I'm trying to build a Resistance Thermometer that varies current/voltage output when heated. It has to be DYI, I don't want to buy anything done, only something like a light bulb, and this has to be easily changed, as the ideal temperature range I want to provide to it is 0-100 oC. I understand all resistors change their resistance with temperature, but I'm looking for something that changes it a lot more than the regular hardware store resistor.
Electronic – What easily acquirable material changes its resistance when heated
resistanceresistorstemperature
Related Solutions
1) Change R8/P1 to get the voltage range you want for the temperatures you want to measure.
2) Set P2 higher to match the values to the range of your ADC.
3) Set sampling time no higher than X times the time constant of your system including sensors. Find a good number for X depending on the requirements of the system. You may however want to average a number of rapid samplings to reduce noise/add bits.
4) A combination of lookup table and interpolation usually works well. Build an excel sheet to show the error of the approximation - massage it until it works for your application.
The base emitter (diode) junction is forward biased just like the two diodes you added and so, in effect, you get 3 lots of variations in junction voltage (for a given current) with temperature. The answer is no, I'm afraid not. Here's how a typical diode alters its voltage, for a given current, against temperature AND, the same is true of the base-emitter junction: -
Three different operating currents produce surprisingly similar slopes that tell you that a diode's forward voltage drop largely reduces at 2mV for a one degC rise in temperature. Resistors don't do this of course and you also have to consider that a diode has got a very low dynamic resistance once biased at some arbitrary operating point of a few hundred micro-amps upwards. This dynamic resistance is lower than a typical emitter resistor: -
Look at the right hand portion of the diagram - I've drawn two horizontal lines at 10mA and 15mA with the corresponding forward voltage drops. The difference (deltas) allows you to calculate the dynamic resistance = 0.1 volts / 5mA = 20 ohms i.e. probably less than the emitter resistor you might choose BUT, it changes with current so you get more gain than you bargained for (gain harder to define) and high signal non-linearity (distortion).
Setting the operating point of the collector at about half the supply voltage is useful to be able to obtain the maximum swing of signal (in terms of Vp-p) at the collector i.e. one side of the output signal doesn't clip much earlier than the other. There are subtleties here but that's the basic rule to maximize output amplitude and no, neither does this affect temperature stability.
Either use negative feedback (with care) or use an emitter resistor to lower the gain of the common emitter amplifier.
Best Answer
You are looking for the temperature coefficient of resistivity. Iron is pretty high, I just looked up 0.0147 at 500C. Copper, in my table, is 0.0042
I use iron wire for a high current load resistor, and it is annoying how sensitive it is to heating.
Here is a link explaining it all.