View Single Post
Old 07-21-2002, 11:03 PM
Mark Wilson's Avatar
Mark Wilson Mark Wilson is offline
Registered User
Join Date: Apr 2002
Location: St. Paul, MN
Posts: 374
Mark Wilson is on a distinguished road
Originally posted by Mike Firth
Well, I read the brochure which is nice and colorful and only moderately useful. There is a chart out on page 15 that shows these things have a 6:1 ohms resistance change from room temp to glass temps (0.4 ohm/mm/m to 2.4) which is very close to a short circuit at cold temps. It's getting late so I won't type what I am thinking and be stupid.
the 6:1 change of resistance with temperature is compensated for by changing the average voltage applied across the elements. the power avaliable to heat the glass is given by the formula

P = V^2/R

so ar R increases with temperature, the average voltage must increase sligthly. it is a slight change since the power goes as the square of the applied voltage. the triac or scr circuit change the average voltage by eliminating a percentage of the power of the AC cycle. this change of resistance with temperature is no different with or without the transformer.
Reply With Quote