“Cut the red wire!” This is a familiar line to any fan of high-stakes action movies like Lethal Weapon. In the movies, characters hoping to diffuse a bomb before it’s too late are faced with a dilemma- what color wire do they cut? While the answer to that question may depend on the film- the difference in color coded wires has real life consequences. How does a color coded system improve electrical safety? Are all color coding systems still used today? Most importantly- what do all of these different colors mean?
Before the modern digital technology we use in computers today, electronic computations were conducted by utilizing both voltages and currents as representations of numerical values. This process needed circuitry capable of carrying out a wide range of analog signal processing tasks, which led to the use of operational amplifiers (often referred to as op-amps). The engineering concept of negative feedback, which forms the basis of practically all automatic control procedures, holds the key to the utility of these tiny circuits.
Kirchhoff’s Current and Voltage Laws are at the center of circuit analysis. With them, we have the fundamental tool set we need to start studying circuits and the formulas for individual components such as resistors, capacitors, and inductors. Named after German physicist Gustav Kirchhoff (1824-1887), Kirchhoff’s Laws are electromagnetic approximations derived from Maxwell’s Equations. Simply put, they are applicable when the size of the components in a circuit are substantially smaller than the wavelength of the signals traveling through the circuit.
Ohm’s Law, discovered by Georg Simon Ohm and first published in 1827, is the earliest and arguably most important connection between current, voltage, and resistance. A straightforward and practical technique for studying electric circuits, Ohm’s Law is very commonly used and has been documented on a broad range of scales. For aspiring electrical engineers studying the basics or for seasoned professionals looking to refresh their knowledge, this scientific law is worth having a deep understanding of.
For circuit operation, the variations between de and ac functions are not too dissimilar. For de signal voltages, the individual stages of the instrument are directly coupled. Direct coupling can also be used for ac voltage signals, but typically capacitors are used to pass only the ac signal voltages from one stage to another. The test indications for ac must be handled slightly differently because of the nature of ac waveforms and the different ways of describing their levels. One ac wave has average, effective (RMS), and peak values. The analog meter movement responds to the average ac value, but we all use the effective or RMS value to describe a waveform, such as 120-V ac line power. The ac meter is calibrated for the effective value, even though it reacts to the average value.
The purpose of the integrator and gate circuits is to pass on the number of pulses that match the test voltage being measured. Since a specific pulse frequency stream is being applied to the gate, the test voltage must somehow control how long the gate will be allowed to pass the pulses; the higher the test voltage, the longer the gate will be open, and the more pulses will be passed through- and the number of pulses will represent the digitized version of the analog test voltage.
Input Networks and Amplifiers
The input network and amplifier perform the same functions as they do for the electronic analog meter. The input network presents a high resistance (11 megohms) to the circuit under test to keep from loading it down; it also attenuates the input voltage with the range switch setting to keep the test signal at the input of the amplifier under 1 volt. Although identical input and amplifier circuits can be used for both digital and analog meters, the example we are using demonstrates the use of an amplifier that can take up to 1 volt of input, and the ranges vary from 2 volts to 2000 volts, in multiplier ranges of 2, 20, 200, and 2000 volts. Since digital measurements use ten digits (0- 9), the counters, and especially the pulse generators deal in multiples of ten for convenience. The follower and amplifier circuits are both op-amps connected to accomplish their functions.
As explained in the previous blog post “Resistance Testing: Part 1,” because of long lines, and because long lines can be buried or otherwise hidden from view, it is difficult to perform some tests without knowing which wires or connections are part of which circuits. A unique tracing system, which is available to quickly identify those parts on the same circuit, uses a hand-held radio transmitter and receiver to trace a circuit with radio-frequency signals. The transmitter can either be plugged into an outlet or connected anywhere in a line with alligator clips. The transmitter sends the signal along the lines to all other lines and components connected to it. The portable, hand-held receiver, which has a pickup antenna, is moved along the suspected path, whether the wires are in walls or buried, and the receiver will give an indication in the form of a light and a beeping tone when it is aimed at all the associated wires, outlets, switches, junction boxes, circuit breakers, etc.
Air Conditioner Current Test
Some motors are designed to reach rated rpm faster than other motors by using a special starting winding or capacitor circuit on startup, then switch over to the normal running circuit once normal rpm is reached. Motors used in air conditioners are of this type. Certain types of test jigs are available commercially.
There are several different types of resistance tests. Resistance tests differ from voltage and current tests because they are rarely performed on a dynamic basis, that is, while the equipment is operating. Resistance tests are usually performed with the power off and usually with the component disconnected to make sure that there are no short circuits to cause misleading readings.