|Laws you should know about in electronics and
other tips for building and testing electronic circuits
The basic law of current flow is
Ohm's law, named for its discoverer, the German physicist Georg
Ohm. Ohm's law states that the amount of current flowing in a
circuit made up of pure resistances is directly proportional to the
electromotive force impressed on the circuit and inversely
proportional to the total resistance of the circuit.
The law is usually expressed by the formula I = V / R, where I is
the current in amperes, V or ( E ) is the electromotive force in
volts, and R is the resistance in ohms (see Electrical Units).
Ohm's law applies to all electric circuits for both direct current
(DC) and alternating current (AC), but additional principles must
be invoked for the analysis of complex circuits and for AC circuits
also involving inductances and capacitances.
Now take a look to a simple circuit
fig.1 - Ohm's
A potential difference of 10 V
(volts) is needed to make a current of 2A (amps) flow through a
conductor with a resistance of 5 ohms.
If a circuit has a number of
interconnected branches, two other laws are applied in order to
find the current flowing in the various branches. These laws,
discovered by the German physicist Gustav Robert Kirchhoff, are
known as Kirchhoff's laws of networks.
fig.2 - The first
The first of Kirchhoff's laws states
that at any junction in a circuit through which a steady current is
flowing, the sum of the currents flowing to the point is equal to
the sum of the currents flowing away from that point.
Now take a look to the equation in
each point of the circuit :
- A: I2 =I1 + I3
- B: I6 = I2 + I4
- C: I6 = I 3 + I 5
- D: I5 = I1 + I4
fig.3 - The second
The second law states that, starting
at any point in a network and following any closed path back to the
starting point, the net sum of the electromotive forces encountered
will be equal to the net sum of the products of the resistances
encountered and the currents flowing through them. This second law
is simply an extension of Ohm's law.
Now take a look to the equation for
each path :
Starting B point :
Starting D point :
Starting A point:
fig.4 - Measuring
The instrument most generally used
to measure potential difference, or voltage, is a galvanometer with
a high resistance in series with the coil. When such a meter is
connected across a battery, or to two points in an electrical
circuit between which a potential difference exists, a small
current (limited by the series resistor) will pass through the
meter. The current is proportional to the voltage, and the latter
quantity can be measured if the galvanometer is calibrated
appropriately. By using the proper values of series resistors, one
galvanometer can be used to measure a large range of voltages. The
most accurate instrument for the determination of voltage,
resistance, or direct current is the potentiometer, which indicates
an unknown electromotive force by comparing it with a known
For the measurement of AC voltages,
AC meters having high internal resistance, or similar meters with
high series resistance, are employed.
Other methods for measuring the value of voltages depend on vacuum
tubes and electronic circuits (see Electronics) and are especially
useful in measurements at high frequencies. One such device is the
vacuum-tube voltmeter. In the simplest form of this meter an AC
voltage is rectified by a diode tube, and the rectified current is
measured by an ordinary galvanometer. Other such voltmeters employ
the amplifying characteristics of electronic tubes to measure
extremely small voltages. The cathode-ray oscilloscope can also be
used for voltage measurements because the deflection of the
electron beam is proportional to the voltage impressed on the
deflection plates or coils.
fig.5 - Measuring
Galvanometers are the main
instruments used to detect and measure current. They depend on the
fact that force is generated by an electric current flowing in a
magnetic field. The mechanism of the galvanometer is so arranged
that a small permanent magnet or electromagnet sets up a magnetic
field that generates a force when current flows in a wire coil
adjacent to the magnet. Either the magnet or the adjacent coil may
be movable. The force deflects the movable member by an amount
proportional to the strength of the current. The movable member may
have a pointer or some other device to enable the amount of
deflection to be read on a calibrated scale.
In the D'Arsonval galvanometer, a
small mirror attached to a movable coil reflects a beam of light on
a scale about 1 m (about 3 ft) away from the instrument. This
arrangement involves less inertia and friction than does a pointer,
and consequently, greater accuracy is achieved. The instrument is
named after the French biologist and physicist Jacques d'Arsonval,
who devised the first reflecting galvanometer. He also conducted
experiments with the mechanical equivalent of heat and in the
high-frequency oscillating current of low voltage and high
amperage, D'Arsonval current, used in the treatment of certain
diseases (diathermy treatment). The addition of a scale and proper
calibration converts a galvanometer into an ammeter, the instrument
used for measuring electric current in amperes; D'Arsonval was also
responsible for inventing a direct-current (DC) ammeter.
Only a limited amount of current can be passed through the fine
wire of a galvanometer coil. When large currents must be measured,
a shunt of low resistance is attached across the terminals of the
meter. Most of the current is bypassed through this shunt
resistance, but the small current flowing through the meter is
still proportional to the total current. By taking advantage of
this proportionality, a galvanometer can be used to measure
currents of hundreds of amperes. Galvanometers are usually named
according to the magnitude of the currents they will measure. A
microammeter is calibrated in millionths of an ampere and a
milliammeter in thousandths of an ampere.
Ordinary galvanometers cannot be used for the measurement of an
alternating current (AC), because the alternation of the current
would produce deflection in both directions. An adaptation of the
galvanometer, however, called an electrodynamometer, can be used to
measure alternating currents by means of electromagnetic
deflection. In this meter a fixed coil, in series with the moving
coil, is employed in place of the permanent magnet of the
galvanometer. Because the current in the fixed and moving coils
reverses at the same instant, the deflection of the moving coil is
always in the same direction, and the meter gives a constant
current reading. Meters of this type can also be used to measure
direct currents. Another form of electromagnetic meter is the
iron-vane meter or soft-iron meter. In this device two vanes of
soft iron, one fixed and one pivoted, are placed between the poles
of a long, cylindrical coil through which is passed the current to
be measured. The current induces magnetism in the two vanes,
causing the same deflection no matter what the direction of the
current. The amount of the current is ascertained by measuring the
deflection of the moving vane.
Meters that depend on the heating effect of an electric current
are used to measure alternating current of high frequency. In
thermocouple meters the current passes through a fine wire that
heats a thermocouple junction; the electricity generated by the
thermocouple is measured by an ordinary galvanometer. In hot-wire
meters the current passes through a thin wire that heats and
stretches. This wire is mechanically linked to a pointer that moves
over a scale calibrated in terms of current.
fig.6 - Digital
We can also measure voltage or
current by using a digital meter.
There are a few rules in measuring
with digital meters :
1.First move the switch to the area
that specifies what do you want to measure.
2.Then choose a measuring range (Ex: If you are trying to measure
220 V AC first move the switch to the alternative voltage area
then in this area move the switch to a number greater then the
voltage you want to measure (in your case you can try 250 ).
3.The next step is the connection of the digital meter to the
circuit. (rules about how you connect the digital meter are
specified above ).
4.Last thing that you have to do is to read the digital screen
when the number displayed becomes steady.
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