Trying to make a transistor cancel another transistor

[dorke]

Hop,I'm not blaming ,just correcting.

Chris,I did catch it on #51 but the difference is hop did say:
"more or less linear over the temperature range in question",it can pass... more or less
while in #54 you said "Thermistors are linear devices",that doesn't pass.

Hop,as usual, is contributing a lot of interesting facts about history of electronics.

Let me add this useful link about temp sensors and measurements.

Nowadays the semiconductor band-gap devices(in the -55 to +150C range) are the easiest to use ,but may not be the cheapest way to go.
They are truly linear, and some have direct uC standard interfaces...even a "pure programmer" can connect and use them.

For pure analog "interface",
I like the 3 pin,sub 1$ devices, like the LM35 and LMT85 etc.

 

[CDRIVE]

Thanks for the PDF. It's a lot of reading but quite informative. I also peeked at the LMT85 datasheet. Wow! The temp curve from -50 degC to 150 degC is damn near a straight line!

Chris

 

[hevans1944]

Yes. @dorke, thank you for the link. That goes in my permanent reference file for temperature measuring. It appears to be quite complete and up to date. What was this extracted from? I guess I can Google the authors... Probably in the text book they mention non-contact temperature sensing too, using radiation detectors. That's a whole 'nother can of worms...

Unfortunately this thread has strayed far away from the original topic and into territory the newbie can probably not comprehend at this time. I have been involved with precision temperature measurements since... well a long time, seems like almost forever, but it never hurts to get refreshed on what the current state-of-the-art is.

We used germanium semiconductor resistance temperature detectors to measure liquid nitrogen temperatures for superconducting thin films experiments at around 77 K IIRC. These are excited with constant-current supplies and the voltage drop across the sensor is proportional to temperature. I have no idea what the linearity is, but these are used all the way down close to 0 K with liquid helium cooling by the "Big Boys" at the National Superconducting Cyclotron Accelerator Lab (NSCL) in Michigan.

I was fortunate to tour that huge facility as part of an annual SNEAP (Symposium of North Eastern Accelerator Personnel) meeting a few years ago. Fascinating people those SNEAPers, all very bright people with a penchant for hands-on accelerator operation and maintenance. Also amazing was getting to walk inside a multi-story sulfur hexafluoride (SF6) tank housing a giant tandem accelerator, the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratories, also part of tour during a SNEAP annual meeting in Oak Ridge, TN. Unfortunately, when we were last there, the facility was in danger of being de-commissioned for lack of funding by Congress. It was finally closed in April 2012 and much of its work moved to the NSCL facility in Michigan. Below is a small image of the tandem accelerator as viewed from inside the SF6 tank. Note the diminutive human for size comparison. A high-resolution exterior view is here. The view of the surrounding valley is awesome from atop the concrete structure.

It is almost impossible to imagine what it costs to initially fill that tank with pressurized SF6 gas. Of course they pumped it out and re-used it whenever the tank was opened for maintenance, as it was that photo. Thousands of tons of SF6 are used worldwide for insulating high-voltage power transformers, but it is a "greenhouse gas" some 29,000 times more potent than CO2... so every effort is made to prevent it from entering the atmosphere to contribute to global climate change... still, the stuff is damned expensive. The only high-voltage insulating gas alternative that I know of is a mixture of CO2 and N2 which doesn't even come close to SF6 in insulating performance. Oh, well, I'm outta that game now except for a few low-voltage "experiments".

 

[dorke]

Thanks Hop,
Interesting as always.
The hole book " Practical Design Techniques for Sensor Signal Conditioning" can be found here
,it is a 1999 book ,not up to date, but still good.
the "Analog-Devices Library" has other very good books as well,here.
Note the 60s Ray Stata op. amp. articles.

 

[hevans1944]

Thanks again, @dorke! Good links to lots of free PDF downloads. I LOVE Analog Devices!