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

fig. 48

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.

fig. 49

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 exceed 40mA.

2. To divide voltages in order to get certain partitions of the original voltage.

3. To translate currents into voltages and vice versa.

fig. 50

fig. 51

Makes of Resistors, used in Electronics:

In general there are two types of it; WIREWOUND and CARBONTYPE resistors.

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 colour rings.

fig. 52

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).

example:

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 is borken.

HOW TO REPLACE RESISTORS?

Make sure that the new resistor has:

- the same RESISTANCE and
- the same or a higher power rating.

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).

VARIABLE RESISTORS

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 different groups:

fig. 53

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.

fig. 54

VARIABLE RESISTORS

(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).

fig. 55

EXERCISE:

You want to know the current flowing in a resistor on a printed circuit.

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.

fig. 56

QUESTIONS

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 the component?