Tempco = thermistor?

[egasimus]

Just a newbie's question - is the 'tempco' used in VCO designs the same as a thermistor?

[CJ Miller]

Technically, I believe the names are synonymous. But as products on the shelf there are differences. What we need to know besides how many kohms resistance is whether it compensates positive or negative, and by how much. This is usually shown as +/- ppm. And what sort of curve/linearity it has.

Rule of thumb: the "classic" part was always the Tel Labs Q81, in either 1k or 2k depending how it was used. I have bought C-2AQ tempcos and those seemed a suitable replacement. Rene Schmitz actually has played with using cheap commonly available thermistors in VCOs, his work is worth checking out.

Ian Fritz http://home.comcast.net/~ijfritz/sy_cir7.htm
Rene Schmitz http://www.uni-bonn.de/~uzs159/expo_tutorial/index.html

[CJ Miller]

One invaluable resource you should be aware of is the long-standing, informative SynthDIY mailing list. You can search on nearly any topic with this link. I can get lost on it for days! http://search.retrosynth.com/synth-diy/

Here is an explanation from Grant "Wiard" Richter. Hopefully it is not trouble if I quote it here...

To: keithw, synth-diy@mailhost.bpa.nl,
From: "Grant Richter
cc:
Subject: Re: Tempcos
Date: Thu, 4 Nov 1999 15:49:36 -0600

Thermistors generally have large and non-linear resistance changes
with temperature, perhaps as large as 2% per degree C.

A temperature compensating resistor (tempco) has a very small and
linear, temperature dependent resistance of +3500 parts per million (ppm)
or 0.0035% per degree C.

The actual math works out to about +3200 ppm but varies depending
on the transistor by perhaps 200 ppm on either side.
Some people prefer +3000 ppm instead of +3500 which is dependent
on what exact silicon you are using for the exponential converter pair.

You can custom order resistors of this type in various compensation
ranges in either positive or negative coefficients and steps of around 500
ppm.
They are also used to compensate analog sensors over long wire lengths.
For instance if you have 500 feet of wire that changes resistance
a +3000 ppm, if you put a tempco with a negative coefficient of -3000
ppm in series with it, the resistance will remain more constant over
ambient temperature.


----------
> From: Keith Winstanley <keithw>
> To: synth-diy@mailhost.bpa.nl
> Subject: Tempcos
> Date: Thursday, November 04, 1999 2:00 PM
>
> I am sorry if I am being a bit thick here but I am assuming tempco
resitors
> the same thing as thermistors?
> If not could someone point me at one in the CPC, Rapid, Farnell, RS or
Maplin
> catalogue.I cannot find a referance to "Tempco's" anywhere
>
> ta..
>
> ....Keith Winstanley(filling small brain up)

[Dan Lavin]

Some of the manufacturers selling these devices will call them thermistors. Tellabs never did to my knowledge...it wasn't until probably the 90's when some of the Asian suppliers started marketing them as thermistors. Some do and some don't.

Traditionally, thermistors just change resistance with temperature, but there are all different kinds. First, they divide by Negative temperature coefficient devices and positive temperature coefficient devices. PTC's further break down into switching types and linear types. Linear types then could break down into silicon-based devices which have slopes of +7000-8200ppm and tempcos (as people here like to call them)....which were traditionally based on metal but now come in thick film and probably other technologies.

The main point is, regardless of what you call them, you need to find devices that are running in the +3000 to +3500ppm/degree C range

[egasimus]

That's great info, folks I've found a source of 2k thermistors yesterday, but I have no data about them - is there any simple way to measure the ppm?

[andrewF]

don't click on the above links

[Uncle Krunkus]