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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