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R4 is thermistor with R25 value of 5 Ohm, Beta is 3150. V should be at most 1V because I want to use internal 1V1 reference voltage. V supply is 5V. I don't really want to use OpAmp to keep it simple. The problem is also to draw as less current from power supply as possible. Accuracy can be 2-3C, range is 20-80C.

I tried to use some Wheatstone bridge calculators, but I don't get the bridge balanced and to draw acceptable current.

enter image description here

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  • \$\begingroup\$ Which resistor are you varying to balance the bridge? \$\endgroup\$
    – user16324
    Jan 9, 2013 at 16:07
  • \$\begingroup\$ I've tried to vary R3. \$\endgroup\$
    – Pablo
    Jan 9, 2013 at 16:08
  • \$\begingroup\$ Thermistor range looks like 0.8 to 6 ohms. Doing this with low current and no amplifier looks like a difficult problem. Did you actually have a question? \$\endgroup\$
    – user16324
    Jan 9, 2013 at 16:16
  • \$\begingroup\$ @BrianDrummond: the question was R1-R3 values. \$\endgroup\$
    – Pablo
    Jan 9, 2013 at 16:17

2 Answers 2

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enter image description here This should get you close. By my calcs, in your range the thermistor will be going from about 1 ohm to about 6 ohms. With a 1K resistor to hold currents down to milliamps, you need a gain of about 35 to keep your max voltage at 1V. This circuit has a gain of 34 using standard 5% resistor values. You might need to change to a rail-to-rail op amp if you're going to power it with a one-sided supply.

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  • \$\begingroup\$ Could you plz give me one rail-to-rail amp example which I can use here? \$\endgroup\$
    – Pablo
    Jan 9, 2013 at 17:38
  • \$\begingroup\$ aldinc.com/pdf/ALD2702.pdf, assuming you need a DIP package, which limits your options. Alternatively, you can just use the tl081 with two power supplies, plus and minus Vcc \$\endgroup\$ Jan 9, 2013 at 17:49
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The resistance change of thermistors is generally big enough that you don't need a Wheatstone bridge. Just put the thermistor in an op-amp circuit. If you need to linearize the thermistor, that's done with an appropriately chosen series or parallel resistor -- http://www.ecircuitcenter.com/Circuits/therm_ckt1/therm_ckt1.htm

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  • \$\begingroup\$ So there is no way to avoid OpAmp? Would it introduce additional non-linearity to thermistor? Would it introduce distortion due to noises? I have TL081. If so then please correct your answer to state that Wheatstone bridge will not do the job and I will open new question with OpAmp. \$\endgroup\$
    – Pablo
    Jan 9, 2013 at 16:45
  • \$\begingroup\$ Do you have a part number for the thermistor?? 5 ohms seems like a low resistance for a thermistor. No need to open a new question. See barrgroup.com/images/glossary/T_ThermistorCircuit.gif on barrgroup.com/Embedded-Systems/Glossary-T for a real simple thermistor circuit. A Wheatstone bridge will certainly do the job, but they're also generally used with a differential amp, any you'll have a tradeoff between proper bridge balance and how much current you draw. In the circuit above, there is a tradeoff between sensitivity and current, but gain can added if needed. \$\endgroup\$ Jan 9, 2013 at 16:56
  • \$\begingroup\$ NTC 5D-20. Did you check datasheet in my original post? \$\endgroup\$
    – Pablo
    Jan 9, 2013 at 16:58
  • \$\begingroup\$ But how can I control this simple circuit to give me max 1V output? Also I'm sure this circuit is not accustomed for 5 Ohm thermistor. \$\endgroup\$
    – Pablo
    Jan 9, 2013 at 16:59
  • \$\begingroup\$ Now I'm confused... should I used only Wheatstone bridge or with OpAmp combination? \$\endgroup\$
    – Pablo
    Jan 9, 2013 at 17:01

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