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'Diodes as temperature sensors'
1997\05\16@150618 by Mike

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Anyone have much experience using ordinary 4cent diodes as temperature
sensors ?

I understand there is some variation between units but, if the device is
calibrated at the time of use - just how reproducable is the voltage vs
temperature characteristics over time ?

Rgds

Mike
Perth, Western Australia


Some say there is no magic but, all things begin with thought then it becomes
academic, then some poor slob works out a practical way to implement all that
theory, this is called Engineering - for most people another form of magic.
                                                                      Massen

1997\05\16@163912 by Bob Fehrenbach

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Mike <spam_OUTerazmusTakeThisOuTspamWANTREE.COM.AU> wrote:
>Anyone have much experience using ordinary 4cent diodes as temperature
>sensors ?

  I believe that Linear Technologies has an app note on this.  If
  memory serves, their circuit does not require calibration.  They
  drive the circuit with a square wave and measure delta Vbe which
  has well defined temperature properties.


--
Bob Fehrenbach    Wauwatosa, WI     .....bfehrenbKILLspamspam@spam@execpc.com

1997\05\16@182710 by Andy Kunz

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At 03:35 PM 5/16/97 -0500, you wrote:
>Mike <erazmusspamKILLspamWANTREE.COM.AU> wrote:
>>Anyone have much experience using ordinary 4cent diodes as temperature
>>sensors ?
>
>   I believe that Linear Technologies has an app note on this.  If
>   memory serves, their circuit does not require calibration.  They
>   drive the circuit with a square wave and measure delta Vbe which
>   has well defined temperature properties.

Analog Devices recommends use of diodes as temperature compensators for
certain micromachined strain guages (I think the new ADXL250??).

Andy

==================================================================
Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
         Hardware & Software for Industry & R/C Hobbies
       "Go fast, turn right, and keep the wet side down!"
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1997\05\17@215207 by Alex Bogdan

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Mike wrote:
>
> Anyone have much experience using ordinary 4cent diodes as temperature
> sensors ?

I don't think that diode is such a good idea for the temperature sensor.
I would use temperature dependent ceramic cap. It will be much more
reliable and easier to convert to some useful characteristic (frequency
for example).
I have been using PHILIPS ceramic caps for two years now exactly for this
purpose. Look in the catalogue and choose the curve you want. Ceramic cap
temperature/capacitance curve is not drifting over the time. (At least I
did not notice it)
Regards,
Alex

1997\05\20@143051 by Tim Crist

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    Mike,

       I have used the intrinsic body diodes in MOSFETs to measure
    the junction temperature during operation.  The technique is very
    similar to looking at the forward drop of your 4cent diodes.  If you
    are not looking for super accuracy, 2.2mv/C is the rule of thumb.
    If memory serves, this number will get you within 10% every time -
    you'll have to measure the initial drop at some given temp to start
    with.  My tests worked very well and were repeatable.

       1.  Run the FETs in circuit for the time desired.
       2.  Forward Bias the Body diode for 200us and read delta_V.
       3.  delta_V/.0022 = Temp rise

       Be careful not to put too much current through to cause self
    heating (or use a very short pulses).  The trick here is to make quick
    and accurate voltage measurements.  Otherwise the technique works
    fine.

    Regards,

    Tim Crist

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