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"What's Your EQ?" (Electronics Quotient, a la IQ - Intelligence Quotient) was a feature run in Radio−Electronics magazine for a couple years in the early 1960's. It was a quiz for challenging your knowledge of basic electronic circuit functions. Most of them are relatively simple exercises in voltage division, resistor-capacitor (RC) and resistor-indictor (RL) time constants, resonant frequency, and transformer input/output ratios (voltage and/or current and/or power). Sometimes, the first step in solving the poser is to rearrange the schematic into a more familiar configuration. Also, read the wording carefully to determine whether parts of the circuit can be ignored. Answers are at the bottom of the page. Note that I do not agree with the magazine's answer for the "How Much Voltage?" circuit. My solution is presented below with the rest of the answers.

Time constant

Suppose you have two 1-megohm resistors and a 1-μf 50-vdc capacitor. Using a 100-vdc supply voltage, how can you connect these resistors to provide a time constant of 0.5 second, without exceeding the 50-volt rating of the capacitor? - Earl H. Rogers

Series Resistor Circuit

In the circuit shown to the right, the vtvm reads 24 volts. A 100-ohm resistor paralleled with R1 raises the voltmeter reading to 40 volts. The same resistor paralleled with R2 reduces the voltmeter reading to 20 volts. What are the ohmages of resistors R1 and R2 and what is voltage E?

- L. B. Hedge

A Problem of Resonance

Inductance L and capacitor C in the circuit to the left form a series-resonant circuit used in a radio command system. The L-C combination is driven by a variable frequency voltage source having a source resistance R and a peak output of 1 volt. What is the minimum permissible capacitor working voltage in such a circuit?

- Phil Cutler

How Much Voltage?

What is the steady-state DC input voltage, Ein, in the circuit to the right?

- E. R. Welsh

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.

Time Constant

The maximum voltage the capacitor sees is the steady-state de voltage of 50 across R2. Using Thévenin's theorem, the resistance that the capacitor sees is between points A and B, looking back into the circuit with the source shorted, or the equivalent of R1 and R2 in parallel, or 500,000. Note the circuit is broken at points A and B when determining the Thévenin's equivalent resistance.

Series Resistor Circuit

The solution is shown in image to the left.

That Resonant Problem

The capacitor voltage is maximum at resonance and equal to Q times the input voltage, E. The resonant frequency is 0.159/√LC = 159 cycles. The inductive reactance at resonance is 2πfL = 1,000 ohms. The circuit Q at resonance is Qr = XL/R = 100. Therefore the peak voltage the capacitor must withstand is QE = 100 volts.

How Much Voltage?

1. E across the two capacitors 0.9Ein - 0.6Ein

2. E across the 1-μf capacitor 2/3 (0.9Ein - 0.6Ein)

3. E across the 10,000-ohm resistor = 0.1Ein

4. 2/3 (0.3in) - 0.1Ein = 10 Therefore, Ein = 100 volts dc.

*** I disagree with the magazine's answer. See my solution to the right.

Posted February 28, 2023

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