50k Thermistor to 0-5 volt with the ADC0838

Sutton Murray

Hi

I need help on getting a 50k thermistor hooked to an ADC0838 IC. I currently am building a engine safethy system. All I/O lines are already used. It leaves me with 2 open channels of the ADC. I tried the voltage divider method with the 2x serial resistors. This is not working as the span slows or fails at·high temperatures(the high temperature reading is a must, this i need from the thermistors maximum capabilities). I have tried many additional hook up·resistors but still no luck. I tried using a adjustable voltage regulator, in stead of a pot changing the voltage outcome i replaced it with the thermistor. For some reason i can't get a full output up to 5 volts. Next i thought of the PWM usages. Using a 555 to give a pulse in place of the PWM command through a resistor(changed with my thermistor) and a capasitor to give me a reading out in volts. My knowledge on the 555 is limited and i am not winning. My guess is i am failing on the correct pulsing from the 555. Can someone help me on hooking this up, or even·any other better ideas for a full convertion usage of the thermistor.

Mike Green

To simplify this ... What range of resistance do you need to measure? Where is the most critical part of the range?

The ADC should work. What did you try (include diagrams or adequate descriptions)? What happened?

Sutton Murray

The full 50 K ohm range is needed with the range at 0 k ohms been very critical for a maximum temperature of +/- a 140 to 150 degree celsius. I will come back with the diagrams.

Bean

I would use an LM35 temperature sensor.
It puts out 10mV/°C
If you need to measure temperature below +2°C you will need a negative supply, or I think they make some others that have an offset so you can measure low temperatures. Something like 1Volt+10mV/°C.
[noparse][[/noparse]edit] The LM50 has a 0.5V offset to allow measureing down to -40°C with a single supply.

Bean.

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Post Edited (Bean (Hitt Consulting)) : 10/3/2006 7:03:24 PM GMT

Mike Green

In a metal package (TO-46), the LM35 is rated for operation to 150C with storage temps to 180C (not rated for measurement, but for continuous presence at 180C). The datasheet indicates that the metal package can be soldered to fixtures as long as electrically it's ok for it to be grounded there. The 140C to 150C range would be 1.4V to 1.5V and an op amp connected differentially could offset and expand this linearly to a 0-5V range.

Sutton Murray

On·6 of the ADC channels i am using the LM34. I understand the offset as mentioned for the LM35. I kept with the farheit output of the LM34 due to the no offset needed. My understanding is if i use the offset all channels will need to work on this offsets. This will set me back on other addional sensors not working on the same span. Two of these channels i need to use the Thermistor as the channels i want to use optional to other sensors like inductive switches, this is why i am looking at resistance input and a full span usage of the input resistance for these channels. i have attached the diagram as required

Sutton Murray

Mike Green-- any thoughts on my circuits?

Tracy Allen

Sutton,

You have to realize that a thermistor that has a resistance of 50,000 ohms at 25 degC (the usual spec point), will have a much much lower resistance at 150 degC. Around 500 ohms, guessing.

You could substitute a lower value for the series resistor, but that would compromise the resolution at lower temperatures. It speaks to Mike's original question, "To simplify this ... What range of resistance do you need to measure? Where is the most critical part of the range?" Do you have the resistance vs temperature curve handy for this particular thermistor? What range of temperatures do you need to measure and with what resolution/accuracy?

Another approach to covering a wider range with the one 8 bit ADC, is to switch in a resistor appropriate to each range, something like the attached circuit. For low temperatures (room temperature), the pin connected to the bottom of the 1kohm resistor floats, but when the temperature goes up, that pin or switch connects the bottom of the resistor to common.



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Tracy Allen
www.emesystems.com

Mike Green

Another option that could be interesting is a logarithmic converter. I don't recall the circuit exactly, but it uses an op-amp with the thermistor in the gain setting feedback loop and produces a voltage proportional to the log of the resistance. This would give you a markedly expanded high temperature (low resistance) range.

Sutton Murray

Tracy--Many thanks for your help. To pick on a range is not what i want. As the resistance measurement may be for different sensors, positions of measurement. From cold ice degrees to hot cylinder head temperature. But your circuit and explanation gave me another idea. Since i have no i/o pins left on the stamp. I will fit jumpers on my pc board. From the jumpers i will make my selection in your circuit for my different sensor and their position being used.

Mike Thanks for your reply. I will be looking into the logarithmic convert. I still need to learn more about this.

Thanks again for all the help