Web Site Says Information Not Found in RDH 4. (Ahem).
- From: Bret Ludwig <bretldwig@xxxxxxxxx>
- Date: Sat, 13 Oct 2007 15:55:11 -0700
Amusing:
http://www.tone-lizard.com/The_Power_Transformer.htm
CLASSIC ARTICLES
Just about everywhere there's AC you'll find a power transformer
stepping the voltage up or down. It's a simple job to wind your own
transformer on a large carriage bolt to see and get the feel of
transformer action. To keep our experiment shock free, we use a 6.3
volt filament transformer as a source of operating voltage.
For the primary, wind 500 turns of No. 28 enameled wire in even layers
as shown. The beginning lead of the secondary, which is wound on top
of the primary, is called the com, or common lead. Wind 250 turns,
bring out a 6-inch loop for tap 1, wind 250 more turns, bring out a
loop for tap 2, then wind 500 more turns for tap 3. Connect the
primary leads to the secondary of the filament transformer and you're
ready to go.
You can make measurements with a 0-10VAC voltmeter (or a 0-1mA DC
milliameter by adding the diode and resistor as shown). Plug in the
filament transformer and measure the primary voltage at A and B. It
should be about 6 volts. Now read the secondary voltage across the com
and tap 1 leads. It should be about 2 to 3 volts. This shows that
voltage has been stepped down. And the amount of step down is equal to
the ratio of primary and secondary turns. Since the secondary we used
has half the number of primary turns, the voltage is half.
Move the meter to tap 2 and the voltage will about the same as the
primary voltage because the turns ratio is one to one. Connect the
meter to tap 3 and the voltage will be twice the primary voltage
because the secondary has twice the number of primary turns. Secondary
voltages are not always in proportion to the ratio of turns because of
poor magnetic coupling between the primary and the secondary.
Transformer action takes place because current changes in the primary
create an expanding and collapsing magnetic field. As lines of this
field cut across the secondary windings, they induce a current in
them. The carriage bolt, however, is not efficient enough to transfer
all the energy. To improve the coupling, lay a screwdriver flat
against the secondary. Now angle the screwdriver so its tip touches
one end of the bolt. The voltage should increase because of the
improved coupling. Another transformer loss occurs because the
magnetic field induces current in the bolt and this just produces
heat. Commercial transformer cores are made of iron laminations which
provide a good path for the magnetic lines of force but resist current
flow.
The transformer's turns ratio also changes current but in the opposite
way. If a 6-volt primary is rated at 1 amp, the available current in a
secondary with twice the number of turns will be .5A. Stepping down
the voltage will increase the current and vice versa.
-H.B. Morris-
Electronics Illustrated
Hopefully you get from this article that different 'grades' of
carriage bolts yield a more efficient magnetic coupling, and iron (as
used in commercial transformers) gives even better efficiency. With
your windings right on top of each other (separated only by the
lacquer insulation), the 'bobbin issue' becomes moot. Try interleaving
your windings if you want, and find out first hand if the efficiency
improves (it doesn't). What makes transformer 'A' any different/better
than transformer 'B' are parameters that aren't taught in college
electronics courses. You won't find any of this information spelled
out clearly in The Radiotron Designer's Handbook either, which is
probably why no tube guru seems to know it.
.
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