How to compute the R values for the Wye-Delta Transform: If you just read the open-circuit voltage at the "input" node and apply a voltage between VOUT and ground, you can easily see that R2/R3 = RC/RB, or R2 = R3 (RC/RB). If you ground both the VOUT and the ground, it's easy to see that the conductance from the input to ground is:
(1/RB + 1/RC) = 1/(R1 + R2||R3)
Substitute the first equation into the second equation to get:
RC = (R1R2 + R1R3 + R2R3)/R3
Similarly:
RB = (R1R2 + R1R3 + R2R3)/R2,
RA = (R1R2 + R1R3 + R2R3)/R1
Likewise, one can compute the reverse transform:
R3 = RARB /(RA + RB + RC),
R2 = RARC /(RA + RB + RC),
R1 = RBRC /(RA + RB + RC)
So now you too can paste a copy of these equations inside your resistor cabinet, and now you know why Tee networks won't give you something for nothing!
All for now. / Comments invited!
RAP / Robert A. Pease / Engineer
rap@galaxy.nsc.com—or:
Mail Stop D2597A
National Semiconductor
P.O. Box 58090
Santa Clara, CA 95052-8090
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Hi Bob:
I took a course in motor controls a few years ago, and I remember the instructor vaguely touching on the concept of an eddy brake as a method of inexpensive low-end speed control. Unfortunately now that I have a need for such an item, I can't find any information on it. Could you shine any light on this concept? Thanks.
Dan Slaughter
via e-mail
I don't think I have this in any book I know. Have you tried the Web? Or a good library? Sliding a piece (or rotating a disk) of aluminum through a strong magnetic field generates a lot of "drag." I'm sure this would work well with an electromagnet, as well as with a permanent magnet. Obviously you can't turn an electromagnet on or off very fast, but this will work, if clean friction is what you need. Have fun!!—RAP