8.1. RESISTORS ELECTRICAL CHARACTERISTICS
voltage/ current relation:
The resistor allows a certain current to flow. According to OHMs
LAW the current increases proportionally if the voltage is increased
behaviour at changing frequencies:
if an ohmic resistor is connected across an alternating voltage
of a varying frequency (while the amplitude of the voltage is kept constant) the
current flowing will only depend on the voltage and the resistance of the
circuit - not on the frequency.
Main functions of Resistors in Electronics:
1. To control the amount of current flowing in a
certain circuit (current path). Example: The current in the circuit will never
2. To divide voltages in order to get certain partitions of the
3. To translate currents into voltages and vice
Makes of Resistors, used in Electronics:
In general there are two types of it; WIREWOUND and CARBONTYPE
Wirewound ones are mostly more expensive and need more volume.
Therefore they are only used where extremly high powers have to be dissipated.
For a power dissipation of less than 3 Watts there are almost exclusively used
the so called MOULDED CARBON TYPE resistors. Clay, resin and carbonpowder is
mixed in fitting proportions and then the whole mixture is poured into moulds
and bibed at very high temperatures. Then the ends of the bodies are given a
small metal solder contact at which the leads are bonded or soldered. Mostly
those resistors get additionally an insulating cover and the specifications are
printed on that cover - Either in form of numbers and letters or in form of
Nowadays those resistors are manufactured at a very good
standard and most of them within a tolerance of five per cents. It would be not
economical to produce them with each and every resistance (like 101, 102, 103
Ohms and so on). Instead of that, they are produced in special rows. The most
often appearing row is the so called E 12 series. The specification E 12 means:
we find per decade 12 different values each of it having a resistance of about
20% higher than the smaller one (100, 120, 150, 180 and so on).
2 decade: 10/12/15/22/27/33/39/47/56/68/82/10
3 decade: 100/120/150/220/270/330/390/470/560/680/820/1000
and so on......
CHECKING AND HANDLING OF RESISTORS IN ELECTRONIC EQUIPMENT:
If you measure the following values:
You can assume:
No voltage across the resistor while the supply is on:
Either a short circuited resistor or an open circuited one.
A voltage very similar to the supply voltage at a resistor which
is connected in series with other resistors:
Either the other resistor is short circuitedor the measured one
HOW TO REPLACE RESISTORS?
Make sure that the new resistor has:
- the same RESISTANCE and
- the same or a higher
If you don't have the fitting values available there are
possibilities to achieve the intended values by series or parallel connections
(but check if the power rating of your resistors is fitting).
At certain points in a radio we find resistors of which the
resistance can be changed. We call them variable resistors. The
resistance between certain terminals of these resistors can be variated by
sliding a slider over the resistor. These variable resistors can be of two
ADJUSTABLE RESISTORS are variable resistors whose slider can be
moved only by means of a screwdriver. Those resistors are normally fixed at
positions where under normal service conditions you cannot reach. They are meant
for setting the circuit to special values before it is handed over to the
customer. During normal service they are not touched anymore.
(real ones) can be operated by turning a control knob or a
control slider and they are fixed so, that the customer can reach them anytime
he wants to.
These variable resistors are produced in two different types.
Very often the resistance (wire or moulded carbon) is brought on
to the body of the resistor, so that the resistance is changing proportionally
to the distance which the handle is moved over the resistor. These types are
called the LINEAR VARIABLE RESISTORS.
For special purposes it is sometimes necessary to have a
different behaviour of the resistance when moving the slider. For volume
controls for example, the LOGARITHMIC VARIABLE RESISTOR is used, because the
(turning it halfway seems to decrease the sound for fifty per cent, even though
the resistance value has been increased four times).
You want to know the current flowing in a resistor on a printed
You do not want to dissolder the component (because of the
reasons explained above).
Therefore you are intending to use an oscilloscope and you want
to measure the current indirectly. You find - with setting of the controls as
shown in the following figure - a signal as shown on the screen.
The resistor has the following colour rings.
A- gold / B- red / C-green / D- brown.
1. What is the resistance of the resistor?
2. What is the peak to peak voltage at the resistor?
3. What is the rms value of the voltage at the resistor?
4. What is the current flowing through the resistor which you
would measure at a analogue multimeter if you would dissolder one terminal of