# PIC08 Circuit To Control 12v PC Fans

#### Exceptional_Joe

Nov 27, 2012
11
Thank you both for your replies. I've added a screenshot of the datasheet I found for the TTC-203 (the thermistor I'm using) below. However I'm struggling to understand what the resistance is (what does the B25/50 value mean etc?) The input to the thermistor will be 12v... will this be a problem? I currently have a 1k resistor before the thermistor so would increasing the resistance of this help in any problems I may have with a too high voltage?

Thanks again.

#### Attachments

• TTC203 Datasheet.jpg
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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
You need to look at the page prior to that one as well. There are 4 graphs, the top right one is the one you need. And the third line from the top is the curve for resistance vs temperature for your thermistor.

You can see that at 25C the resistance is 20k, so the voltage you get from your voltage divider (assuming a 5V supply) is:

5*20/(20+1) = 4.76V. If you have an 8 bit ADC, the value you'll get is 255*20/(20+1) = 243.

looking at the graph, you can read the following values:

temp resistance (k)
30 15
40 10
50 7
60 4
70 3
80 2
90 1.3
100 1

and so on... (remember it's a log Y axis, do take care in interpolating!)

to pick one of those values... say 80C to repeat the calculation:

voltage = 5*2/(2+1) = 3.33V, and ADC count = 255*2/(2+1) = 170

However, remember that the values may be lightly different due to tolerances in components.

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