6.1. Primitive power supply realization

When designing, it is necessary to pay special attention to the selection of components that can meet the power needs. The goal is to create a 5V power supply with a maximum current of 0.5A. Hence, the selected components are as follows:

power supply 220V
breadboard
fuse 63mA
2 x capacitor 0.015µF
transformer with the following ratio

4 x IN4007 diode – able to dissipate 3W of power
electrolytic capacitor 1000µF
4 x resistor 2.2Ω, 0.25W power dissipation – 2 x 2 network will be able to handle 4 * 0.25W = 1W
Zener diode 1N5338B – 5.1V, 5W power dissipation, ~1A maximum current, ~100mA minimum current (~10% of maximum)

Figure 6‑12 Primitive power supply schematic

Explanation: Let’s try to find a justification for the listed components. When calculating the current that will flow through the circuit, we must take into account the minimum amount of current that the Zener diode needs to conduct. If we look in the Zener diode 1N5338B datasheet, we can notice the values:

nominal voltage 5.1V
maximum current 930mA
minimum current – approx 10% of maximum current = ~ 100mA = 0.1A

Figure 6‑14 Zener diode as a voltage regulator
(Source: referenced)

Let us calculate the total amount of current needed, by looking at the previous figure:

according to the Kirchoffof’s current law, the sum of currents flowing into the node is equal to the sum of currents flowing out of the node

for the target current IL = 0.5A and minimum Zener diode current Iz = 0.1A

Thus, the total current required is Is = 0.6A. Let us now see whether the components can withstand that amount of current. The fuse we have chosen can withstand 63mA (0.063mA) which is a lot higher than needed but acceptable. Since the primary winding voltage is 23 times higher than the secondary winding, the current on the primary winding is 23 times less than the secondary winding:

The IN4007 rectifier diode is capable of dissipating 3W. Since the voltage drop across the diode is ~0.7V, it is easy to calculate the dissipative power

Further, let us calculate the needed resistor value Rs:

input voltage Vin = 6.5V,output voltage Vout = Vz = 5.1V
according to the Kirchoffof’s voltage law, the sum of the potential differences around any closed loop is zero, or

therefore, we have a resistor voltage

now, we can calculate the value of the resistor Rs

2.2Ω is chosen because we expect small increases in load current. Since the resistors can dissipate a power of 0.25W, we connected 4 resistors in a 2 x 2 network to withstand 4 * 0.25W = 1W, which should be enough. The calculations below should substantiate our claim, as we need ~0.8W:

let’s calculate whether the selected resistor corresponds to the specifications of the Zener diode. We will do this by calculating the minimum and maximum value of a resistor that can be connected in series with a Zener diode, without connecting a load. We will calculate the resistances based on the fact that the current is identical throughout the length of the series circuit. Therefore, if we take the conduction limit currents of the Zener diode, we can calculate