent: Thu, Sep 12, 2019 10:44 pm
Subject: Re: [GreenKeys] Loop supply rheostats Watt Rating?

On 9/12/19 11:17 PM, hwhall at compuserve.com wrote:
> It seems worth observing that in my experience (I bet others,
> too) resistors, including variables, are far more often found
> with watt ratings stamped on them than with max current
> stamped on them. Apparently, max current data may be in
> the manufacturer's datasheets but we probably don't often
> seek it out. ??<shrug>
It's simple enough, P = I^2 * R So it follows that SQR(P/R) =
I.
For example, a 5K carbon 5 watt resistor is limited to no more
than (about) 30 mA, regardless of where the wiper is.

If you wanted to use a 0.1 A rated variable resistor in a loop
supply, assuming 120 VDC. First off, you would limit the total
loop current to .1 A. 120 / .1 = 1200 ohm series resistance.
To adjust the current down to .06 A, you need and additional
800 ohms.

So you'd have a 1200 ohm fixed resistor in series with a 1000
ohm pot. (Only because they make those, and not 800 ohm
ones.)

If you wanted to be able to go down to .02 A, use a 5000 ohm
rheostat.





-- 
"I am a river to my people."
Jeff-1.0
WA6FWI
www.foxsmercantile.com

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