Now that we’ve looked at the theory behind current and volt- age measurement, let’s go on and build a few circuits to put it into practice. Figure 3.10 shows the breadboard layout of the circuit of Figure 3.6, where a single resistor is connected in series with a multi-meter and a battery. With this circuit we can actually prove Ohm’s law. The procedure is as follows: 1) set the meter’s range switch to a current range — the highest one, say, 10 A,
2) insert a resistor (of say 1k5) into the breadboard, and connect the battery leads,
3) touch the multi-meter leads to the points indicated. You will probably see the pointer of the multi-meter move (but only just) as you complete step 3. The range you have set the multi-meter to is too high — the current is obviously a lot smaller than this range, so turn down the range switch (to, say, the range nearest to 250 mA) and do step 3 again. This time the pointer should move just a bit further, but still not enough to allow an accurate reading. Turn down the range switch again, this time to, say, the 25 mA range and repeat step 3. Now you should get an adequate reading, which should be 6 mA give or take small experimental errors.
Is this right? Let’s compare it to the expression of Ohm’s law:
Figure 3.10 A breadboard layout for the circuit of Figure 3.6: the meter is in series with the resistor; only the meter itself makes the circuit complete
If we insert known values of voltage and resistance into this we obtain:
Not bad, eh? We’ve proved Ohm’s law!
You can try this again using different values of resistor if you wish, but don’t use any resistors lower than about 500 Ω
as you won’t get an accurate reading, because the battery can’t supply currents of more than just a few mA. Even if it could the resistor would overheat due to the current flowing through it.
In all cases where the battery is able to supply the current demanded by the resistor, the expression for Ohm’s law will hold true.
Take note — Take note — Take note — Take note Make sure that you start a new measurement with the range switch set to the highest range and step down. This is a good practice to get into, because it may prevent damage to the multi- meter by excessive currents. Even though circuits we look at in Starting Electronics will rarely have such high currents through them, there may be a time when you want to measure an unknown current or voltage in another circuit; if you don’t start at the highest range — zap — your multi-meter could be irreparably damaged.
Hint:
The scale you must use to read any measurement from, depends on the range indicated by the meter’s range switch. When the range switch points, say, to the 10 A range, the scale with the highest value of 10 should be used and any reading taken represents the current value. When the range switch points to, say, the 250 mA range, on the other hand, (and also the other two ranges 2.5 mA and 25 mA), the scale with the highest value of 25 should be used. It all sounds tricky doesn’t it? Well, don’t worry, once you’ve seen how to do it, you’ll be taking measurements easily and quickly, just like a professional.
Note that current (and voltage) scales read in the opposite direction to the resistance scale we used in the last chapter, and they are linear. This makes them considerably easier to use than resistance scales and they are also more accurate as you can more easily judge a value if the pointer falls between actual marks on the scale.
Voltages
When you measure voltages with your multi-meter the same procedure should be followed, using the highest voltage ranges first and stepping down as required. The voltages you are measuring here are all direct voltages as they are taken from a 9 V d.c. battery. So you needn’t bother using the three highest d.c. voltage ranges on the multi-meter, as your 9 V battery can’t generate a high enough voltage to damage the meter anyway. Also, don’t bother using the a.c. voltage ranges as they’re — pretty obviously — for measuring only alternating (that is, a.c.) voltages.
As an example you can build the circuit of Figure 3.9 up on your breadboard, shown in Figure 3.11. What is the measured voltage? It should be about 4.5 V.
Now measure the voltage across the other resistor — it’s also about 4.5 V. Well, that figures, doesn’t it? There’s about 4.5 Vacross each resistor, so there is a total of 2 x 4.5 V that is, 9 V across them both: the voltage of the battery. This has demonstrated that resistors in series act as a voltage divider or a potential divider, dividing up the total voltage applied
across them. It’s understandable that the voltage across each resistor is the same and half the total voltage, because the two resistors are equal. But what happens if the two resis- tors aren’t equal?
Build up the circuit of Figure 3.12. What is the measured voltage across resistor R2 now? You should find it’s about 2.1 V. The relationship between this result, the values of the two resistors and the applied battery voltage is given by the voltage divider rule:
Figure 3.11 A breadboard layout for the circuit in Figure 3.9: measuring the voltage across R2. This will be the same as R1, and each will be around 4.5 V — half the battery voltage
Figure 3.12 A circuit with two unequal, series resistors. This is used in the text to illustrate the voltage divider rule, one of the most fundamental rules of electronics
where Vin is the battery voltage and Vout is the voltage meas- ured across resistor R2.
We can check this by inserting the values used in the circuit of Figure 3.12:
In other words, close enough to our measured 2.1 V to make no difference. The voltage divider rule, like Ohm’s law and the laws of series and parallel resistors, is one of the fundamental laws which we must know. So, remember it! ok?
Take note — Take note — Take note — Take note By changing resistance values in a voltage divider, the voltage we obtain at the output is correspond- ingly changed. You can think of a voltage divider almost as a circuit itself, which allows an input voltage to be converted to a lower output voltage, simply by changing resistance values.
Pot-heads
Certain types of components exist, ready-built for this voltage dividing job, known as potentiometers (commonly shortened to just pots). They consist of some form of resistance track, across which a voltage is applied, and a wiper which can be moved along the track forming a variable voltage divider. The total resistance value of the potentiometer track doesn’t change, only the ratio of the two resistances formed either side of the wiper. The basic symbol of a potentiometer is shown in Figure 3.13(a).
A potentiometer may be used as a variable resistor by con- necting the wiper to one of the track ends, as shown in Figure 3.13(b). Varying the position of the wiper varies the effective resistance from zero to the maximum track resistance. This is useful if we wish to, say, control the current in a particular part of the circuit; increasing the resistance decreases the current and vice versa.
These two types of potentiometer are typically used when some function of an appliance e.g., the volume control of a television, must be easily adjustable. Other types of po- tentiometer are available which are set at the factory upon manufacture and not generally touched afterwards e.g., a TV’s height adjustment. Such potentiometers are called preset po- tentiometers. The only difference as far as a circuit diagram is concerned is that their symbols are slightly changed. Figures 3.13(c) and (d) show preset potentiometers in the same con- figurations as the potentiometers of Figures 3.13(a) and (b). Mechanically, however, they are much different.