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Wiring thermal probes (diode) to obtain an average?

Discussion in 'Electronic Basics' started by Toby Lawrence, Jan 11, 2006.

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  1. I'm trying to wire four thermal probes in a way that I can get an
    average temperature between them all, but I really can't figure out how
    I'd do that, exactly.

    For example, I stick all four probes into a big block of ice, and leave
    them in the freezer, until each probe's readout is stable within 0.01.
    (doesn't fluctuate more than that) If I wire them in parallel, and keep
    adding thermal probes, the value readout goes down, but it doesn't go
    down in a pattern and I can't derive any function to fit it. I would
    much prefer a factual way to average them as accuracy is important to me.

    If there is some alternative way I could wire the probes, or if you know
    the math required to get an average, please please let me know.

    Thanks in advance. :)
  2. Diode probe instruments, I think, provide a fixed current and measure
    the junction voltage that results. The derivative of the solved diode
    equation gives something like (8.617 uV/K * ln(1+I/Is)). So with an
    operating current (I) of, say 10uA, and using a guess of Is=1e-15, the
    slope would be something like 1.98 mV/Kelvin -- call it 2mV/K. That's
    what the instrument will be looking at, for changes. It will also
    need at least one calibration point.

    Paralleling junctions would firstly operate each junction at a current
    lower than the instrument expects, so the conversion of voltage to
    temperature would be misguided. A new table may be needed or, at
    least, a two-point calibration step. But placing junctions at
    different places, meaning that they are potentially at different
    temperatures, and paralleling them would force all their junction
    voltages to be the same and thus, to simply adjust the balance of the
    current branching through them. And what that would look like would
    require some analysis. I think it would roughly mean that:
    T1*ln(I1/Is1+1) = T2*ln(I2/Is2+1) = T3*ln(I3/Is2+1) = ...
    And that all the probes would adjust their relative I1, I2, etc., so
    as to balance that equation at their respective T1, T2, and so on,
    with their somewhat varying Is1, Is2, etc. But that probably wouldn't
    behave they way you want.

    If you placed them in series, you might get something closer to an
    average (a sum of the voltages, anyway), but then your instrument
    probably wouldn't be able to support the voltage compliances required
    and wouldn't really drive them properly. And in any case, its tables
    or math probably wouldn't apply, and again you'd be in trouble.

    I don't have any experience with this, so I'm just thinking off the
    top of my hobbyist head. Perhaps someone can be specific for you.

  3. Bob Masta

    Bob Masta Guest

    Since you mention that these probes are diodes, I assume
    that the readout is a current source; the resistance of a
    diode is a *very* linear function of temperature (negative
    slope), so by applying a constant current the readout just
    has to measure the voltage across a diode and adjust
    for a particular slope and offset. But the actual slope constant
    differs among diodes, so if you put them in parallel they won't all be
    seeing the same current. Putting them in series would be better, but
    then the readout would see the sum of the 4 voltage drops
    and would definitely not read right.

    So you either need to make your own current source and
    measuring circuit, or tweak with the one you have and hope
    it can handle the larger voltage range. Alternatively, you
    can always use 4 separate circuits and sum them together
    afterward, with a divide-by-4 scale factor thrown in.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator

  4. Diode probes are not passive components like resistors. Most of them require
    a well defined constant current through it so the voltage accross it can be
    measured to find out the temperature. That's what the electronics in the
    instrument do for you. If you wire some probes in series or parallel, the
    measured voltage will be out of its normal operating range and the result -
    if any - makes no sense. So to obtain your objective, the average
    temperature, you'll have to adapt some other measurement technics. As I have
    no idea of you application (and your budget), I can only shoot at random:

    - Find a resistor based instrument. Four NTC (or PTC) probes can be wired to
    look like one probe.

    - Use four independent instruments. You'll have to find a way to read them
    out and do the calculation. If you can read out the voltage for instance you
    can "calculate" by some opamps. If the instruments provide some serial
    output, you can use a PC to do the calculations for you.

    - Build a system that connects the probes to the instrument one at a time.
    Reed relays may do a good job for you. You'll still need a PC or a micro to
    control the relays and do the calculations.

    Next time provide some more information about the real problem and the
    equipment available.

    petrus bitbyter
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