possible tempco with KTY81

[Rene Schmitz]

Hi all!

At 07:58 17.11.99 -0700, Ian Fritz wrote:
>UNLESS R(T0)=C1*T0, which I believe is pretty close for the wirewound
>Q81. (Terry Michaels showed some results from a Q81 spec sheet once, but
>I don't have them any more.) Different kinds of tempco's would have to
>be checked separately.

I think this is correct. 

I've done some calculations comparing an ideal tempco resistor and 
the KTY81 + series 1350 Ohm. I've chosen the Ohms/K factor for the wirewound 
so that both devices have equal resistance at 25 deg.

T    R1(KTY+1.35k)       Relative error in percent
			R2=C*T
-55	1825		1.712,13 	-4,8236	
-50	1850		1.751,37 	-4,2150	
-40	1902		1.829,85 	-3,0832	
-30	1959		1.908,34 	-2,1650	
-20	2019		1.986,82 	-1,3751	
-10	2083		2.065,31 	-0,7562	
0	2152		2.143,79 	-0,3509	
10	2224		2.222,27 	-0,0738	
20	2300		2.300,76 	 0,0324	
25	2340		2.340,00 	 0,0000	
30	2379		2.379,24 	 0,0103	
40	2458		2.457,73 	-0,0117	
50	2542		2.536,21 	-0,2474	
60	2628		2.614,69 	-0,5686	
70	2719		2.693,18 	-1,1035	
80	2812		2.771,66 	-1,7238	
90	2909		2.850,15 	-2,5151	
100	3009		2.928,63 	-3,4346	
110	3112		3.007,11 	-4,4823	
120	3217		3.085,60 	-5,6155	
125	3269		3.124,84 	-6,1607	
130	3320		3.164,08 	-6,6632	
140	3415		3.242,57 	-7,3690	
150	3495		3.321,05 	-7,4338	

C was chosen to be 2340 ohms/298.15 K (as for a 2.34K TC)
The value for R1 is the value for typical R of the KTY81-1xx from the 
datasheet plus a series resistor of 1340 ohms.
The relative error calculates to ((R2-R1)/2340)*100

Now what we can see here is that the KTY+R combination behaves pretty much
like the wirewound within resonable limits. (-10..60 degrees, less than 1%
error.) This shows that the KTY arrangement will pretty much work like the
wirewound, within that range. Assumed you never leave that range it will
behave like a wirewound resistor.
I think if you'd take your measurements only within that range and you'd
fit a line into the values it will also go thru the origin of your R vs. T
graph. It is not required that the values outside the ususal range fit on
that straight line.

I think the -10..60 degrees range is even more than we actually require.
The 1% error 
is 1% error of the total compensation (i.e. 33ppm!). This means the drift
will be compensated to >99% within that range. So we get >99% of the
performance for 10% of the price of a Pt resistor! 

That remaining error probably gets swamped by other factors, like opamp
drift, capacitor drift, mismatch of temperatures (which also applies to
Q81!)...

What do you think?

Bye
 René







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