[SOLVED] Thermistor setup for battery charger -- how?

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lee321987

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Hello.
I've just breadboarded a battery charger based on the MAX713 IC -- datasheet: **broken link removed**

It's working but I'm not sure how to set up the thermistors.
I purchased some 100K@25C NTC thremistors.
Now I have to choose values for R3,R4,R5.
Any help?

Here is the relevant paragraph and image from page 12-13 of the datasheet (the diagram on page 1 doesn't use temperature to cut-off charge current):
 

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At ambient temperature, you want the bridge to be "balanced" so the T1/T2 = R3/R4. In theory this should give not hot and not cold. AS the battery gets hotter T1 will decrease in value so the "hot" goes to "1", adjust R3 to make this happen at the right temperature. Conversley R5 needs to be adjusted so that the "cold" goes to a "1" at the correct temperature. So look up the data for the battery, find out what its max and min case temperature is, then find out what your thermistors are at these temperatures and calculate the various resistance values.
Frank
 
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