April 1974 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
OK, I am ashamed to admit that with just a quick guess I thought Rx in question 6 would be 180 Ω rather than the correct value of 20 Ω. I knew the ratio of 150 Ω to 50 Ω (3:1) would be the same as for 60 Ω to Rx, but stupidly went the wrong way. In order for the bridge to be balanced, the voltage division between the left and right arms of the bridge had to result in the voltages on both sides of the meter to be 0 V. I did manage to get the equivalent resistances of Q8, 9, and 10 right, though, by re-drawing the circuits (Hint: it helps a lot to number the nodes and resistors when doing so). Q10, by the way, is similar to the resistor cube problem I demonstrated a solution approach for back in 2010 (wow, that long ago?!).
What Do You Know About Resistors?
By Robert P. Balin
The electronics technician and hobbyist must know a great deal about resistors. Besides being able to read the resistor color code, they should be able to identify some of the common applications of resistors, know how to use a resistance bridge, and know how to combine resistors to obtain a desired equivalent resistance or power rating.
To test your knowledge of resistors, see how many of the following problems you can solve. The numbers on the problems correspond to numbers on the diagrams below. The answers are at the bottom of the page.
1. The color code on the axial-lead resistor shown indicates that it has a rating of how many ohms and what tolerance percentage?
2. The color code on the radial-lead resistor shown indicates that it has a rating of how many ohms and what tolerance percentage?
3. 4. 5. What is the function of the resistor (R) in each of these three circuits?
6. If the bridge circuit shown is balanced when the adjustable resistor is set for 60 ohms, what is the value of the unknown resistor, Rx?
7. If a 500-ohm, 5-watt resistor and a 500-ohm, 10-watt resistor are connected in series, the combination is the equivalent of a resistor of how many ohms and what wattage rating?
8. 9. 10. What is the total equivalent resistance between points A and B for each of these three circuits? All of the resistors are identical and rated at 6 ohms.
1. 27,000 ohms, ±5%
2. 3.6 megohms, ±10%
4. Parasitic suppressor
5. Ballast or dropper
6. 20 ohms
7. 1000 ohms, 10 watts
8. 4 ohms
9. 2 ohms
10. 6 ohms
Posted April 11, 2017