A diode is to electricity what a check valve is to the water. It allows current to go in one direction and blocks it in the opposite.
We can get the diode V-I curve with the test circuit below, where a variable voltage is applied to the diode, and the resultant current is measured.
The real characteristic shows that when the voltage is negative, no current flows. When voltage turns to positive, no significant current exists until V reaches over 0.5 to 0.6Volt. From there, with a slight increment in voltage, the current rises abruptly.
For practical uses, we will assume that the diode does not allow current to pass in inverse sense, and in direct sense the current can flow, but with a small voltage drop of 0.7 volts.
The diode´s ability to block an inverse voltage is not infinite, every diode has a breakdown voltage, which is the maximum inverse voltage that it can resist. This must be taken in account at the time of buying a diode.
The following table shows some common diodes with their commercial identification and main characteristics.
We will usually choose them by their current capacity (IF) and the breakdown reverse voltage (VR).
A diode is easy to check with a multimeter or tester. When we apply the probes as in the picture, the tester injects a very low current and senses the voltage. Let us say that 0.4V to 0.7V are normal values in forward connection, while on reverse the multimeter will indicate overflow.
In fact, the diode has arrived here in order to continue the job started by the power transformer. As we learnt in the previous chapter, the transformer gives us a power source with lower voltage than the line, but it is still an AC signal. For a secondary rms voltage of 12Vac, we get the following output:
It can be noticed that there is a positive hemicycle and a negative one. As we are aiming to a positive DC signal, it is not a bad idea to put a diode to allow the current flow only in one direction, and eliminate the negative hemicycle.
When AC voltage at the secondary winding is on the positive hemicycle, the diode allows current to flow to the load. On the negative hemicycle, the diode is in reverse polarization and does not conduct.
As a result, only positive hemicycles are present at the output, and we obtain a half of the available power, because half of the time, voltage and current over the load are null. In addition, it is not a good condition for the transformer, and the signal is too discontinuous or "noisy".
You'll remember that for a AC signal, given the rms value, it is possible to calculate the peak value with this expression:
Then, for a 12 Vac voltage, the peak value would be 17 V. But now we need to take into account about 0.7V voltage drop in the diode, that's why the half-wave rectifier peak voltage results 16.3 Volts.
Actually nobody uses the half-wave rectifier, if we buy a transformer we will want to get both hemicycles. The circuit below is a diode bridge, which provides a path for the current when the transformer's output is positive (during the positive hemicycle) and another path for the negative hemicycle, but both times the current arrive to the load (a resistor in this case) by the positive side. This is full-wave rectification.
We represented positive hemicycles in red, and negatives in blue. This way, looking at the red arrows you can follow the current's path during the positive hemicycle and the same thing with blue arrows on the negative hemicycle.
At the output, both positive and negative hemicycles converge to positive terminal, so the voltage over the load is always positive. This DC signal obtained at the output is still rippled, but not so discontinuous as in half-wave rectifier.
If we want to know the peak value of this output dc signal, we must multiply the transformer secondary rms voltage value by square root of 2, and subtract the voltage drop on diodes (there are two diodes on the current's path):
The rectified signal output would look like this:
The maximum value is a required parameter to design a power supply, here you have peak values calculated with the previous expression for a full-wave rectifier with many typical secondary transformer voltage:
Center-Tapped Full-Wave Rectifier
Certain transformers have three secondary terminals instead of two. They are called center-tapped transformers. The center terminal can be named ground; when the upper terminal is on its positive hemicycle, the bottom terminal is in the negative hemicycle, and vice-versa.
For example, a center-tapped transformer with two 12VAC secondary windings is known commercially as "line to 12+12VAC transformer". If we took this transformer, and forget its center terminal, what we have is a 24VAC transformer (between the extreme terminals).
In order to rectify the 12VAC signal with this kind of transformers, the following circuit applies:
Once again, we can follow the current's path during the positive hemicycle looking at the red arrows, and the same thing with blue ones on the negative.
Output is almost the same, with the difference that current flows through only one diode each hemicycle, so the voltage drop is 0.7 V instead of 1.4V. For example, with the transformer shown on the picture the peak value at the output is:
This is the waveform:
And some typical values:
Thus, with center-tapped rectifiers we need only two diodes instead of four, and we get 0.7 volt more than with the diode brigde. As a counterpart, center-tapped transformers tend to be a little bigger than normal transformers for the same power.
You will need to use one of these configurations every time you start a new project, so please ask if there is anything that is not completely clear.
On the next chapter we will introduce the capacitor and learn how to use it to filter the rectified output in order to get a smoother signal.