MAILBAG
Dear Russell,
In your column in the September 2008 issue
(pages 26 and 27), you gave an incorrect explanation
of the operation of transformers (and by implication,
inductors). Your explanation is a widespread
misconception about Faraday's Law. Faraday's Law is
based on the phenomenon that a time-varying magnetic
field produces an electric field. The EMF induced in an
inductor or the secondary winding of a transformer is
due to this electric field, not the magnetic field. This is
proved by examining the operation of a toroidal
transformer. Because of the essentially total cancellation
of the magnetic field external to the primary winding
due to its geometry, if the secondary winding is wound
on top of the primary winding, it is not even in the
magnetic field of the primary winding. Yet, the
replace a 100 watt bulb for about $100, which is cheaper
than building one yourself.
I use 40 watt fluorescent tubes in my house; two of
them give 10X the illumination of a 100 watt bulb, so I
think that is the best solution although perhaps not the
most efficient.
A CYCLING CIRCUIT
QI urgently need a circuit to cycle four pairs
of thermocouple signals for a four cylinder
air-cooled engine (CHT&EGT) at the cycle rate
of 30 to 40 sec for the four cylinders. As each
cylinder cycles on, I need it to also indicate the cylinder
1-2- 3-4 on a seven segment LED. A selective hold switch
would also be helpful for extended viewing of any
particular cylinder. The power source is 12 VDC.
The circuit doesn't need any signal conversion. Also,
there isn't any common ground. This circuit would replace
having to manually switch from cylinder to cylinder.
— Art Schwedler
AI remember answering this question about a year
ago, but can't find it in any issue. Anyway, I got
feedback that I could have done it simpler, so I
will try to do that this time.
There are eight thermocouples (four cylinder and
four exhaust) which are to be read in sequence: cyl1,
exh1, cyl2, exh2, etc. You evidently already have the
thermocouple circuit and display; all you need is an
automatic switching circuit.
In Figure 7, I am using a Fluke model 80TK
thermocouple interface but if you are using an
instrumentation amplifier and cold junction compensation,
you can use that instead. The input impedance of the 80TK
is high enough that the resistance of the analog switch will
not cause significant errors. I don't know much about
thermocouples, but I believe that the negative lead has to
QUESTIONS & ANSWERS
toroidal transformer works.
— Michael S. La Moreaux
Thanks for giving me the opportunity to correct any
misconception. You are correct that the flux does not have
to "cut" the turns of the secondary, as I may have said, but
the induced voltage is due to the time varying magnetic
field. If it makes sense to you that the electromagnetic field
extends beyond the magnetic flux field and thereby induces
the secondary voltage, I won't argue with you. But, it makes
more sense to me to think in terms of flux coupling the two
coils of a transformer. Faraday's Law says that an EMF
cannot exist without a time varying magnetic field, so it
really doesn't matter which you assume to be the driving
force, as long as you are consistent. I know how (in principle)
to calculate the induced voltage from a time varying magnetic
field; I don't know how to calculate the induced voltage
from an EMF without going through the magnetic flux.
have the same interface as the positive. That means eight
connections on the PCB; same as the positive. The
schematic shows CD4051, one of eight multiplexers
because it is available in thruhole plastic DIP; but
74LV4051 is a better device, available in surface-mount
TSSOP. The program for the PIC16F627A controller is
Figure 8. As usual, I can send you a programmed
PIC16F627A for $5 if you don't have access to a compiler
and programmer.
NV
February 2009 31