Hi everyone, I have some 0402 surface mounted thermistors which are causing me some bother. I am measuring an object in contact with the thermistor but am obviously measuring the temperature inbetween that of the PCB and the object. I would like to use another thermistor placed in the circuit (that will measure the temperature of the PCB) to allow me to offset this middle temperature to compensate. Can anyone help with a circuit example or ideas?
Many thanks in advance
David
Given the very small thermal mass of an 0402, and the huge thermal conductivity of the alumina body, solder, and copper pads, your readout signal error is going to be dominated by the quality of the thermal contact between the 0402 and the object being sensed. Given that, you're going to have real problems getting a good calibration from a second thermistor.
Think about it as a voltage divider problem: analogically speaking, your sensor has a 1-ohm resistor go ground across its input, and you're connecting it to an unknown voltage (the object temperature) across a resistor of poorly controlled resistance that is certainly a good deal larger than 1 ohm (the thermal contact).
This is not a recipe for a good measurement, regardless of how closely matched the two thermistor curves may be. You ought to figure out how to insulate the thermistor from the board.
Cheers,
Phil Hobbs
Thanks for the reply Phil. At the moment my jig is conducting the heat to the sensor well and repetitively. (I have a look up table to convert the resiatance into degres C and I'm getting a value approaching 0.1 degree C Accuraccy) I can read the temperature accurately with the sensor but it does vary with ambient.
I have tiny PCB tracks (minimum my manufacturer will let me have 0.127) to try to minimise heatsinking. Unfortunately insulating an 0402 from the PCB is nigh on impossible. However I can improve the thermal conductivity of the ambient compensation sensor to the PCB (to accurately measure the temperature of the PCB).
The offset of ambient is the final problem really. My thermistors have less than 1% variation.
The layout is somthing like below:
Object Thermal paste [ Real sensor ] ================================PCB [ambient sensor] Do you think I'm fighting a loosing battle?
Do you have room to route a slot on each side of the sensing thermistor? I think if you could do that, and add a patch of thermal insulation (foam tape) to the back side of the board (and move the ambient sensor outside of that thermal insulation) you could improve your accuracy by an order of magnitude or more.
If you add the second thermistor and calibrate the fudgefactor somehow, you should be able to improve accuracy by, I'm guessing, 5:1 or so. You'll have to do the thermistor curve thing to each thermistor independently, giving two temperatures, and then apply the fodge.
I've seen similar stuff done in poorly-designed ovenized crystal oscillators, like from S**.
John
Many thanks to both Johns, I'll look in to routing some of the PCB away. It is the fudge suggested by John Larkin I would prefer to add but electrically using perhaps a potential divider circuit but am having trouble getting my head around what I'm after. As the ambient increases the target sensor reads closer to the correct temperature so i need to further decrease the resistance of the target sensor using the ambient sensor. Many thanks again
David
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