[sdiy] Current source for tempcos in small quantities

[René Schmitz]

Hi Martin et al.,

> Sorry to drop in, I think all this Farnell stuff is no wire wound 
> PTC, but some semiconductor stuff.

I doubt that. Certainly these aren't wire wound anymore, but there are
also non wound platinum resistors available. (Think of the sensors from
Sensor Nite at Conrad.)

> So you can go as well with Philips KTY8x  (x= 1...4).

Good point, I always wondered if I'm the only person who uses that.


> They are easier to obtain. And they are small, look like a 1N4148 
> diode.

Some of these come in a T092 case, which is very good to couple these to
the japanese dual transistors. I just mount them face to face with a
very thin layer of heatsink compound between them, and then wrap the
entire arrangement with a piece of heatshrink tube.

> The tempco is too high (8000ppm from my head), but this is a good 
> thing since it can always be made lower, but not higher.

The tempco is 7900ppm. So a series resistor of 1340 Ohms is needed for a
1k KTY-8x-1xx to put it to the desired 3350ppm. Of course you need to
scale up the other resistors apropriately to use these (x2.34 that is.) 
There is some information about the KTY-81 on my website. (Look at the 
expo tutorial.) In the temperature range of 20-40 degrees the resultant 
temperature behaviour is even better than for an ideal platinum resistor 
[!!!!], in the usual divider scheme. (There is a link to a comparison 
table at the bottom of that document. I think I also once posted that 
table here.)


Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159