"new" tempco sensors

[Rene Schmitz]

At 14:29 20.05.99 +0200, Martin Czech wrote:
>I always wondered what these KTY positive temperature coefficient
>things are...  now Rene Schmitz pointed us to www.conrad.de where he
>bought some other PT tempcos (quite expensive, order number 171778-62,
>5 Euro, +0.385%) (thank you, Rene!!)
>
>and I suddenly came across these devices:
>
>Philips: KTY84-xxx order number 183458-62
>Siemens: KTY10x    order number 183440-62
>                   order number 183407-62
>
>These guys are quite cheap: About 1.5 Euro.
>Philips specs the tempco to be +0.61%
>Siemens has no spec for that (!!!?), but an app note shows
>about +0.5%.
>
>They are not based an metal alloys, but on doped silicon, so they
>are not 100% linear, but cheap. I think the nonlinearity is
>not that bad at all (0C-100C). Other errors (thermal) may be much larger.
>
>The Philips part looks more interesting to me, it's speced and it's in a
>diode glass housing, like an 1n4148.
>
>Since these are positive tempco devices, they are used in the lower leg
(ground
> leg) for our purposes. The tempco value is too high, but that could be
easily
> changed by a series resistor. This way the tempco could be trimmed.
>
>I think these little things are quite interesting, I should have a look at
them.
>I need a new 2.2Ohm 25W resistor anyway... better order now.
>
>Rene, you mentioned experiments with the KT-120, I can't find them at
conrad any
>more (full text search). I guess these silicon things are all more or less
the
> same. What where your results?
>
>
>m.c.

Whoops that was a typo, of course they were KTY-81-120s. As you say, these
KTY-8x's are pretty much the same. 
I've build a VCO which uses this as tempco resistor, see
http://titan.cs.uni-bonn.de/~schmitz/vco3.html
for a detailed description. I have not done any precise temperature
analysis, since I lack a thermo-chamber. I put them into the feedback loop
of the opamp, because these have no inductive effects. 
These have a tempco of 7900ppm @ 25deg, which changes about 20ppm/K in the
neighborhood of 25deg. When scaling this down with a series resistor the
error also scales down. So far I'm quite satisfied with this VCO, but I've
found that it *detunes* slightly but the *scale* stays right, so I blame
the offset drift of the input summing opamp for that. If used in a divider
scheme (vs. inside the loop) this drift would be less of a problem. Maybe
I'll change the scheme. 
I was afraid at first if these devices are really ohmic, of if they have
some nonlinearities in the V/I dependance as well (they are made of Si!)
but this is not the case.


Bye 
 René





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