Robotics · Electronics basics · Grade 8

Diode and rectifier

Why does a gadget not burn out when you put the battery in the wrong way round? How is alternating current “rectified”?

The situation

At home you once put a battery into a radio or toy in a hurry — the wrong way round. Some devices simply do not work, others break. Why? Inside there is one small black cylinder — a diode — that lets current through in one direction only and protects the device. The same diode does one more wonder: it turns the alternating current from the socket into direct current, like a battery’s. On a breadboard, test the diode, break the device on purpose, then rectify 9 V~ from a safe adapter and watch it on the oscilloscope.

Topic

Diode and rectifier. Protection against a battery connected the wrong way round: a diode passes current in one direction

Key ideas

Diode
Half-wave
Bridge and smoothing

Formulas

U_{fwd} \approx 0.6 - 0.7 V, I_{rev} \approx 0

A diode is a one-way valve: in the forward direction it “loses” ≈ 0.7 V and conducts, in the reverse direction it is shut (1N4007: 1 A, withstands up to 1000 V).

U_{m} = \sqrt{2} U_{\sim}

The amplitude of an alternating voltage: the 9 V~ written on the adapter is the effective (RMS) value, the peak is 12.7 V. The frequency is 50 Hz: the period is 20 ms.

U_{avg} = \frac{U_{m} - U_{D}}{π} (half-wave), U_{avg} = \frac{2 (U_{m} - 2 U_{D})}{π} (bridge)

The average output voltage without a capacitor: with half-wave rectification half of the sine wave is lost, the bridge uses both halves (two diode voltages are subtracted).

Δ U \approx \frac{I}{f C} (half-wave), Δ U \approx \frac{I}{2 f C} (bridge)

The ripple with a smoothing capacitor: the load current $I$ discharges the capacitor between the peaks; in a bridge the peaks come twice as often.

Theory

The diode — a one-way valve

A diode is a semiconductor of two layers (a p–n junction). Current flows only from the anode to the cathode; the white stripe on the body is on the cathode side. Connected the right way, it drops ≈ 0.6–0.7 V (silicon) and the rest goes to the circuit; connected the wrong way, there is no current — the diode takes the whole voltage. The 1N4007 is the simplest rectifier diode: current up to 1 A, reverse voltage up to 1000 V. An LED is a diode too: ≈ 1.9 V forward (red), but it withstands only 5 V in reverse — above that it breaks down.

Battery the wrong way round: the protection diode

If you put a series diode at the input of a device, then with the battery the right way round it is open (0.7 V is lost — 8.3 V is left of 9 V), and with the battery reversed it is shut: no current, and the device does not even notice. Without the diode the reverse voltage gets inside: the electrolytic capacitor heats up and bursts, the LED breaks down, the chips are ruined. If you fit the diode the wrong way round (stripe on the wrong side), the device does not work even with a correct battery and does not protect against a reversed one. So look at the stripe.

Alternating current and why an adapter

The socket gives 220 V, 50 Hz alternating voltage: it reverses direction 50 times a second, the peak is 311 V. This is extremely dangerous — a current of 50 mA stops the heart, so the student never connects anything to the socket. The experiment uses a transformer adapter: inside are two windings that step 220 V down to a safe 6–12 V~, and the 220 V side is sealed. The 9 V~ written on the adapter is the effective value; the amplitude is $U_{m} = \sqrt{2} \cdot 9 \approx 12.7$ V. The oscilloscope shows it: a sine wave with a period of 20 ms.

Half-wave and bridge rectifier

One diode passes the positive half-waves and cuts off the negative ones — the output is a row of “humps” with a 10 ms gap between them: a half-wave rectifier, the average voltage is only $(U_{m} - U_{D}) / π$ ≈ 3.8 V. A bridge is four diodes in a diamond: in each half-cycle two of them open, and the negative half-wave is also “flipped over” and sent to the output — the humps follow one another with no gaps, the average is $2 (U_{m} - 2 U_{D}) / π$ ≈ 7.2 V. In a shop the bridge is sold in one four-pin package (+ ~ ~ −) holding the same four diodes; it can also be built from four 1N4007s.

The smoothing capacitor and ripple

A capacitor connected across the output charges at the peak, and when the diodes shut it feeds the load itself — the voltage does not drop to zero, it only “sags” a little. The drop between the peaks is the ripple $Δ U \approx I / (f C)$ (half-wave, $f$ = 50 Hz) or $I / (2 f C)$ (bridge: peaks at 100 Hz). A heavy load (large current) gives large ripple; a large capacitance gives small ripple. A 100 Ω load, a bridge and 2200 µF: ≈ 0.5 V. Every phone charger and radio contains such a unit.

About the model

The diode is a piecewise model: a 0.65 V threshold, then 0.35 Ω (≈ 1 V at 1 A — matches the 1N4007 datasheet), reverse leakage 1 ns. The device’s capacitor has a leakage resistance of 200 kΩ — that is why a shut diode takes the whole voltage. Failure: an electrolytic capacitor with reversed polarity bursts in ≈ 2.5 s, an LED breaks down in ≈ 0.3 s at a reverse voltage above 5 V (burns out). The alternating current is slowed down 100 times (a 20 ms period lasts 2 s on the screen), the oscilloscope is 1 MΩ, the adapter winding is 1 Ω. A real LED flickers at 50 Hz and the eye does not notice.

Did you know?

In 1874 Karl Ferdinand Braun discovered that a contact between a metal wire and a metal sulphide crystal conducts current better in one direction — the first observation of a semiconductor diode; Braun later received the Nobel Prize for radio (1909, together with Marconi).
The 1N4007 diode has been made since the 1960s and is still one of the best-selling electronic parts: it withstands 1 A of current and 1000 V of reverse voltage, costs a few cents — and is found in almost every power supply.

Inside the experiment

A real board in a hand-inked isometric drawing: turn it, place the parts, wire the circuit, write and upload the code — current and voltage are computed at once, and a wrong connection burns a part. Inside are missions, theory, a quiz and a guided tour.

More experiments in this section

Breadboard: rows — Grades 5–6
LED and resistor: Ohm’s law — Grade 7
Resistors: the colour code — Grade 7
Voltage divider and potentiometer — Grade 7
Capacitor: charging and the RC time — Grade 7
The transistor — a switch — Grade 8
Measuring with a multimeter — Grade 8