The rectification is the process of converting an alternating signal into a signal with a single polarity.
The electrical grid supplies us with a sine wave that changes polarity 50 times per second.
- For 10ms, wire A is positive with respect to wire B.
- For the next 10ms, wire A is negative with respect to wire B.
If we connect an electronic chip to this, we will burn it during the negative half-cycle. Digital electronics requires that the positive side is always positive.
To achieve this, we use the most basic semiconductor component: the Diode.
The Diode
Think of it as a gate that only opens in one direction.
- If the water pushes in the correct direction (Forward Bias), the gate opens and allows the flow to pass.
- If the water tries to flow backwards (Reverse Bias), the gate slams shut and blocks the path.
The diode does exactly this with electrons.
- It has an anode and a cathode, often identified on many packages by a band.
- In forward bias, it allows conventional current to flow from the anode to the cathode, with a voltage drop that depends on the type of diode, the current, and the temperature.
- In reverse bias, it blocks as long as its maximum rated voltage is not exceeded.
Half-Wave Rectification
The simplest thing we can do is place a single diode in series with the load.
- Positive Half-Cycle (+): The diode conducts and current passes to the load.
- Negative Half-Cycle (-): The diode blocks and, in the ideal model, the current is zero.
Result: At the output, we only have the “hills” at the top of the sine wave. The valleys at the bottom have disappeared (they are 0V).
- Advantage: Very cheap (1 diode).
- Disadvantage: We are throwing away 50% of the energy. Furthermore, the current arrives in widely spaced “bursts”.
Full-Wave Rectification: The Graetz Bridge
To use both half-cycles, we use a configuration of four diodes connected in a bridge.
The objective is clever: No matter which wire the current enters through (A or B), the bridge takes care of redirecting it so that it always exits from the same point towards the load.
Step-by-Step Operation
Positive Half-Cycle (A+, B-):
- Current enters through A. Diode D1 allows it to pass to the load.
- It returns from the load. Diode D2 channels it towards B.
- (The other two diodes are blocked).
Negative Half-Cycle (A-, B+):
- Now current enters through B. Here is the trick!
- Diode D3 (which is cross-connected) channels that current to the same positive input of the load.
- It returns from the load and leaves through D4 towards A.
Result: What used to be a negative valley, the bridge has “inverted” upwards. Now we have a succession of positive hills with no gaps in between.
- Output frequency: 100 Hz (double that of the input).
- Efficiency: 100% (minus the small voltage drop across the diodes, about 1.4V).
Filtering: The Capacitor
We now have a current that always flows in the same direction (it’s pulsating DC), but it’s terrible. It rises to 325V, drops to 0V, rises to 325V… If you power a computer with this, it will turn off every time the voltage hits zero.
We need to “fill in” the gaps. We need a buffer reservoir. This is where our old friend from Module 3 appears: The Capacitor.
If we place a large capacitor (“Filter Capacitor”) in parallel with the bridge output:
When the wave rises, the capacitor charges fully.
When the bridge’s wave starts to drop towards zero, the capacitor discharges its stored energy, keeping the voltage high.
The result is no longer a roller coaster, but an almost straight line with a small fluctuation called ripple. The larger the capacitor (
