[GreenKeys] Loop supply rheostats Watt Rating?

[Jeffrey Angus]

On 9/13/19 12:32 AM, hwhall at compuserve.com wrote:
> 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.
It's NOT the wattage rating, it's the maximum current rating.

Using a part rated at 100 mA in a circuit centered around 60 mA
is a good design. It allows a 40% margin on the rating.

But if you're going to use ONLY the 2500 ohm rheostat, you run
the risk of exceeding it's current rating if you reduce the value
too much.

For example, suppose you turn it the wrong way to 300 ohms.
The total loop resistance at this point is 500 ohms. 120 v /
500 ohms = 240 mA.?? AT 300 ohms, that's 17.28 watts. Only
69% of the wattage rating. But 240% of the current rating.

At 120 volts, with a selector magnet resistance of 200 ohms, the
Rheostat needs to be set at 1800 ohms. That's only 6.48 watts.
While that is only?? 25.9%n of the power rating, that's still 60% of
the current rating.

The wattage rating is ONLY across the entire resistance.
The Maximum current is the critical rating.


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

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