# Question about the importance of 1% resistor in thermostat circuit

Discussion in 'General Electronics Chat' started by tracecom, Jun 6, 2010.

Apr 16, 2010
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2. ### retched AAC Fanatic!

Dec 5, 2009
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I suppose if you are trying to get an accurate temperature, you want to build it from accurate parts.

here tis:

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3. ### Bychon Member

Mar 12, 2010
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I'd say, "yes". All the thermistors I've met were 1% accuracy. No reason to throw that away for a 12 cent resistor.

4. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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Does RV1 compensate for any variance in R4?

5. ### Bychon Member

Mar 12, 2010
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No. It sets the switching temperature.

6. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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I guess that is where I am confused. The circuit doesn't actually display the temperature anywhere. In use, you have to have a separate thermometer in order to set RV1 to switch at the temp you want it to, so it seems that the precise value of R4 doesn't matter. I'm not disputing what you say; I just don't understand.

7. ### Jaguarjoe Active Member

Apr 7, 2010
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Maybe they want 1% because it might have a better temperature coefficient than a garden variety resistor. The actual value does not need to be 1% accurate because if its not, you'll just tweak RV1 a tad to make up for the slight difference in value.

8. ### Bychon Member

Mar 12, 2010
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I have a chart of a 10k 1% thermistor and it shows 2 1/2 degrees (Centigrade) error for a 10% difference in R4 (at a center point of 25 C). If that doesn't matter to you, throw the 1% resistor away and put in a 10% resistor.

9. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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That's what I thought, but I'm no expert. As a matter of fact, the R4 resistor that ships with this Velleman kit has a 5% tolerance.

10. ### Bychon Member

Mar 12, 2010
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Gee...you had all of us thinking a 1% resistor came with the kit and you were thinking about throwing it away.

11. ### tracecom Thread Starter AAC Fanatic!

Apr 16, 2010
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No, just the opposite; I was wondering if I needed to change to a 1%. Seriously, is that chart you mention something that you can provide a link to? Maybe I could learn something.

12. ### Bychon Member

Mar 12, 2010
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I don't know how to do a link to my filing cabinet. Maybe I can scan it.

• ###### 1%thermistor.PDF
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tracecom likes this.

Jul 7, 2009
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Bychon's scan looks like a sheet I have for an old YSI thermistor. Don't assume it applies to any other thermistors except the indicated model. You can go to Measurement Specialties' site and get data sheets on their different thermistors (they bought YSI).

14. ### SgtWookie Expert

Jul 17, 2007
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Well, a metal film resistor would have better stability than a carbon or carbon film, and would also have lower noise.

However that circuit isn't aimed for precision anyway, so it's really "Much Ado About Nothing".

BTW, 1N4148's were poor choices for D1, D2. Expect them to burn up after the circuit's been in use for awhile. You could swap them out for any 1N400x series, or if you prefer some 1N5817's.

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