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Here are a few brain exercises for starting a new week. They appeared in the 1964 issue of Radio-Electronics magazine in the "What's Your EQ?" feature (EQ = Electronics Quotient). E.D. Clark created over the span of a many years based on suggestions from readers. It's easy to over-think the first one, but the provided answer is more of an Occam's razor approach. The Black Box challenges can be difficult, partly because IMO the problem statement is not always clearly presented regarding what is an is not permitted as part of the solution. Here, for instance, it begins "Voltmeter tests read zero between any two terminals, so an ohmmeter (VTVM type) is connected to measure resistance." Since sometimes the generic term "voltmeter" is used in reference to a multimeter, the shift in the sentence from volts to ohms can be confusing. The zero is volts. Note that only semiconductors (diodes, transistors, etc.) are ruled out as part of the solution, not electromechanical devices. For the last one, be sure to note the restriction on frequency response.

Conducted by E. D. Clark

Three puzzlers for the students, theoretician and practical man. Simple? Double-check your answers before you say you've solved them. If you have an interesting or unusual puzzle (with an answer) send it to us. We will pay \$10 for each one accepted. We're especially interested in service stinkers or engineering stumpers on actual electronic equipment. We get so many letters we can't answer individual ones, but we'll print the more interesting solutions - ones the original authors never thought of.

Write EQ Editor, Radio-Electronics, 154 West 14th Street, New York, N. Y. 10011.

Answers to this month's puzzle are [at the bottom].

Noise!

We needed a clean, 6-volt 60-cycle test signal in an industrial electronic circuit. The old dodge of using the reactive drop of a capacitor to reduce the 117-volt line voltage to 6 was tried. This method uses a capacitor instead of the heat-producing resistors or expensive transformer normally used. The resistor and capacitor form a voltage divider, the reactance providing the required drop.

This technique was tried on a slightly noisy power line. The 117-volt waveform looked good on the scope, but the output waveform was terrible! Why? - Donald E. Lancaster

Another Black Box

Voltmeter tests read zero between any two terminals, so an ohmmeter (vtvm type) is connected to measure resistance. Between A and C, infinite resistance is indicated. Between A-B or C-B, the meter indicates 14 ohms on R x 1 range, 140 ohms on R x 10 range, 14K ohms on R x 1K range, and 135K ohms on R x 10K range. On the highest range (R x 1 meg), the indicated resistance is 6 megohms. What do we have here that seems to change resistance? No semiconductors are involved. - Mivko Vozniak

Input and Output

The attenuator in the diagram is designed so that the ratio of the output voltage to input voltage is independent of the frequency of the input. What restriction does this place on the values of the components? - J. A. Chambers

Quizzes from vintage electronics magazines such as Popular Electronics, Electronics-World, QST, and Radio News were published over the years - some really simple and others not so simple. Robert P. Balin created most of the quizzes for Popular Electronics. This is a listing of all I have posted thus far.

This month's puzzles are on page 48

Noise!

A capacitor's reactance decreases with increasing frequency. The high-frequency noise and harmonics on the power line (almost invisible on the 117-volt input) saw a much smaller reactance than the 60-cycle ac did, and passed pretty much unimpeded. This "amplified" the noise with respect to the ac waveform. It is important that a clean input waveform be always used in this type of circuit, or else the output will be noisy.

Another Black Box

The box contains a running vibrator with B connected to the moving contact while A and C are connected to the stationary contacts. As A-B and C-B are never closed simultaneously, infinite resistance is indicated between A-C. An apparent resistance is indicated between A-B and C-B due to the average time per cycle that the contacts are closed.

The contacts act as an interrupter or chopper which, along with the ohmmeter internal battery, produce a pulsed input to meter. The meter deflects to approximately the same point between half scale and infinity on any ohmmeter range with the exception of the highest, where leakage and the comparatively long time constant of the meter-input circuit become factors.

Input and Output

First, where frequency is zero, the capacitive reactances of C1 and C2 become infinite, and the circuit reduces essentially to two resistors with the attenuation factor of (R1 + R2)/R2.

At the opposite extreme, where the frequency approaches infinity, the circuit reduces essentially to two capacitors with the attenuation factor of:

The circuit will be independent of frequency if and only if the results at the two extremes form the relationship:

R2C1 + R2C2 = R1C1 + R2C1.

The R1C1 terms appear on both sides of the equals sign and drop out of the equation, so the attenuator meets the requirement if the component values comply with the relationship: R1C1 = R2C2

Posted March 4, 2024