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Date:         Thu, 17 Feb 2000 16:13:47 -0500
Reply-To:     David Beierl <dbeierl@IBM.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@IBM.NET>
Subject:      Re: Overheat thought
Comments: To: Davidson <wdavidson@THEGRID.NET>
In-Reply-To:  <000601bf7984$feada4e0$7530a2d1@dell>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 03:24 PM 2/17/2000 , Davidson wrote: >what is a thermistor?? >Bill

It's a special resistor made of a material that exhibits an unusually large resistance change with temperature, usually less resistance with higher temperature (which is backwards of most materials). This slope direction is called "negative temperature coefficient." The device may be the size of a pinhead or much larger, and often is embedded in a case like the Vanagon Temp I and Temp II and gauge senders are.

Among other uses, it's one of the two common methods of measuring temperature electrically -- the other way uses a thermocouple, which is in essence a pair of dissimilar wires joined at the ends. They generate a (tiny) voltage which is proportional to the temp. difference btw the hot ends and the cold ends. The thermistor method is generally cheaper and less accurate, and requires external power (easily available in a vehicle). Also a thermocouple will probably withstand temps that would destroy a thermistor.

Incidentally if you look at the resistance vs temp charts in Bentley, you'll see that Temp I and Temp II have the identical characteristics, though their external appearance is different. Very likely the actual thermistor is identical in both.

david David Beierl - Providence, RI http://pws.prserv.net/synergy/Vanagon/ '84 Westy "Dutiful Passage" '85 GL "Poor Relation"


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