[sdiy] Current source for tempcos in small quantities

[Czech Martin]

Sorry to drop in,
I think all this Farnell stuff is no wire wound PTC,
but some semiconductor stuff.
So you can go as well with Philips KTY8x  (x= 1...4).

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

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.

Some of these guys are unipolar (i think #4), so watch out!

These days some of these special items seem to vanish,
though there seem to be some manufacturers who 
live from bringing "obsolete" stuff back again,
like ON semiconductor.

Better buy a pile before they are completely gone.


m.c.



-----Original Message-----
From: Ian Fritz [mailto:ijfritz at earthlink.net]
Sent: Montag, 10. März 2003 04:38
To: harrybissell
Cc: Jozsef Baksay; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Current source for tempcos in small quantities


Hi Harry --

At 04:46 PM 3/9/2003, harrybissell wrote:
>I thought the Q81 were 3300 ppm/K ???

The info I have says 3500 ppm/K. There is some +/- tolerance on this, but I 
don't remember what. To get the best compensation I have needed to add a 
series resistance roughly consistent with the 3500 value.

>In the common use of the tempco (shunting the base of the expo tranny)
>will adding the non-tempco in series cause any problems with the value...
>or do you just change the series resistor to the base to match ?

You have to change the other leg of the voltage divider (whichever 
configuration) proportionately.  In other words, you have to readjust the 
tracking.

   Ian