Digital Circuits

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.




Op-Amps and ICs

The op-amp is a differential amplifier on an integrated circuit (IC) chip, with terminal access to all points in both stages. Connections can be made to these terminals to have the op-amp perform a wide variety of functions. It can be an ordinary amplifier with one input, in which the outputs can be inverted or noninverted or both. The two outputs can be used independently or as a differential output. The op-amp can also be used as a source follower. With some amplifiers, if the alternate input is used, the amplifier is the same, except that the inverted and noninverted outputs reverse. The two stages in the op-amp can also be used to gate an input. The input applied to the first stage will be fed internally to the second stage, which will block the signal until it receives a gate voltage.


Pulse Generators

The pulse generator circuits are the principal circuits which determine the accuracy of the digital meter. The clock is a signal generator, or stable oscillator, that creates and supplies the steady stream of pulses to the variable gate. This stable signal source is also passed through a sequence of decade dividers that each reduce the pulse frequencies to Y10 of their value.

In this example, the original clock frequency of I megahertz is subdivided three times, first to I 00 kilohertz, then to I O kilohertz, and then to I kilohertz. The subdividing can continue down to I hertz so that a gate pulse of any width will be accessible. Also, the basic clock frequency and all frequencies subdivided by 10 are available for use individually for selective counting in different ranges.


Oscillators and Waveshapers

The clocks and the decade dividers, as well are not usually just one stage. With the clock, for example, in addition to the oscillator, there are generally amplifiers and wave shaping circuits used to get each wave in the pulse train properly shaped. When a sine wave oscillator is used, clipper and clamper circuits, or a Schmitt trigger, are used to reshape the sine waves into square wave pulses.

There are countless varieties of oscillator circuits. The main requirement for a circuit to oscillate and produce a stream of waves is that positive feedback, also called regenerative feedback, must take place. The feedback from the output must be in phase with the input for regeneration to occur. Several types of oscillators include: a two-stage oscillator, using the noninverted (in phase) output for feedback; a one-stage oscillator using RC circuits to shift the feedback phase 180 degrees to make it positive; and an LC tuned or resonant circuit to cause regenerative feedback. The tuned oscillator also has a crystal, which is not always used, but which has a natural vibrating frequency to control the oscillator.


Continue reading “Digital Circuits”

Resistance Testing: Part 2

Circuit Tracing

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.

Short Tests

When a short occurs in an electrical line, wear and abrasion of insulation on a wire generally exposes the bare conductor and allows it to contact either a grounded metal or another damaged wire. Any damage to a hot line’s insulation is likely to cause a short, since in nonmetallic sheathed cable, the grounded wire is bare, and in BX cable or conduit, the metal sheathing is grounded.

Testing for shorts is a lot like making continuity tests. If a short exists between a hot wire and ground, then an ohmmeter will show continuity along that line, through the short, and back through the ground (with the power disconnected). Finding the exact location of the short is another matter. On long lines, the resistance reading of the wire and the normal ohms-per-1000 feet of the wire will be a guide to the location of the short. For relatively short lines, the signal tracer test can be used. The receiver, when moved along and near the hot line, will show signal reception before the short is reached, but the reception will fade out as the short is passed.


Open Tests

“Open” tests are continuity tests in which the user is looking for open trouble in a circuit. Continuity, or little or no resistance, affirms a good, continuous circuit, and high or infinite resistance indicates an open circuit.

A power switch is tested with a voltmeter can be tested for continuity (with the power disconnected) with an ohmmeter in a similar way. With the ohmmeter connected across the switch terminals, the ohmmeter should read zero ohms, or continuity with the switch turned on. It will read infinite or very high resistance if the switch is bad, indicating an open circuit in the switch.



Current Tests: Part 2

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.

The initial surge current of an air conditioner that requires a 15-ampere circuit is over 15 amperes, and this high current flows momentarily every time the motor starts up, when you first turn the equipment on, and whenever the compressor comes on. Therefore, it is recommended that a time-delay circuit breaker be used, and the air conditioner be on a separate, dedicated circuit. Set the ammeter range higher than 15 amps to allow for the surge; the normal run reading while the compressor is on will likely be less than 10 amps, and while the compressor is off, around 3 amps if the circulator fan is running, and zero if it is not.


Usage Tests

Usage testing refers to power usage more than it does to current usage, and it is the measure of the power used over a period. Essentially, a wattmeter is connected into the circuit, either permanently or for prolonged periods, to keep track of power consumed over specific lengths of time.

The most common type of usage tester, or wattmeter, is the utility’s monitor that is connected across the main input service line before it enters the building or, inside the building, before it enters the main service box. It monitors the line voltage and all current flows to the building and gives usage readings in kilowatt hours (kWh).

In other applications, separate circuits or equipment can be monitored to compare the power usage of the different systems. In a shopping mall, for example, the heating and/or cooling systems could be monitored separately, as well as the lighting systems, and people-moving systems, such as escalators and elevators. In industrial applications, individual production systems could be monitored. All of these usage monitoring systems give readings in kilowatt hours.


Load Tests

Typically, when a circuit is rated to carry a certain maximum current, say 20 amperes, there is little or no difference when a low current flows or when a higher current flows- as long as the current stays within limits. However, when certain problems exist, such as a high-resistance connection, the higher current that flows through such a connection will cause a voltage drop that will result in the line voltage in that circuit being reduced. How much the line voltage drops depends on how bad the connection is. In a case such as this, if the line voltage was tested when there was little current flowing, a normal reading might be obtained, because the line voltage drops under heavier load currents.

When circuits are suspected of this problem, a load tester can be used to simulate load conditions and read out the percentage that the line voltage drops as loads are simulated. A typical example of a load current simulator is one that can be set to draw 0, 10, 15, or 20 amperes in sequential steps. Do not set the simulator for a 20-amp load on a 15-amp circuit.



Resistance Testing: Part 1

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.

Resistance testing of a component that has pure resistance is straightforward. The component resistance reading should be the same as it would be in both ac and dc circuits. But many components used in ac circuits have both a pure resistance and an impedance, which affect circuit operation. Any component containing coils or capacitances has an ac impedance. Ohmmeters measure only the DC or pure resistance of a component. There are, however, AC component testers, to measure impedance.


Wire Continuity Tests

Many electrical troubles are caused by breaks, or opens, in wire continuity, as well as increased resistance in a connection. Continuity testing is merely a test to show that there are no complete breaks in a wire, whereas resistance testing generally measures for a more specific reading in ohms. Since a reading is not always necessary in a continuity test, many continuity testers sound an audible to notify the user when continuity exists, while an analog or digital meter would show zero or very low resistance.

Testing the continuity of a wire requires connections to both ends of that wire. With a short wire, this is not difficult, but with a long wire, where one end might not be accessible, that end might have to be tied to another return wire so that the continuity of both wires can be tested in series.


Wire Resistance Tests

Continuity tests are general indicator tests that are useful in most cases. But, in some cases, particularly where a stranded cable is used, a wire can have continuity but too high a resistance if one or two or more strands are broken. Also, a bad connection will allow continuity, but a high resistance reading. Resistance tests are made the same as continuity tests, except ohm readings are obtained on a meter. For long wire lengths, the normal resistance of the length of wire should be known.


Infrared Tests

Because of some long-line testing, and the fact that wiring is often buried or hidden from view, many ordinary continuity and resistance tests are challenging. There are several infrared scanners available which allow the user to test for high resistance effects while the equipment is operating. The effect of high resistance in a circuit carrying current is that the resistance dissipates power and generates heat. The infrared testers are also called thermal testers. The tests are made by first aiming the tester at a known normal temperature target to get an ambient reference reading; and then aiming the tester at various suspected targets in a circuit, usually connections. An audible tone on some units may guide the user to a suspected target, while visual displays, such as lights, show the severity of the heat. Other testers can give the actual temperature readings.