  Radio and Electronics (DED Philippinen, 66 p.)  10. BLOCKS OF RADIOS / -1- / POWER SUPPLIES  10.1. GENERAL CONSIDERATIONS 10.2. TRANSFORMER 10.3. THE RECTIFIERS. 10.4. SMOOTHING AND FILTER CIRCUITS 10.5. STABILIZATION

10.3. THE RECTIFIERS.

GENERAL REMARK

- So you will hardly meet a valve radio in practice nowadays, here we will deal only with semiconducting rectifiers.

- So the different kinds of rectifier circuits are already taught in Form III they will not be a matter of this chapter.

- This chapter will deal with questions which might arise, if you have to check or to repair a rectifier in a radio.

HOW TO FIND OUT THE TYPE OF RECTIFIER CIRCUIT?

A) By checking the components and their number of terminals:

The following table shows the different cases you can meet and the types of rectifier circuits, used in those cases:

 number of rectifying components or units number of terminals of these components or units type of rectification achieved in this case 1 2 one-way rectification with one diode 1 4 double-way rectification with bridge unit 2 2 double-way rectification with a centre tapped transformer 1 3 double-way rectification with a centre tapped transformer with two double diode units 4 2 double-way rectification with four diodes

B) By checking the identification of the components:

There are two cases possible:

- Either you find only diodes used in the rectification circuit - then you will find on them two letters and a figure.

The first letter determines the material used: A= Germanium / B = Silicon

The second letter determines the normal use of this diode: Y= rectification / Z = Zenerdiode

The number tells You where in the data book, you will find the specifications for this very diode:

- or You find a rectifier unit big enough to carry specifications on itself: for example: B250 C 100

Here the first letter shows the type of the unit (B = bridge type / M - centre tapped type / DB - threephase bridge type) the figure 250 shows the maximum reverse voltage allowed

The letter C shows the allowed type of load (here a capacitor/see next chapter). The last figure 100 gives the permissible forward current (here 100 mA) fig. 92

WHICH VALUES ARE VERY IMPORTANT?

For this question you can find an answer very easily if you look to the specifications of rectifier units explained in the last chapter.

It is: the reverse voltage, and the forward current. fig. 89

If You recollect the characteristics of a diode you will easily understand why a diode is limitted just with those two values:

Find out yourself:

What can you do, if you want to repair a radio and you do not find: a) a diode with a satisfying forward cur rent? b) a diode with a satisfying reverse voltage?

ADDITIONAL REMARKS ON THE PROBLEM OF “REVERSE VOLTAGE”.

As you saw in the chapter above on bridge units you find a special letter showing, which kind of load is permissible for this very unit.

This fact you will understand after considering the following chapter:

a) rectifier loaded by a resistor:

In forward direction the current flows through the diode, causing almost no voltage drop at the diode. In reverse direction there is no current flowing through the diode, but the total voltage drop at the diode is equal to the supply voltage. fig. 90

b) rectifier, loaded by a capacitor:

In forward direction the conditions are the same as if the load is a resistor. But after the capacitor was charged during the positive halfwave, during the next negative halfwave, the reverse voltage will be the sum, of the voltage at the capacitor and the supply voltage. So the reverse voltage will be twice as high as the voltage of the supply fig. 91

This shows, the diode supplying a circuit with a capacitor has to withstand higher reverse voltages.

CONNECTION OF RECTIFIERES

On a diode we find always a ring on its cover. This ring shows the ANODE of the diode.

At rectifier units we find at the terminals the following signs:

- means ac-input
+ and - show the de-output.