Transformer question (puzzled)


Honeywell's published documentation for their R182C switching relay http://customer.honeywell.com/techlit/pdf/95-0000s/95-6762.pdf contains the following warning:
IMPORTANT: The transformer [120VAC primary, 24VAC secondary] on the R182C may overheat when used with a series 20 thermostat if the total resistance of the thermostat circuit exceeds 2.5 ohms. If the measured resistance of the thermostat (including thermostat wire and thermostat contact resistance) exceeds 2.5 ohms, add a 100 ohm, 10 watt resistor between the W and R terminals.
I don't understand that -- if the total resistance is, say, 10 ohms, clearly the current drawn will be *lower* than it would be at 2.5 ohms, or at 1 ohm.
How is the transformer in danger of overheating at a *lower* current? Seems to me that the greatest danger of overheating would occur with a circuit resistance of near zero ohms, i.e. a dead short across the transformer secondaries. Someone please explain this to me.
[Please note that I *do* understand the purpose of the 100 ohm 10W shunt resistor, in reducing the total resistance of the connected load. What I *don't* understand is how a *lower* resistance avoids overheating the transformer.]
--
Regards,
Doug Miller (alphageek at milmac dot com)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Maybe this "transformer" is really some sort of constant-current device, and pushing current into a higher resistance requires more power, which could overheat it. Some control circuits work that way, eg 4-20 mA loops which supply constant current regardless of wiring resistance.
Maybe the "transformer" just supplies enough current to operate the R182C relay, regardless of wiring resistance?
Nick
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Possible salespeak.
Nick
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.