dave@onfcanim.UUCP (Dave Martindale) (03/21/89)
In article <2308@lll-lcc.UUCP> rzh@lll-lcc.UUCP (Roger Hanscom) writes: >We've all seen single >value devices that have a common pin with resistance between >that pin and all the others equal to x ohms. Just recently, >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 'common' pin and the pin >at the other end of the package seem different than the others, >but they are not like regular sips. How are they configured? Pin one of an ordinary SIP resistor can be considered a common "bus", and all other pins have a particular value of resistor between that pin and the "bus". 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. These are often used for TTL bus terminators - the SIP is connected to +5V and GND. 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.
cdl@mplvax.EDU (Carl Lowenstein) (03/28/89)
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 {decvax|ucbvax} !ucsd!mplvax!cdl cdl@mplvax.ucsd.edu