Q&A
■ WITH RUSSELL KINCAID
I was saddened to hear of T.J. Byer’s death; I
have been in awe of his range of knowledge and
enjoyed his column. I am honored to be asked
to continue the column and hope to be as
interesting as he was. My experience has been as
an analog design engineer, although, I have
recently discovered microprocessors and will use
one when applicable and simple enough that I
can do it. I will no doubt make some mistakes, but
there are certainly readers that can set me straight.
Feedback is welcome. — Russ Kincaid
A Short Biography of Russell Kincaid:
WHAT’S UP:
Join us as we delve into the
basics of electronics as applied
✓A current limiting mystery.
✓Happy fish.
✓A game show quizzer.
✓Vespa flywheels.
Send all questions and comments to:
Q&A@nutsvolts.com
I became interested in radio while in high school. I saw an ad in Popular Science for a battery-less, tubeless radio and bought a crystal radio kit. A neighbor, who was a college student, helped me build a short
wave radio and I was hooked. I got a book from the library and built some radios of my own. I talked my
parents into sending me to the Commercial Radio Institute, in Baltimore, Maryland, but was drafted and
spent a year teaching basic electronics at the Infantry School, Fort Benning, Georgia. I took advantage of
the GIBillandwentto Rensselaer Polytechnic Institutein Troy, New York, gettingan MEEdegree. I worked
for 22+ years as an analog design engineer at Sanders Associates and subsequent assigns, and seven
years as Manager of hybrid electronics for Sprague Electric. I told myself that I would never retire, but
the demand for analog engineers is greatly diminished, and I found myself out of work at 59 years old, so
I retired. I have been retired for 18 years and will never get to the bottom of my to-do list.
CAUSE OF CURRENT
LIMIT?
QI am using a coil to
generate a magnetic field
and in turn, control the
torque in a clutch. The
current control uses an H-bridge to
apply a PWM duty cycle. My coil
resistance = 1.3 ohms, the inductance
= 4.1 mH and I expected to move
more than 10A through the coil at a
frequency of 5 kHz at 15 VDC. For
some reason, I can not get more than
8A to flow. I wonder if there’s some
power consumption in the PWM
switching semi-conductors?
Where can I measure the power
consumption in the PWM circuitry?
It seems to me the junction resistance
is just 1.3 ohms and I am supposed
26 May 2007
to be able to generate a current
15/1.3 = 11.5A.
— WR Hwang
AThe resistance that is limiting
you to eight amps is: R =
15/8 = 1.9 ohms. The coil is
1.3 ohms which leaves 0.6
ohms in the H-bridge, which is perfectly
reasonable. You need a better H-bridge
or more supply voltage.
BANDSTOP FILTER
QI need a reject filter for off-air
Channel 7. One of the area
DTV signals is on 9, and I
have a severe intermod
problem when I amplify it enough to
be useful.
I haven’t been able to find a
manufacturer — or individual — with a
sweep setup that sells or will build a
75 ohm, 170-184 MHz reject filter.
— David Schoepf,
Marianna, FL
AI designed a three-pole
bandstop filter to see what
it looked like: check out
Figure 1. The design is for
perfect inductors; the real thing
won’t be as good. Note the 1.36 pF
capacitors; the stray capacitance will
be significant and such a small
value can be made by twisting two
enameled wires together. Cut
the wires to trim the value. You
will need a sweep generator to
tune the filter. This equation can
be used to wind the inductors:
L = N2(RADIUS/2( 9*RADIUS +
10*LENGTH)). Dimensions are in
inches, inductance is in micro-
