Why Electronics Can't Run on Wall Power ? — AC to DC Conversion

Main Ideas

AC, DC, and Rectification

Electronics generally need direct current (DC), while wall power is alternating current (AC).

• DC flows in one direction.
• AC repeatedly changes direction. The power grid uses AC, while most electronic devices ultimately require DC.
• Converting AC into one-directional current is called rectification.

Half-Wave Rectification

A diode allows current to pass in one direction and blocks it in the other. With one diode in series with a load, only one half of each AC cycle reaches the load.

The result is pulsating DC: the output falls to zero during the blocked half-cycle. This method is simple but makes poor use of the available power.

Full-Wave Rectification with a Bridge

A bridge rectifier uses four diodes for full-wave rectification. The AC source connects to two bridge terminals, and the load connects across the other two.

• On one half-cycle, one pair of diodes conducts.
• When the AC polarity reverses, the other pair conducts.
• In both cases, current flows through the load in the same direction.

This uses both halves of the AC waveform.

Diode Voltage Drop

Each conducting diode has a forward voltage drop of about 0.7 V. In a bridge rectifier, current passes through two diodes at a time, for a total drop of about 1.4 V. The output peak is therefore roughly 1.4 V below the input peak.

This loss matters especially in low-voltage circuits. The video also notes that the bridge will not conduct when the input magnitude is below about 1.4 V.

Smoothing the Output with a Capacitor

Rectification alone produces a voltage that rises and falls in rounded humps rather than remaining steady. A capacitor placed across the output charges near each voltage peak, then discharges through the load between peaks, reducing the voltage drop.

The diodes prevent the capacitor from discharging back into the AC source. Smoothing reduces—but does not eliminate—voltage variation. The remaining variation is called ripple.

The capacitor stores energy according to:

[ E = \frac{1}{2}CV^2 ]

Estimating Ripple and Choosing a Capacitor

The video derives this approximate relationship:

[ V_{\text{ripple}} \approx \frac{I}{C f_{\text{ripple}}} ]

where:

• (I) is the load current,
• (C) is the capacitance, and
• (f_{\text{ripple}}) is the ripple frequency.

This estimate assumes that the capacitor charges almost instantly, the load current stays roughly constant, and the capacitor voltage falls approximately linearly between peaks. It is most useful when ripple is small compared with the DC output voltage.

The derivation proceeds as follows:

1. Start with the capacitor relationship (Q = CV).
2. Divide by time to relate the rate of charge change to current: (I = C(V/T)).
3. Rearrange to get (V = IT/C).
4. For a discharge interval, the voltage change is the ripple voltage, and the interval is approximately (1/f_{\text{ripple}}). Substituting gives the formula above.

The ripple frequency depends on the rectifier:

• Half-wave: (f_{\text{ripple}}) equals the AC supply frequency.
• Full-wave: (f_{\text{ripple}}) is twice the AC supply frequency, because both halves of the AC waveform contribute.

Example: A 12 V Supply

For a full-wave rectifier with a 120 Hz AC frequency, a 1 A load, and a ripple target of 10%:

1. The permitted ripple is (0.10 \times 12\text{ V} = 1.2\text{ V}).
2. The ripple frequency is (2 \times 120 = 240\text{ Hz}).
3. Estimate the capacitance:

[ C \approx \frac{1\text{ A}}{240\text{ Hz} \times 1.2\text{ V}} \approx 0.0034\text{ F} ]

1. This is about 3,400 µF. The video suggests choosing a nearby standard value, such as 3,300 µF or 4,700 µF.

As a rough starting point for basic projects, the video gives a rule of thumb of about 2,000 µF per amp of load current. A 2,200 µF capacitor may suit a 1 A load, but more sensitive electronics may need more capacitance or an additional voltage regulator for a steadier output.

Speakers and Sources

• Speaker: One unnamed narrator presenting the lesson.
• Other speakers or named sources: None identified in the subtitles.