[GreenKeys] Loop supply rheostats Watt Rating?

[hwhall at compuserve.com]

Are we talking about the watt rating or the watts dissipated during use?
The rheostat is apparently designed to handle 0.1A thru the full 2500ohms, thus it appears 25 watts is the designed figure for max power dissipation. This seems borne out by other rheostats the same physical size that are specifically labeled "25 Watt."

But if we're looking at actual power dissipated... Using only half the resistance but applying sufficient voltage to run 0.1A thru that, then the power converted to heat is indeed 12.5 watts.
As I understand it, wattage rating refers how much power can be dissipated without overheating given certain assumptions about ambient temperature, etc.
If we only used half the resistance windings, the entire unit's mass & area is still there as a heat sink/radiator.  It's certainly not as heat-efficient as when the power is dissipated over the entire length of the windings, but using only half the windings doesn't, in effect, make the other half of the rheostat's mass & area dissappear.  BUT... that said, assuming so is quite conservative & sidesteps trying to guess how efficiently heat is dissipated by the whole device when only half the winding volume & area are generating the heat.
.................I think.  :-)

WayneWB4OGM
-----Original Message-----
From: Peter Gottlieb <hpnpilot at gmail.com>
To: Charles <charlesmorris800 at centurytel.net>
Cc: hwhall <hwhall at compuserve.com>; greenkeys <greenkeys at mailman.qth.net>
Sent: Thu, Sep 12, 2019 11:14 am
Subject: Re: [GreenKeys] Loop supply rheostats

Only 25 watts if you use the full resistance though. 


Peter
On Sep 12, 2019, at 1:04 PM, Charles <charlesmorris800 at centurytel.net> wrote:



2500 ohms * 0.1 amp squared = 25 watts <wlEmoticon-smile[1].png>  From: Paul Heller Sent: Thursday, September 12, 2019 11:51 AMTo: Steve Garrison Cc: hwhall at compuserve.com ; greenkeys at mailman.qth.net Subject: Re: [GreenKeys] Loop supply rheostats I can’t really read the picture on ebay clear, but it is either 300 volts or 500 volts. I can read .10 AMP clearly  Paul

 On Sep 12, 2019, at 10:06 AM, Steve Garrison <steve.n4tty at gmail.com> wrote:     I’m assuming a pretty hefty wattage?    How much each plus shipping?    Steve G./N4TTY    From: greenkeys-bounces at mailman.qth.net <greenkeys-bounces at mailman.qth.net> On Behalf Of hwhall at compuserve.com
Sent: Wednesday, September 11, 2019 7:43 PM
To: greenkeys at mailman.qth.net
Subject: [GreenKeys] Loop supply rheostats        Recently, there was a brief discussion of having variable resistors in the loop, to tweak the loop current.  With that in  mind, today I came across a box of brand new rheostats marked 2500ohms, 0.1amps. They have screwdriver adjust shafts.  Anyone want some of these?      Wayne  WB4OGM______________________________________________________________
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