Temperature with Capacitors

Amaral

Hello all.
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I’ve been using a Capacitor as a thermometer , a simple RC circuit an some factor conversions to debug the temperature, at the beginning it looks really reliable a precise, with of course a ·small range, I put my finger over it and the numbers start to move.
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Then today I went to buy some capacitors (0,1uF) and the thermometer did not work with this new ones,· Does any one has tried to do that, and does any one knows why this is working with just parallax ( the ones that came with the Boe Bot ) capacitors ??? ( I mean , why this or some capacitors are more sensitive to temperature variation than others ?)
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Thanks
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Ricardo Amaral

Philip Gamblin

You mentioned "putting my finger on it" it is possible that you actually changed the value of the capacitor by proximity to your body. your body can act as an antenna and cause oscillation and all kinds of weird stuff.

Amaral

yeap ! it can , but capacitance changes with temperature too, I also tested as a thermometer ! with a heat font close to it. do this test yourself ! when you take the heat font from it it takes some time to start cooling and changing numbers , also with a small piece of ice , the variation is not linear but for small changes it can be considered as a segment o straight line.

DO

HIGH 7
PAUSE 4

RCTIME 7,1,time

DEBUG "time = ",DEC time," " ,"freq = ",DEC freq," ","temp = ",DEC temp, CR

LOW 7
freq = (166-time)*231
FREQOUT 0,28,freq
temp = 190-time*5/8
LOOP

the circuit is pretty close to the LDR with a capacitor (RC on the Boe Bot)


Ricardo Amaral

Philip Gamblin

Still, what relationship there is, not by design such as light and the resistance photocell. Tanatalum capacitors are more stable than ceramic disc, for example, and would most likely have much less of the temperature effect you are observing. A thermistor is designed to be a temperature dependent but typically the resistance values are so small ( a few hundred ohms or so) that they are not particularly useful wit RCTIME.

Tracy Allen

Ceramic capacitors are rated by the type of ceramic dielectric. Ceramics with very high dielectric constants usually also have large temperature coefficients. Z5U dielectric is often used in high capacity ceramics, and you would see much more effect with that than you woulld with X7R. NPO capacitors have near zero drift, but are only available in small caps. Browse the Digikey catalog and download data sheets and you can see the effects. Or look at the discussions in books like the Art of Electronics.

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Tracy Allen
www.emesystems.com

Amaral

Fantastic Web site ( www.digikey.com ).
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I may have a worst case capacitor, represented by the line F of the graph. Than for a variation on 10 to 12 degrees Celsius(the weather is about 20 deg. Celsius·now over here and I touch it, its going to about 32 deg. C) , I'm Having a variation of 15% on its value,·witch is exactly the difference that I'm having; the time is varying from 160ms to 132ms!

The graph was taken from : http://dkc3.digikey.com/PDF/T052/1002.pdf

Thanks

Ricardo Amaral.