Resistor TCR in parallel and series

[nukie]

I am trying to make a 102K 5PPM resistor by soldering a 100K+2K 5PPM in series. Is this a feasible method? Does the TCR increase when resistors are connected in series?

I searched high and low, all I can find on the net is this formula no.1
http://rsi.aip.org/resource/1/rsinak/v79/i12/p123304_s1?view=fulltext&display=print&bypassSSO=1

But then it doesn't really help because I am a maths idiot and it isn't really what I am after. Can someone please point me out in the right direction?

[IanB]

If the TCR of each of the resistors is the same, then when you connect the resistors in series the TCR of the combination will be the same as any resistor individually. You can visualize this by imagining the 100k resistor to be made up of 50 identical 2k resistors in series (divide the 100k track into 50 equal segments). Since the TCR is a property of the track width and not of the track length, then any length of track will have the same TCR including a length of 102k made from 51 equal segments.

If the TCR of each of the resistors is different, then the situation is more complicated and you will have to use the formulas given in the link you referenced. If you do not understand the math then try to find someone who does. Sometime mathematics and formulas are the only answer to certain questions. (I'm sorry but I do not have time to go into it here as it is well after midnight for me, but maybe someone else can assist.)

[nukie]

Thanks heaps Ian, that's what I need to know. I was worried if there's a pitfall for something simple like that but TCR is new to me.

[ejeffrey]

The other thing to remember is that if you have one resistor much bigger than the other, the TCR will be mostly determined by the large resistor.  This is simply because 100 PPM of a 100 k resistor is a lot more than 100 PPM of a 1 K resistor.

This is useful because it means you can use large fixed resistors with low TCR in series with a small trimpot with a larger TCR and still get an accurately trimmable resistor with a low TCR.