How do we measure resistance tolerance
Resistance - an obstacle to the current
|1||Battery / voltage source 9V|
|2||Resistance 470 ohms|
|2||Resistance 1.0 kOhm|
In electronic circuits it happens again and again that the current has to be limited in some way or the voltage has to be reduced. The easiest way to do this is through resistance. As a rule, 2 types of resistance are used. Carbon film resistors and metal film resistors. The resistors are available with different loads. I will assume a load of 0.25W in the courses. These are easy to get as mass-produced goods. The figure shows carbon film resistors with different values.
Carbon film resistors with 0.25W
How much a resistor limits the current depends on the value. This is given in ohms (Ω). The value is coded with colored rings printed on the resistor. To determine the value, place the resistor in front of you so that the last ring, which is furthest out, points to the right. I now assume that this is a carbon film resistor. Then you can start coding. Corresponding values are assigned to the individual colors. These values can be used in my Table (to be found in the 'Library' section). You can see e.g. the following color combination:
Now the corresponding values result:
1st ring = green = 5
2nd ring = blue = 6
3rd ring = brown = 1
4th ring = gold = tolerance 5%
The values of ring 1 & 2 can be written down in plain text. The 3rd ring indicates how many zeros still have to be appended to the value. The last ring shows what tolerance this resistor can have. So here we have a resistance of:
5 6 0 ohms with 5% tolerance.
So the resistance has a value between 532 and 588 ohms.
Let us take another example to deepen it. In front of us is a resistor with the color combination yellow - purple - orange - gold. From the table we read from it:
1st ring = yellow = 4
2nd ring = purple = 7
3rd ring = orange = 3rd
4th ring = gold = tolerance 5%
This results in:
4 7,000 ohms with 5% tolerance.
Instead of 47000 it is also allowed to write 47k (pronounced kilos). When it comes to values in the millions, one says mega. The value is then abbreviated with M. e.g. 47000000 = 47M.
If you want to practice using the color table a little more, here are some color combinations:
Red - red - orange - silver
Brown - green - black - gold
Orange - Orange - Yellow - Red
Red - purple - blue - gold
The effect of resistance
What exactly does a resistor do now? It is best to investigate this time with the help of an experiment.
If this circuit is set up and put into operation, the measuring device shows a current of approx. 19 mA. In this circuit, the resistor limits the maximum current that can flow. This current depends on the strength of the resistance. But what happens now if you change the value? Let's replace the resistor with one with 1 kOhm.
The measuring device now shows a value of approx. 9 mA. The current has halved itself a little more than. If we look at the two values, we can see that the value has been more than doubled. So there is a direct relationship between the current and the level of the resistance value. A further increase in the value would result in a further decrease in the current.
For example, if you need a 1 kOhm resistor. But if you don't have it at hand, you can create a replacement resistor by connecting two 470 Ohm resistors in series. If we take the adjacent circuit into operation, the measuring device shows a current of 9.5 mA. When connected in series, you only need to add the individual values to get the total value. So we now have a resistance of 470 + 470 = 940 ohms. In many circuits the difference compared to 1 kOhm is negligible.
|When connected in series, the same current flows through all resistors. But what about the tension? Doesn't the existing tension have to be split up? This is shown in the adjacent circuit. The multimeter shows a voltage of 4.5 V here. So half of the existing operating voltage. Does R1 have the same voltage?|
The device also shows 4.5V for this measurement. The voltage is divided equally between the two resistors. This principle also shows another property of resistors. The voltage divider. In this way it is possible to reduce the existing tension. This property is described in more detail below.
|Does the knowledge you acquire also apply to other values? We measure it. With this circuit we experience the same effect as with a single resistor. The current is reduced by more than half compared to the 470 ohm resistors. And the tension?|
|This measurement shows that the voltage is also 4.5 V as with the 470 ohm resistors. This should also apply to the 2nd resistance.|
|This measurement confirms it. For the voltage, it doesn't matter whether we use two 470 ohms or two 1 kOhm resistors. The voltage of 4.5 V is always set. Even if we were to change the value further, the voltage remains constant. Only the current changes.|
|If the resistors are connected in parallel, there are a few other laws. As we already know from a previous experiment, a current of 9 mA flows through a 1 kOhm resistor at a voltage of 9 V. Since here 2 resistors of 1 kOhm each are connected in parallel, the currents have to be added. This corresponds to 9 + 9 = 18 mA. Our measuring device also shows us this value. If further resistors were to be connected in parallel, the current would increase by the corresponding additional value.|
The voltage at the resistors in this type of circuit is always the same. We prove this with this measurement. This circuit can also be compared with our house electricity network. All sockets within an apartment are wired in parallel and are supplied with a voltage of 230V. This value does not change if consumers are additionally connected or removed.
|The current to be measured increases when the value of one or more individual resistors is reduced. When measuring within this circuit, a current of 38 mA results. Equivalently, the current would decrease accordingly if the values were increased.|
|To make it clear that the voltage does not change when the value changes, we measure it again here. Our multimeter confirms this. If you want to know how to calculate the total value of the resistance, there is a simple option for the same resistance. You simply divide the individual value by the number of resistors present. The last attempt then results in: 470/2 = 235 ohms. If the values are different, you have to use a different formula. You can do this in Look up formulas.|
|Of course, it is also possible to mix several types of interconnection, as was done in this experiment. Here we now have a total resistance of 970 ohms (1 kOhm / 2 + 470 ohms). According to Ohm's law, a current of approx. 9.2 mA should arise. The measurement will confirm it. Such circuits are usually not necessary, as there is a suitable resistor for almost every required value. In very few circuits, a value is required to be 100% accurate and it is therefore sufficient to select the next higher or next lower value. You can find out which values are commercially available from the international series of standards in my See table.|
Another property of resistors is the ability to create voltage dividers. Voltage dividers are often used when a voltage has to be reduced. The disadvantage, however, is that the current of the corresponding consumer must be firmly defined. If this is not the case, the voltage limitation must be achieved in other ways.
Here a voltage divider with a division not quite 2: 1 is shown. In this example, our multimeter shows a voltage of around 6.1 V. So about 2/3 of the operating voltage. As a result, the rest of the voltage must be present on R2.
|As this measurement shows, the rest of the voltage of approx. 2.9 V is applied to resistor R2. If you change the resistances, the tensions change accordingly. The ratio of the voltages corresponds to the ratio of the resistances. This can easily be determined using the rule of three.|
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