Check out Melexis products, AFAICR they can measure upto 350 degrees
celcius, non-contact IR measurement.


On Mon, Apr 22, 2013 at 10:45 AM, KPL <kpl.listes@gmail.com> wrote:

> Could this (diode junction) be (cheapest | easiest) way to measure
> temperatures up to about 200C? Looks like PT100 and similar need about
> the same amount of analog circuitry to work, but those are more
> expensive than diodes.
>
> I want to add more advanced regulator to my laminator, so I could use
> it for PCB toner transfer, but could still use it for it's original
> function as well.
> Great precision is not required, but temperatures can reach close to
> 200 degrees celsium.
>
> I have no experience with any of these, as usually ds18b20 were good
> enough until now.
>
> >
> > Diode bandgap measurement  does most of what you want and cost can be
> > lowish if you work at it.
> >
> > Wikipedia covers basics:
> > http://en.wikipedia.org/wiki/Silicon_bandgap_temperature_sensor
> >
> > If you sequentially apply two currents to a silicon diode the
> > delta-voltage is a function of only temperature and the two currents
> > and a constant.
> > You can notionally swap sensors using any silicon diode and get the
> > same result without recalibration - but using the same type of diode
> > is better in practice.
> > This is a substantially more accurate and generally superior method to
> >  just measuring diode forward voltage drop.
> >
> > The method and the formula are not "hard" - just perhaps unexpected.
> >
> > The sensor is in theory ANY siliicon diode, swappable with out
> > calibration or recalibration,  but using say 1N4148 as standard will
> > improve ease of getting good results.
> >
> > Any diode material can be used if due allowances made
> > Si is good for up to about 200C with SiC being better above 200C
> >
> > Delta_Vdiode =3D KT/q * ln ( i2 / i1 )
> >
> > or
> >
> > T =3D Delta_Vdiode / (k * ln (i1/i2))
> >
> > ln may be annoying but is not intractable.
> >
> > Where
> > K =3D Boltzmann's constant
> > T =3D temperature in degrees K
> > q =3D electron charge
> > i1 =3D current 1
> > i2 =3D current 2
> > k =3D K/Q
> >
> >  > Accuracy is .5C  + or - .25 C
> >> Range is - 20C to 75C
> >> Long term stability
> >
> > Achievable, yes, yes.
> >
> >> simple interface such as  i2c, SPI , voltage , current , resistance
> .....
> >
> > Needs two switched precision currents and suitably accurate voltage
> > measurement.
> > OR two non precisely produced currents which can be precisely measured.
> >
> >> simpler is better
> >> Low price
> >
> > Sensor is a silicon diode. Cost is a diode for the sensor plus a
> > suitably accurate ADC (assuming digital) for voltage and two accurate
> > currents OR a means of accurately measuring currents.
> > eg imagine that you inject I1 then I2 into a series combination of D1
> > + R1 to ground.
> > Measure VR1 and then V(D1 + R1)
> > If R1 is stable and known then this allows measurement of 2 x current
> > and Vdiode with a single switched ADC input or two 'unswitched' ADC
> > inputs.
> > In this case current switching could be achieved with eg applicaton of
> > a dfigital high or low to a resistor network using relatively low
> > precision resistors.
> > With D and R in series the relative voltage drops of R and D can be
> > adjusted to best use ADC range.
> >
> >> no external parts
> >
> > Deep ends what you mean by "external".
> > Needs above parts. Can be integrated into whatever as required.
> >
> >> no calibration
> >
> > Yes. Neither additional or for sensor swap.
> >
> >> Can be slow ..... 30 second update   .... can be as large as golf ball
> >
> > Easily met.
> >
> >
> >   Russell
> > --
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>
>
> --
> KPL
> --
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