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Relay affects thermistor — Parallax Forums

Relay affects thermistor

cparkercparker Posts: 14
edited 2006-08-11 16:10 in BASIC Stamp
I have a question about making my simple thermistor circuit more percise.

Long story short: I am using a BS2, and a cheap radio-shack thermistor to build a simple cooling thermostat (fridge controller). I am using RCTIME to measure the capacitor discharge time through the thermistor as described in the BS2 manual. I'm using an LM317 3-terminal voltage regulator to provide 5 vdc to the stamp, and to the thermistor, and to a relay which controls the fridge compressor. Vin is provided by a 12v(ish) AC wall-wort adapter.

I've noticed that when voltage is flowing through the relay in order to close the relay ciruit, this has a somewhat significant affect on the RCTIME measurement of the thermistor. The affect is 3-4 degrees F.

Any ideas on how I can reduce the affect of the relay on the thermistor? Would it make sense to use two LM317s, one for the more-sensitive thermistor circuit, and another for everything else?

I'll draw a quick schematic if needed.

Thanks!

CP

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2006-08-11 05:36
    Having a separate and/or isolated supply to your thermistor in a situation like this is not only a good design practice, but probably required for an application such as this.

    Go ahead and post a schematic, just in case something might be missed in the text description.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-08-11 15:18
    Along the lines of what Beau is getting at, what voltage is the Relay circuit and the thermistor circuit running from?· In a situation where the Relay was 12V but the BASIC Stamp and thermistor circuit was 5V the relay would be less likely to affect/impact the thermistor circuit.· Another option is to increase the resolution/sensitivity/stability of the readings by using an ADC0831 rather than RCTIME.· I hope this helps.· Take care.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • metron9metron9 Posts: 1,100
    edited 2006-08-11 16:10
    You could use a Zener to regulate the voltage through the RC timer. Use a Zener to limit the charge to say 4V maximum. As long as the voltage with the relay on is above the zener voltage you should have the same time constant.

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    Think outside the BOX!
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