watt-hour meters

Watt-hour meters
The utility company is not too interested in how much power you’re using with one appliance,
or even how much power a single household is drawing, at any given time. By
far the greater concern is the total energy that is used over a day, a week, a month or a
year. Electrical energy is measured in watt hours, or, more commonly for utility purposes,
in kilowatt hours (kWh). The device that indicates this is the watt-hour meter
or kilowatt-hour meter.
The most often-used means of measuring electrical energy is by using a small electric
motor device, whose speed depends on the current, and thereby on the power at a
constant voltage. The number of turns of the motor shaft, in a given length of time, is directly
proportional to the number of kilowatt hours consumed. The motor is placed at
the point where the utility wires enter the house, apartment or building. This is usually
at a point where the voltage is 234 V. This is split into some circuits with 234 V, for
heavy-duty appliances such as the oven, washer and dryer, and the general household
fines for lamps, clock radios and, television sets.
You’ve surely seen the little disk in the utility meter going around and around,
sometimes fast, other times slowly. Its speed depends on the power you’re using. The
total number of turns of this little disk, every month, determines the size of the bill you
will get—as a function also, of course, of the cost per kilowatt hour for electricity.
Kilowatt-hour meters count the number of disk turns by means of geared, rotary
drums or pointers. The drum type meter gives a direct digital readout. The pointer type
has several scales calibrated from 0 to 9 in circles, some going clockwise and others going
counterclockwise.
Reading a pointer type utility meter is a little tricky, because you must think in
whatever direction (clockwise or counterclockwise) the scale goes. An example of a
pointer type utility meter is shown in Fig. 3-11. Read from left to right. For each little
meter, take down the number that the pointer has most recently passed. Write down
the rest as you go. The meter in the figure reads 3875 kWh. If you want to be really precise,
you can say it reads 3875-1/2 kWh.

wattmeters

Wattmeters
The measurement of electrical power requires that voltage and current both be measured
simultaneously. Remember that power is the product of the voltage and current.
That is, watts (P) equals volts (E) times amperes (I), written as P
EI. In fact, watts
are sometimes called volt-amperes in a dc circuit.
You might think that you can just connect a voltmeter in parallel with a circuit,
thereby getting a reading of the voltage across it, and also hook up an ammeter in series
to get a reading of the current through the circuit, and then multiply volts times amperes
to get watts consumed by the circuit. And in fact, for practically all dc circuits,
this is an excellent way to measure power
Quite often, however, it’s simpler than that. In many cases, the voltage from the
power supply is constant and predictable. Utility power is a good example. The effective
voltage is always very close to 117 V. Although it’s ac, and not dc, power can be measured
in the same way as with dc: by means of an ammeter connected in series with the
circuit, and calibrated so that the multiplication (times 117) has already been done.
Then, rather than 1 A, the meter would show a reading of 117 W, because P
EI
117
 1
117 W. If the meter reading were 300 W, the current would be 300/117
2.56 A.
An electric iron might consume 1000 W, or a current of 1000/117
8.55 A. And a
large heating unit might gobble up 2000 W, requiring a current of 2000/117
17. 1 A. This
might blow a fuse or breaker, since these devices are often rated for only 15 A. You’ve
probably had an experience where you hooked up too many appliances to a single circuit,
blowing the fuse or breaker. The reason was that the appliances, combined, drew too
much current for the house wiring to safely handle, and the fuse or breaker, detecting the
excess current, opened the circuit.
Specialized wattmeters are necessary for the measurement of radio-frequency
(RF) power, or for peak audio power in a high-fidelity amplifier, or for certain other specialized
applications. But almost all of these meters, whatever the associated circuitry,
use simple ammeters as their indicating devices.

Learn Electronics - Relays

In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radioteletype.

In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radioteletype.


In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radioteletype.In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radioteletype.In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radioteletype.In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radioteletype.In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radioteletype.In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radioteletype.In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radioteletype.In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radioteletype.In some electronic devices, it is inconvenient to place a switch exactly where it should
be. For example, you might want to switch a communications line from one branch to
another from a long distance away. In many radio transmitters, the wiring carries
high-frequency alternating currents that must be kept within certain parts of the circuit,
and not routed out to the front panel for switching. A relay makes use of a solenoid
to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature,
is held to one side by a spring when there is no current flowing through the electromagnet.
Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient
current is applied, the armature is pulled over to the other side. This disconnects
terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for
use with dc, and others are for ac; a few will work with either type of current. A normally
closed relay completes the circuit when there is no current flowing in its electromagnet,
and breaks the circuit when current flows. A normally open relay is just the opposite.
(“Normal” in this sense means no current in the coil.) The relay in the illustration (Fig.
8-8) can be used either as a normally open or normally closed relay, depending on which
contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one
state (either with current or without) for a long time, while others are meant to switch
several times per second. The fastest relays work dozens of times per second. These are
used for such purposes as keying radio transmitters in Morse code or radio teletype.


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