A power supply is used to operate the electrical power for the equipment that may not be connected directly to the 220V socket, but require a different voltage, typically much lower, similar to that provided by batteries.
To make sure that the alternating voltage provided in the outlets of the house be equal to that of a battery, the power supply uses several components, each with a specific function: we will see what are these components, examining the construction of the simplest of the power supplies.
The transformer has the task of lowering the supply voltage; it is composed generally of two distinct windings: one, entry, said primary, which is connected to 220V; an output, said secondary, which provides a voltage lower than the inlet voltage , suited to the needs of the user, that is, the device that you want to feed.
Depending on the types, the transformer may have one or two secondary windings; we will see how to make the best use either one type than the other.
The voltage coming out of the transformer can not supply a device made to run with the batteries; while the batteries have in fact a DC voltage, the voltage that comes from the transformer is still an alternating voltage, which means that changes continuously polarity (to be exact: 50 times per second). It should then “straighten” this voltage, to obtain that the user uploads a direct current flow always in the same direction. The task of blocking the current at times when the flow is reversed is entrusted to the diode; you can use one, two or four diodes, according to various circuit that we will soon see.
The Capacitor Levelling
The AC voltage which comes from the transformer, as made unidirectional by the diodes, has not a constant value: its value changes continuously, going from zero to a maximum value, and this happens, as we said before, fifty times in a second . The capacitor that is added to the circuit operates as a reserve tank: stores energy when the voltage is maximum, and returns when the voltage tends to drop.
We observe the chart that follows: at the top we see the waveform that has the 220V mains voltage to the input of the transformer
At the center we see the voltage that is obtained in output after straightening with a half-wave rectifier, or to only one diode
The bottom part shows the voltage that is obtained in output after it is rectified with a full wave rectifier, such as those that use 2 or 4 diodes.
Three Circuits for a Power Supply
The circuit to be used depends on the transformer you have.
If the transformer has only one secondary winding (such as that of Figure 1a) is possible to realize the diagram of Figure 5, which uses a single diode, or that of Figure 6, which uses four.
The circuit of Figure 5 is simpler, but since sees only one half-wave of the alternating voltage is more suitable for users that absorb little current (not more than 50 mA).
When necessary a stronger current is good to use the pattern with four diodes (Figure 6), which uses both half-waves and therefore allows a better leveling of the output voltage.
If the transformer is equipped with a secondary winding double, that is, with center tap (such as the transformer of Figure 1b), it is possible to straighten both half-waves of the alternating current using only two diodes (circuit of Figure 7). In the case of Figure 1b, the diodes are connected to pins 3 and 6, while the center tap is obtained by connecting together the pins 4 and 5.
The described power supply is very simple, so do not have a voltage regulation system that the user arrives; to obtain the desired output voltage, the only way is to use a transformer whose secondary give a very precise voltage. Let us see how the transformer secondary voltage will be calculated:
- VU call the tension that has to come to the user
Transformer Output Voltage Transformer type 4,5 220/4,5 6 220/5,5 9 220/8 12 220/10,5 15 220/12,5 18 220/15 24 220/20
- Add 1 to the value of V to take account of the voltage drop in the rectifier diodes
- We divide this number by 1.41 to switch from the maximum value to the RMS value
- We multiply the value obtained for 1.1 to take account of the voltage drop in the transformer during operation under load
With these calculations you are obtaining Vs and that is the value of the voltage that needs to have the secondary of the transformer; Who does not want to do calculations , the table on the right are the values already calculated. It should be noted that the values shown are approximate, partly because, unless you do it on purpose winding, hardly able to find a transformer with the exact tensions.
The transformer must then be suitable to the power required: It must multiply the user’s operating voltage for the current that it requires; the value obtained must be increased by about 20% if the power supplies must operate intermittently, or by 40% in the case of prolonged or continuous operation.
Example: a device must operate at 12 V and consumes a current of 1.5 A; the power of the transformer is:
- 12 x 1.5 x 1.2 = 21.6 VA (for intermittent operation)
- 12 x 1.5 x 1.4 = 25.2 VA (for continuous operation)
|1N4001||50 V||1 A|
|1N4003||200 V||1 A|
|1N5400||50 V||3 A|
|1N5401||100 V||3 A|
|1N5402||200 V||3 A|
|6A4||400 V||6 A|
The diodes must be chosen based on the voltage and the current flowing through them. For the voltage, there are no problems, considered that any rectifier diode can operate safely up to a voltage of at least 50V. The current should be calculated in view of the fact that, when turned on, the diodes are crossed by the strong current spike that goes to charge the capacitor completely discharged electrolyte; for this reason it is good to use diodes can withstand higher currents than those required by the user, and the more so the greater the capacity of the smoothing capacitor. In particular, in the rectifier to a single diode, it must be considered that the current flows in the diode itself only for half the operating time, so its flow is discontinuous, with the double value peaks.
The adjacent table shows the rectifier diodes most commonly used.
The capacitor must be suitable to the power supply output voltage; about its capacity, this depends on both the current required by the user, is used by the circuit: with a rectifier to a half-wave, for example, a capacitor should be of double capacity compared to a rectifier in two half waves.
The capacitor calculation is quite complex, and also takes into account the percentage of residual ripple you are willing to accept outgoing; for this reason advice then to proceed experimentally, using for example a value of the initial capacity of about 1000 uF. In some cases we find that this value is too low: for example, using the power supply to operate a radio transistor , you hear a hum in the speaker; in which case it is permissible to gradually move to higher values, like 2200, 3300 or 4700 uF (you will notice that the buzz decreases as).
Many times, the devices that run on batteries have an outlet intended specifically for connecting to a power supply unit; it is almost always a coaxial jack, near which, on the casing of the device, is marked polarity of the wires to be connected (positive center, negative contact on the external contact – or vice versa). To connect to this socket, we must equip our corresponding power supply unit (Figure 8), taking care to connect correctly in the positive and negative wires.