The rectification is the process of converting an alternating signal into a signal that flows in a single direction. If we add filtering, we get a much more stable voltage, useful for powering real electronics.
So far, we have assumed we have perfect 5V or 12V available for our circuits. But the reality is that energy comes to us as 230V AC (or 110V AC, depending on the country) from the wall outlet.
If you connect your board directly to the wall… well, smoke will come out and breakers will trip.
To power our electronics, we need transformation, rectification, and filtering. Today we will focus on the last two, which are the basis of energy conversion.
Although we explain the theory here, working with the mains electricity involves a risk of electrocution. If you are a beginner, always use sealed commercial transformers that already deliver a safe, isolated low voltage.
The Transformer
The first step is to step down the voltage. The transformer takes the 230V AC from the mains and, through magnetic induction, delivers a more manageable voltage, for example, 12V AC.
It is still alternating current. The signal completes 50 cycles per second (50 Hz) and changes polarity in each half-cycle: it goes up to positive, crosses zero, goes down to negative, and goes up again.
Our chips need a fixed polarity (stable positive and negative). If you feed negative voltage to a chip, you kill it.
Rectification: The Diode Bridge
We need to “flip” the negative part of the wave so that everything becomes positive.
Half-Wave Rectification (The Bodge)
If we use a single diode, we block the negative part.
- Result: We have voltage half the time and zero the other half.
- Problem: It is very inefficient. We are throwing away 50% of the energy.
Full-Wave Rectification (The Graetz Bridge)
Here we use the famous rectifier bridge (4 diodes, like the 1N4007, connected in a diamond shape).
This arrangement of “valves” works like this:
When the current flows in the positive direction, two diodes conduct it to the output.
When the current flows in the negative direction (reverse), the other two diodes “redirect” it so that it also comes out as positive.
Result: We utilize both half-cycles. We obtain a succession of positive “humps” that still drop to zero between peaks. We have made the current always flow in the same direction, but the voltage constantly rises and falls from 0V to 17V (approx.).
Filtering: The Electrolytic Capacitor
No digital circuit can work with those “humps”. It would turn off 100 times per second every time the voltage hits zero.
We need to fill the valleys between the mountains. For this, we use a filter capacitor (the water tank).
We place a large electrolytic capacitor (e.g., 1000
Rising: When the bridge voltage rises, it charges the capacitor to its maximum.
Falling: When the bridge voltage starts to fall (the hump decreases), the capacitor begins to release its charge to the circuit.
Result: The voltage no longer drops to zero. It stays high, dropping only a little until the next pulse from the bridge arrives and recharges it.
The Ripple
That small drop and rise that remains is called the ripple voltage.
- Small capacitor: discharges very quickly. There is a lot of ripple and poorer output quality.
- Large capacitor: discharges more slowly. There is less ripple and a much more stable line.
Additionally, in full-wave rectification, the capacitor recharges twice per cycle, so the ripple appears at double the mains frequency. This helps significantly in making the output easier to filter.
The Peak Voltage Trap (
This is mistake number 1 when designing power supplies. You buy a 12V transformer. You put the bridge, the capacitor, measure with a multimeter, and… it reads close to 17V! What happened?
No. Transformers are sold by their root mean square voltage (
If we subtract the voltage drop of the two diodes that conduct in each half-cycle (approximately 1.4V total), we are left with a real voltage of about 15.5V - 16V.
If you need a stable 12V DC, a 12V AC transformer is perfect because after rectification, you will have plenty of margin (16V). But be careful: make sure your capacitors and regulators can withstand at least 25V, otherwise they will explode from overvoltage.
Stages of the Process
- Transformer: 230V AC → 12V AC (still alternating).
- Diode Bridge: 12V AC → pulsating signal (all positive, humps).
- Capacitor: pulsating signal → DC with ripple (almost flat, but unstable).
Now we have a direct current, yes. But it is not regulated. If you connect a heavy load, the capacitor will discharge faster, the ripple will increase, and the average voltage will drop. If the mains voltage rises a bit, your output will rise.
