6. Power supply

For the last experiment of the presentation of basic building blocks, we will present a simple power supply circuit. It is highly recommended not to implement this on your own because the voltage of the power main is deadly! Doing it wrong can cause serious injuries and death! I will build a primitive power supply, according to the block diagram below, and then use the professional one, which is much more complex and stable. It is important to notice that I have succeeded to understand the workings of the power supply circuit through the brilliant explanation of Mr. Stan Gibilisco, in the book Teach Yourself Electricity and Electronics.

Figure 6‑1 Power supply block diagram
(Source: referenced)

For the voltage source, a standard 220V AC power source is used. The fuse and transient suppressor help protect the circuit being supplied, or the load. The transformer reduces the input AC voltage and the rectifier turns it into a fluctuating DC voltage. The fluctuations are reduced with a filter, and the voltage regulator provides constant, stable direct voltage.

Fuse

In the power supply block diagram, we can notice that the fuse is the first component. A fuse is a very simple piece of wire that melts if the current exceeds a certain value. The fuse, therefore, ensures the safety of the source and the load, so that its components do not burn out in the event of a sudden and excessive increase in current.

Figure 6‑2 Fuse

Transient suppressor

Although the standard voltage source is constant and stable, the possible appearance of voltage spikes of much higher values puts the load in serious danger. In addition to external causes such as thunder, magnetic fields, and static electricity, voltage spikes also occur due to sudden changes in the load connected to the source, so we must protect ourselves with a stabilizer. I will build a simplified stabilizer from capacitors that, due to their properties, can absorb these sudden changes. Capacitors with a value of 0.01 µF ground both poles of the source, according to the figure.

As already explained, the capacitor is a passive electronic element that serves as a reservoir of static electricity. It consists of two electrically conductive bodies separated by an insulator. Since conductive bodies do not make a connection, the capacitor blocks direct current. The charges are therefore stored at the poles of the capacitor, creating an electric field. However, for alternating current at a certain frequency, the capacitor acts as a short circuit and conducts it. According to the formula

for the reactance of a capacitor, by which we express its resistance to the passage of current, it is clear that it increases with increasing frequency. The voltage spike is very short, so it is easy to compare it to high-frequency alternating current due to the formula for frequency

Therefore, in these short moments, the capacitor is conductive and easily grounds the voltage spikes and preserves the circuit. In this context, a capacitor is also called a bypass capacitor.

Transformer

In order to reduce the input voltage, the transformer is used. A transformer is a device that inductive couples two alternating current circuits. In our example in the figure, the iron core of magnetic properties is organized so that the primary and secondary windings of the wire are not in direct connection.

The alternating current with its flow through the primary winding generates a magnetic field that fluctuates around the secondary winding and induces an alternating current of the same frequency. We call this extraordinary phenomenon electromagnetic induction. The iron core has good magnetic permeability, so it facilitates the transfer of electricity from the primary to the secondary winding. Voltage amounts are referred to as winding numbers which tells us the transformation ratio formula

If the number of secondary windings is less than the primary, as in our case, the voltage will also decrease. In this very simple way, we can, for example, reduce the input voltage from 220V to 10V. If we are interested in the transformation ratio, it is

It is important to note that the transformer does not violate the principle of conservation of energy. It means that the power of the primary winding must be equal to the power of the secondary winding, considering there are no losses in the transformer. Let us express this with the formula:

It follows that the current relationship is the inverse of the voltage relationship.

Finally, for our example, since the voltage on the primary winding is 22 times higher than the voltage on the secondary winding, the current on the primary winding is 22 times less than the current on the secondary winding.

Rectifier

Once the AC voltage is reduced, we need to figure out a way to convert it to DC. This is the purpose of the rectifier and it will convert AC to fluctuating DC voltage. The operation of the rectifier is based on the properties of the diode, which we already explained in detail. If we connect our transformer to the diode according to the schematic below, we will partially achieve our goal.

Figure 6‑5 AC to DC conversion with diode
(Source: referenced)

It is not difficult to conclude from the graph that the effective voltage is significantly lower than the peak voltage because we use only one of the half-cycles of AC voltage. To take advantage of both half-cycles, e will construct a Graetz rectifier, a matrix of four diodes, as shown in the figure.

Figure 6‑6 AC to DC conversion with rectifier
(Source: referenced)

The graph shows us that the effective voltage has increased. The working principle is explained in the figures below. To use the positive half-cycle, we use the first two diodes and realize a circuit.

Figure 6‑7 Rectifier usage of AC positive half-cycle
(Source: referenced)

To use the negative half-cycle, we use the second two diodes and realize a circuit.

Figure 6‑8 Rectifier usage of AC negative half-cycle
(Source: referenced)

Filter

To reduce fluctuations, the capacitor is used again, but this time electrolytic, of higher capacity. The principle of operation of the filter is very simple. As the voltage rises, it is stored on the capacitor. When the voltage drops, the stored voltage corrects fluctuations, acting in the same direction.

Figure 6‑9 Removing the fluctuations with the capacitor
(Source: referenced)

Voltage regulator

Finally, we have a stable voltage, but much higher than the required value of 5V. In order to reduce the voltage, we will use a Zener diode. The Zener diode is specially and precisely made so that its breakdown voltage is strictly defined, low and constant. The figure shows the principle of operation of a simple voltage regulator.

It is extremely important that we do not miss that the Zener diode is turned in the opposite direction because it is driven by breakdown voltage. Also, the Zener diode must be connected in series with the resistor Rs, which serves to limit the current to which the Zener diode, and then the load, will be exposed. It is therefore necessary to check from the specification the minimum current of the Zener diode that causes conductivity, and the maximum current to which the Zener diode can be exposed. The Zener diode will easily absorb changes in current value within the prescribed current limits, keeping the voltage constant. The load is connected in parallel to the Zener diode, so it will be exposed to a voltage equal in value to the breakdown voltage of the Zener diode. When realizing the resistor Rs, it should be ensured that the resistor can dissipate the power to which it is exposed. For example, if we want to ensure that a 30Ω resistor can withstand a current of 0.5A, the power to which the resistor will be exposed will be

If we use a resistor with a rated power of 1W, it is obvious that it cannot dissipate a power of 7.5W. To solve this problem, we can create a series-parallel network of resistors of the same values, according to the figure below. If we make an n x n matrix, the rated power will increase n² times. In the presented case, a 2 x 2 matrix would allow 4W, so we would opt for a 3 x 3 matrix that can dissipate 9W power.