High temperature diode?

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chuckey

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I am attempting to fault find a Maverick electronic thermometer model #732. the problem is that the temperature probes do not last very long ~200 hours. I took apart one which had an unstable resistance ( .1 - <10 Mohms) and the active device looks exactly like a miniature diode. Glass envelope about 3mm long by 1.5 mm diameter. It had the characteristic red portions where the metal to glass seals were and a very narrow band between them, .5mm?. It had iron or iron plated leads - they had a little rust on them.
So my question is "as meat can be cooked at 220 degs C, does anyone know of a diode that can have a useful life at this temperature or more". Or even if its not a diode, what could it be?
Frank
 

It could be a silicon PTC like KTY83 which comes in the same diode glass package. It should be quite easy to decide if it's a diode or a PTC by using a multimeter.

Although it's beyond specified temperature range, both diode and PTC can probably work at 220 degree C. I fear that the package might be not leakproof enough to withstand steam pressure. An industry standard thermocouple or Pt100 in a hermetical stainless steel capillary tube would be better suited.
 

Maybe this why the sensors did not last long. The sensors are glass encapsulated in the SOD68 (DO-34) package

KTY83 is rated to 175'C http://pdf.datasheetcatalog.com/datasheet/philips/KTY83-1SERIES_4.pdf
KTY84 is rated to 300'C http://pdf.datasheetcatalog.com/datasheet2/e/0l2lc3p1dl8e5dgghsfh2oee43py.pdf

Philips sold semi. business to NXP with no change in limits **broken link removed**

Bias current is critical @ 2mA


You can buy the KTY84/130 at your favourite stocking disti.
the /130 is 3% tolerance while the /150 is 5% tolerance.
. . . . . . . +-10.9@220'C . . . . . . .+-15.2@200'C

But I think the biggest error is the my heritage united kingdom style of cuisine cooking meat at 220'C, where a longer time at 170'C is preferred.
 

Not sure why a glass body diode woild fail at low current,
may be more about the temp cycles than the temperature.
Glass doesn't much like cycling. Hi-Rel packages with glass
seals or glass eyelets are always a hurt waiting to happen.

A metal can device (maybe a simple BJT, pick E-B or C-B
and tie off the other, to B) might be more reliable. You will
not like the pricing for anything JAN or MIL-B or QML-Q but
it ought to last.

There is no intrinsic reason why a silicon diode should fail
that soon at 200C; I ran an A-D converter at 300C for
about a year, before the test oven burned up. I'd bet the
problem is with the packaging technology.
 


Without a forensic analysis of Philips's design or a query to NXP, who can say except there is a distinct difference in the specs for the KTY83 and the KTY84 guaranteed specs. Perhaps the CTE matching from passivation materials contribute to the reliability differences. The glass seal appears to be the same.

Indeed, the current sensor profile is odd, but who should doubt it?

These are your options.
 

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