Got questions like how to get a LED on? How to decrease the RPM of a fan?

The aim of this article is to search for answers and solutions to the questions mentioned above. To try to keep the terms simple and easily to understand and of course and for sure, to rise new questions. After you have read this, you should have the basic skills to calculate required resistances, power usage, etc. Let's see how we manage there.

Index:

Voltage
Current
Ohm's law
Kirchhoff's first law
Kirchhoff's second law
Power
The resistor
  Serially connected
  Parallely connected
  Color codes
  The E-series
The LED
The transistor
Examples
  Connecting two superbright blue LEDs in parallel

Voltage

The quantity is U and the unit is volt (V).

Voltage means the differences in charges in an electric state between two points. Difference in potential. When you connect these points, the state electric strives to get balanced, thusly creating an electric current.

 

The difference in potential can be visualized with a model of a waterfall. The higer the fall, the higher the force of the water which it walls with. If you check with a point in a middle of the fall, we notice that the force of the water isn't as strong as lower down.

The same phenomenon applies in the PC's power supply. The potentials most commonly used are +12, +5 and ground. A common connection where you benefit from the difference of potential is a case where you want to give a fan a lower voltage than 12 volts, but higher than 5 volts. Both of these voltages are the ground compared to the potential. But, if we compare them with eachother instead, meaning we connect the fan between the 12 volt and 5 volt wires, the difference of potential is now only 7 volts. A bit like in the waterfall-model; the water doesn't have to fall all the way to the ground, making its force smaller. This way, also the difference in potential is smaller. 12 v - 5 v = 7 v.

Current

The quantity is I and the unit is ampere (A).

The leveling of electric charges appears as current. It is caused by the movement of electrons in the conductor. The electrons travel from the negative (minus) pole to the positive (plus) pole, but it is a common agreement that the current goes in the direction from plus to minus.

Ohm's law

Probably one of the most important laws that electronics contains. With the help of this law, you can calculate the relations between voltage U, current I and resistance R.

When you cover the desired quantity from this formula, you have the required calculation in front of you.

When you know the voltage and resistance, the current is calculated with the formula I= U/R

When you know the voltage and current, you can calculate the resistance with R= U/I

When you know the current and resistance, you calculate the voltage with the formula U= I*R

Example:

We want to light up a red LED from a line with five volts. Each LED has its own specific voltage threshold and current. In our example LED, let's use a voltage of 1.6 volts and the current is usually 20 milliampere in LEDs. (20 mA = 0.02 A).

As said, the power source supplies us with five volts, but our LED needs only 1.6 bolts. We need to get rid of this excess voltage, for which we will use a resistor, which transforms this voltage into heat.

So, the LED takes 1.6 volts, so the resistor should remove 5 V - 1.6 V = 3.4 volts from the connection. To get the resistor to eliminate the correct amount of voltage, we will calculate a resistance value for it. Because we know how much the resistor must eliminate of the voltage, and we know that a current of 0.02 ampere will go through the connection, we can use a familiar formula for this:

R= U/I

With the right values placed in their rightful places, we get 3.4 V/0.02 A = 170 ohm. The closest resistor available for this situation is 180 ohm. As we are forced to connect a resistor with 10 ohm more than we calculated for, the LED will shine a tad darker and the current going through the connection is a bit smaller than 0.02 A.

Kirchhoff's first law

This law presents you a model according to which the currents of components connected in parallel are summed up with eachother. The current going to the circuit is the same as the one going out from it.

If we now switch the power on, a current of 0.02 amperes would go through each LED. So the whole circuit requires 0.02 A + 0.02 A + 0.02 A = 0.06 amperes.

Kirchhoff's second law

According to the law, the sum of losses of voltage within a circuit equals the voltage of the power source.

Because we have two LEDs and one resistor, and because we know that the voltage threshold of the LED is 1.6 volts, we are able to calculate mow much the excess voltage will be. 5 V - 1.6 V - 1.6 V = 1.8 volts. We know that the current in the connection is 0.02 amperes, because we have a serial connection. This way, we can calculate the value for the resistance with the help of Ohm's law. The voltage that exceeds the resistance divided by the current going through the resistor: R= U/I= 1.8 V / 0.02 A = 90 ohm. The closest resistor available is 82 ohm.

Power

Different resistors have different endurance to power. This is important to remember, as the function of a resistor is to turn the exceeding voltage into heat. Because of this, it is important to know the power which is going through the resistor, so that we can choose the right resistor with the right power endurance.

So, you count the power using the voltage U and current I. For example, a serial connection including one resistor and one LED. The resistor is affected by 3.4 volts and the current going through it is 0.02 amperes. Thus the power affecting the resistor is 3.4 V * 0.02 A = 0.068 watt. Here you have an example with several LEDs.

Example:

This circuit includes one resistor which limits the voltage to suitable levels for three LEDs. The LEDs have to be similar. This time, we have a 12 volt powersource.

First, let's calculate the total current flowing in this connection. According to Kirchhoff's first law, we sum the current of three parallel LEDs with eachother.

0.02 A + 0.02 A + 0.02 A = 0.06 amperes.

Then we calculate the voltage affecting the resistor with Kirchhoff's second law. The voltage going through the LEDs subtracted from the voltage from the powersource.

12 V - 1.6 V= 10.4 volts.

The power affecting the resistor will thusly be:

10.4 V * 0.06 A = 0.624 watts.

The smaller and cheaper resistors can endure about 0.25 W, so one of those wouldn't last for long in this connection. So, we are forced to pick a resistor which can endure at least this 0.6 watt. One-watt resistor would be enough, for example.

The resistor

As its name would indicate, the function of a resistor is to resist the flow of electric current, so it causes a loss of voltage. This treat is caused by the material it is fabricated out of, which leads poorly electricity. The physical property of a resistor is resistance, which is labeled with R and the unit is ohm.

Resistors connected serially.

The resistance of this connection is 150 + 150 + 250 = 550 ohms.

Parallel connection of resistors.

The result is: 1 / (1/150 + 1/150 + 1/250) ~ 57.69 ~ 58 ohms.

Color codes

The values of resistors are indicated with color codes. The code is read by starting in the end which is closer to the first colored ring.

The first ring indicates the first number value of resistance, and the second ring indicates the second number of resistance. The third ring indicates how many zeroes are to be added ('the place of the comma'). This value is in ohms. The fourth ring indicates the tolerance, which means the accuracy of the amount of ohms in percents. A table which helps you to read the value follows:

E-series

To avoid constructing a resistor for every amount of ohms, there is something called the E series. This table enables you to find the closest available resistor. The most common one is the E12-series which defines that each decade has 12 different values of resistance. A decade means tenfolding of a value. For example, from 10 to 100.

You get the the real resistance values by moving the comma. E.g. 0.15 ohm, 1.5 ohm, 15 ohm, etc.

So, if you want to find a closest match for a calculated value of 175 ohms, you multiply the values of this table by 10. This way, the closest match is 180ohm.

Examples of resistors:

  Yellow, violet, orange and silver. 4 and 7 multiplied by 1000. 47 kilo-ohms with a tolerance of 10 percent.

  Red, red, brown and silver. 2 and 2 multiplied by 10. 220 ohms with a tolerance of 10 percent.

The LED

The LEDs (Light Emitting Diode) are these funny things that light up when the required voltage threshold is exceeded. The LEDs can be found in various colours, brightnesses, diameters and so on. A LED requires always a resistor to limit the current going through it. You can see every now and then connections where this has been ignored. In these cases, the LED isn't going to last for long and often causes functioning problems for the rest of the circuit. When you are calculating the resistance, you need to know the voltage threshold and current of the LED. Exceptions are those blinking LEDs which have a built-in resistor, so they don't need an external one. General values for threshold voltages of LEDs:

The current is usually about 20mA. This is a good value to remember, if you don't know the exact value. The calculation of resistance you will use Ohm's law, which is presented earlier in this article.

The transistor

A transistor is often used to enhance a signal. This way, it is suitable for example for an electric switch which controls a greater current. Simplified, you could say that a transistor is a variable resistor, controlled by electricity. With a small controlling current you can alter the resistance of the resistor, which affects the greater current which is desired to be controlled.

There are two types of transistors, PNP and NPN. The most common one is the NPN transistor. This transistor is suitable for controlling a large amount of LEDs than you would be able with direct connection, for example. Here you have an example of a connection which would enable a NPN transistor to control six leds from the hard drive connector of your motherboard.

The value of a preresistor of a LED is counted by assuming that each of the LEDs have a threshold voltage of 1.6 volts and the current is 20 mA. Three LEDs in a serie, so the voltage is 4.8 v. The transistor take some voltage too, a suitable average would be 0.3 volts. The resistor is left with 12 V - 4.8 V - 0.3 V = 6.9 V. The LED-series are two in parallel, so the current through the resistor is 20 mA + 20 mA = 40 mA. According to Ohm's law, the value of the resistor should be 6.9 V / 40 mA = 172.5 ohm (180 ohm). The power through the resistor is P= U*I, which means 6.9 V * 40 mA = 0.276 watt. Because of this you maybe wouldn't be okay with one of those cheaper resistors, as they can endure only 0.25 watt.

Examples

Two superbright LEDS in parallel

We'll make a connection including two superbright LEDs in parallel and those connected to a five volt line. This can be applied to the KeySpot, GlowPad mods, and of course illuminating the case interior, and much much more.

The voltage threshold for the LEDs are 3.8 V and the current 20 mA. Because the LEDs are in parallel, they are affected by the same voltage, but the current is doubled to 40 mA. The voltage affecting the resistor is therefore 5 V - 3.8 V = 1.2 V. The value for the resistor should therefore, according to Ohm's law, be R= U/I= 1.2 V / 0.04 A = 30 ohm. Because the E-series doesn't include a resistor of this value, we have to choose the closest match, which is 33 ohms.