[Table
of Contents]People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
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Here
is another of the Electronic Sticklers challenges from
Popular Electronics (see May
Electronic Sticklers). These are fairly basic
circuit analysis problems that often can be solved by inspection, but sometimes a pencil and paper are necessary.
Re-drawing the circuit in a different configuration to make the connections more obvious often helps when solving
total resistance, capacitance, inductance, etc., as in question #1. In this case, though, you need to be able to
recognize a common configuration to be able to simplify the circuit; otherwise, you'll be writing and solving mesh
equations. #2 has a simple answer and a more elaborate possibility. #3 and #4 are simple inspection problems.
See all articles from
Popular
Electronics.
Electronic Sticklers
These four thought-twisters are arranged in order of increasing difficulty.
1)
Harvey Matrix discovered this network covered with solder, in his junk box (to the right). Rather than trust his
ancient ohmmeter, he tried unsuccessfully to compute the resistance mathematically. Show Harvey you are slicker
and quicker by solving the problem in one minute.
--Dennis Wroblewski
2)
Joe Whatsit had a black box with only two terminals showing (to the left). To find out what was in the box, Joe
connected a 1 1/2-volt dry cell to the two terminals and noted the current flow. He then connected a second identical
dry cell in series with the first cell and repeated the experiment. The same current was noted. With this information,
Joe figured out what was in the box. Can you?
--David Borenstein
3)
Mr. Pennypincher, in order to save money on batteries for his portable radio, built this little voltage supply (to
the right) to substitute for the batteries. When he plugged in the unit, he was running a risk of a blown-out component.
Any idea why?
--Ronald Wilensky
4)
With the setup shown (to the left), a diode in series with a capacitor, Harold Tinkertoy applied 100 volts r.m.s.
across the circuit. Then he used his vacuum-tube voltmeter to measure the peak voltage across the diode. How much
did he measure? To make the problem easy, assume that a sine wave is applied to the circuit.
--Louis E.
Garner, Jr.
Answers to Electronic: Sticklers
1. Two ohms. Redraw the network schematic in the form of a bridge circuit. You will find that the
resistance values of the bridge legs result in a balanced bridge. Hence the 3-ohm resistor is an inactive component
and can be omitted from the circuit. All that remains are two series resistance circuits in parallel.
2.
A short circuit. The current remains the same since the two dry cells provide not only twice the voltage but also
twice the internal resistance. (Kirt note: It could also contain a constant current circuit)
3. Since there
is no surge resistor, the charging current of the capacitor would pull too much current through the rectifier. Without
the surge resistor (20 cents), there's a good chance the rectifier ($1.00) would burn out.
4. About 282
volts! How come? On one half-cycle, when the diode's plate is positive, the capacitor charges to peak line voltage...
or about 141 volts (1.41 multiplied by line voltage). On the next half cycle, the capacitor's voltage is in series
with the peak line voltage and thus adds to it... and 141 plus 141 equals 282! This arrangement, incidentally, is
basic to voltage-doubler power supply design.
If you know of a tricky Electronic Stickler, send it with
the solution to the editors of POPULAR ELECTRONICS. If it is accepted, we will send you a $5 check. Write each Stickler
you would like to submit on the back of a postcard. Submit as many postcards as you like but, please, just one Stickler
per postcard. Send to: POPULAR ELECTRONICS STICKLERS, One Park Ave., New York 16, N. Y. Sorry, but we will not be
able to return unused Sticklers.
Posted November 12, 2013