🚀 go-pugleaf

In article <17813@onfcanim.UUCP> dave@onfcanim.UUCP (Dave Martindale) writes:
>In article <2308@lll-lcc.UUCP> rzh@lll-lcc.UUCP (Roger Hanscom) writes:
>>I've come across some that are identified as 220/330 ohm.  They
>>look just like the others, but I can't find any two pins that
>>show resistance of 220 or 330.  
>
>The ones you have should use pin 1 and the one at the other end as
>two separate busses.  Each of the in-between pins should have 
>a 220-ohm resistor between it and one bus, and 330 between it and
>the other bus.  
>
>If you can't measure either 220 or 330 between an end pin and
>any inner pin, perhaps the SIP is defective or marked wrong.

After a few days, I can't resist answering.  This is much like the puzzle
problems we used to get when I was studying electrical network analysis
back in the early 50's.

Call the values of resistor R1 and R2, assume that there are N of them.

Then the resistance from end to end is the parallel combination of N
series combinations, and is equal to (R1 + R2)/N

The resistance from an inner pin to one end is R2 in parallel with
(R1 in series with (N-1) parallel (R1 + R2)'s).  Interchange R1 and R2
for the other end.

The resistance from one inner pin to another inner pin is trickier, until
you realize that the two end pins are at the same potential by symmetry.
So the answer is twice the parallel combination of R1 and R2.

Going to my junk drawer, I find a resistor pack (R1=220, R2=330, N=12),
and get the following results:
	end to end:	calculated 45.8, measured 46.5
	inner to hi end: 139.3, 139.0
	inner to lo end: 148.5, 148.6
	inner to inner:  264, 270 (Ohmmeter resolution 2 digits here)

So don't be surprised that you can't measure anything that corresponds
directly to the markings on the device.
-- 
	carl lowenstein		marine physical lab	u.c. san diego
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