The classic double alternation circuit, studied in the previous chapter, is very interesting because it operates as an intensity doubler and the DC output voltage is easy to filter, the frequency of the AC component being 100 Hertz.
This type of power supply however has a disadvantage : the need to use a transformer having a secondary with a midpoint, that is to say, almost two secondary. As a result, the transformer is expensive and bulky.
To overcome this disadvantage, there is realized another type of power supply, having the advantages of full-wave mounting and operating with a transformer having only one secondary, or even directly on the sector.
This new circuit, called bridge circuit is shown in Figure 22.
It uses four separate rectifiers of the same type, or a bridge rectifier (single box with four rectifiers).
The operation of the circuit is as follows :
a) - During the positive alternation of the voltage, that is to say when the points A and D have the polarities shown in Figure 22, D1 is blocked and D2 leads.
The current then flows from point A to point B, and through the load reaches the mass (point C), then thanks to D4 reaches the point D, to return to secondary.
b) - During the negative alternation of the voltage, that is to say when the points A and D have the polarities indicated in Figure 23, D3 leads and D4 is blocked.
The current flows from point D to point B via D3 and reaches the mass through the load, then, thanks to D1 reaches point A, to return to secondary.
The two half-cycles of the period being rectified, the circuit operates as a doubler of intensity.
The DC voltage without a capacitor at the head is :
US = 0,9 x U eff.
With a capacitor, this voltage goes on, as in the case of the "full wave" rectification at its value :
US = 1,41 x Ueff.
The reverse peak voltage, on the other hand, is less important than in the assemblies studied so far.
Indeed, by examining Figures 22 and 23, we see that for each alternation, there are always two rectifiers that lead (instead of one in the previous montages).
As a result, the reverse peak voltage is reduced by half.
So we have :
Peak reverse voltage : 1.41 x U eff (instead of 2.82 x U eff for single and full-wave rectifier mounts).
Viewed from a different angle, it can be said that from the alternative point of view, the zero level passes successively from point A to point D, depending on the alternations.
These two points being connected directly to the mass via D1 and D4, we can consider neglecting the internal resistance of the rectifiers, that during negative alternations, the points A and D are at the potential of the mass, it is at zero potential.
Each rectifier thus supports only the peak value of the positive half-wave.
As for the value of the DC output voltage, with a normal load, it is equal, as in the case of full-wave mounting to :
US = 1,2 x U eff
The table in Figure 24 summarizes the features and benefits of bridge feeding.
All that has been said elsewhere in the previous chapter remains valid for this type of power supply (filtering, load calculation, protection resistance, etc.).
Under the name of voltage multiplier, we find a series of assemblies, whose essential property is to double, triple, quadruple, etc ... the value of the applied voltage.
The most used circuits are of the doubler or voltage tripler type.(If you do these montages, be very careful because there is very high voltage can cause death, read our lessons carefully).
The most well-known voltage doubler assembly is called the LATOUR doubler. This one, due to the French engineer Marius LATOUR, is represented Figure 25.
The operating mechanism is as follows :
a)During the positive alternation
D1 leads, while D2 is blocked. The electrochemical capacitor C1 charges, the current flowing through D1 - C1 and returning to secondary by the point B (conventional direction).
For example, if the AC effective voltage is 100 Volts, the peak DC voltage between A and B is :
100 x 1,41 = 141 Volts
b)During the negative alternation
D1 is blocked, while D2 is driving. The electrochemical capacitor C2 charges, the current flowing from point B to C2 and returning to the secondary through D2 (conventional direction).
The peak DC voltage between B and C is then :
100 x 1,41 = 141 Volts.
Capacitors C1 and C2 being connected in series, the DC voltage between A and C is :
141 + 141 = 282 Volts (at empty).
Components C1 and C2 therefore behave as two DC voltage generators in series (see Figure 26).
Of course, the doubling effect occurs in the presence of a load between A and C, if the capacitors C1 and C2 have a sufficient capacitive value.
The exact value is related to the time constant of the usage circuit.
For an AC voltage of 50 Hertz and for normal data rates in electronics (on the order of 50 to 400 mA), this value is generally 100 µF.
The maximum flow rate requested from the power supply must not be greater than the maximum current that can be supplied by a single rectifier.
It should be noted that in this arrangement C1 and C2 provide doubling of the voltage and fulfill the role of filter input capacitors (snoring voltage = 100 Hz).
Each of these capacitors, however, has to support only half of the maximum value of the output voltage (1.41 x U eff).
The table in Figure 27 summarizes the features and benefits of the voltage doubler assembly.
It should also be added that the voltage doubler assembly can be used directly on the mains without the intermediary of a transformer. On the other hand, two DC voltage values can be output :
1) between A and B : US = 1,2 x U eff
2) between A and C : US = 2,4 x U eff.
Example for the mains voltage 220 Volts : 220 x 1,2 = 264 Volts and 220 x 2,4 = 528 Volts or 264 + 264 = 528 Volts (Be careful).
The typical failure occurring in this assembly is due to the aging of C1 or C2.
Indeed, when the capacitive value of these components becomes relatively low, as a result of the drying of the dielectric, the DC output voltage no longer has a value equal to 2.4 x U eff.
It is then necessary to replace these two components.
Another type of well-known voltage doubler is the SCHENKEL doubler (Figure 28).
The operation of this assembly is based on the same principle as that of the doubler of LATOUR, that is to say by load of C1 and C2.
Let us first specify that the lower end of the secondary being grounded, we will always have for point B, the zero reference level (zero potential).
However, in the alternative, point B (zero level) will be very positive compared to point A, when it will be negative
Indeed, if the secondary AC voltage is 100 Volts, the point A will pass relative to the point B from - 100 Volts to + 100 Volts, according to the alternation considered.
So, when we get to A = - 100 Volts, we can say that point B is positive in relation to point A.
Now let's see the behavior of the elements, according to each alternation.
a)During the negative alternation (Figure 28).
Point A is negative, so D2 is blocked.
On the other hand, the point B is positive with respect to A, which amounts to saying that the anode of D1 is positive with respect to its cathode.
Therefore, D1 leads and charges C1 with the polarities shown in Figure 28.
The voltage across C1 is equal to the peak voltage of the secondary.
b)During the positive alternation (Figure 29).
Point A is positive with respect to B.
So D2 drives, while D1 is blocked.
The voltage applied to D2 results from the series setting of the mains voltage during the positive half cycle and from the previous voltage, to which C1 has been charged (note the polarities between B and C1 for Figures 28 and 29).
So, D2 straightens E2 = 2 E1
Capacitor C2 therefore loads at this value, that is to say at 2.82 x U eff (twice the peak value).
As before, the capacitors are subjected to voltages of invariable polarities ; however, C2 supports the value of the doubled HT (2.82 x U eff), while C1 only supports a voltage equal to the peak value of the sector (1.41 x U eff).
The reverse peak voltage applied to D1 and D2 is 2.82 x U eff.
The doubling of the voltage however only depends on C1, the capacitive value of this component increases with the current.
C1 = 100 µF for a maximum current of 200
mA,
C1 = 150 µF for a maximum current of 300
mA,
C1 = 300 µF for a maximum current of 400
mA.
C2 also acts as a filter input capacitor.
This capacitor however is recharged only with each positive alternation. The humming voltage is therefore 50 Hz, which implies a very efficient filtering cell.
Voltage multipliers
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