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Power Supply, Oscillator, Pulse and Modulation Circuits

A power supply turns AC into steady DC: a rectifier flips the negative half, a capacitor filter smooths the bumps and a regulator makes it flat. An oscillator makes a wave by itself using feedback. A pulse circuit switches ON and OFF with a set duty cycle. Modulation puts a message on a high-frequency carrier, and demodulation takes it back.

🎬 Step-by-step story

  1. Mains power is AC: it goes up and down, positive then negative (faint line). A diode bridge flips every negative bump upward. The red line is now always above zero: bumpy DC.
  2. A capacitor stays full between bumps and cuts the dips. A bigger capacitor gives a smoother line. A regulator then makes it perfectly flat. Try the regulator button.
  3. An oscillator makes a wave with no input signal. Feedback grows a tiny wobble into a steady wave. Change the frequency slider to make it faster or slower.
  4. A pulse circuit, like a 555 timer, switches the output ON and OFF. The ON share of each cycle is called duty. Slide it and see the pulses get wider or thinner.
  5. Modulation: a slow message (faint line) changes the height of a fast carrier wave. This is AM radio. Press Demodulate and the message comes back.
  6. Your turn. Switch between the five circuits and move the sliders. Which one makes DC? Which one makes pulses? Which one carries a message?

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why is the rectifier output still bumpy?

It only flips the negative halves up. The bumps remain until a capacitor fills the dips.

Does a bigger capacitor always help?

It reduces ripple, but a very big one draws large charging current each peak and costs more. Slide it in step 1.

Where does the oscillator's wave come from if there is no input?

A tiny noise is amplified, fed back in step and grows until the amplifier limits it. See the growing wave in step 2.

Is a 20% duty pulse weaker than a 50% one?

Its average value is lower, so a lamp looks dimmer. This is how PWM dimmers work.

Why do we modulate at all?

A slow voice signal cannot travel far or share the air. A fast carrier can, and each station gets its own carrier frequency.

Power-supply circuits

Most electronics need steady DC, but the wall socket gives AC. A power supply has four blocks:

  1. Transformer: lowers the AC voltage (for example 230 V to 12 V).
  2. Rectifier: diodes let current go one way only. A half-wave rectifier keeps only the positive half. A full-wave rectifier (a bridge of 4 diodes) flips the negative half up, so there are twice as many bumps. Output is pulsating DC.
  3. Filter: a big capacitor charges at each peak and gives current in the dips. The leftover wobble is called ripple. Bigger C or a lighter load means less ripple: ripple voltage ≈ I / (f × C).
  4. Regulator: a Zener diode or an IC (like 7805) holds the output at a fixed voltage, even if the mains or the load changes.

Ripple frequency: half-wave = 50 Hz, full-wave = 100 Hz (for 50 Hz mains). A diode circuit is explained in PN junction diode.

Oscillator circuits

An oscillator makes a repeating wave by itself, from DC power alone. It uses an amplifier plus positive feedback: a part of the output is sent back to the input in step with it, so the wave grows until it settles at a steady size.

Barkhausen idea: oscillation needs loop gain of at least 1 and the feedback in phase.

More about the LC pair in LC oscillations.

Pulse circuits

A pulse is a signal that jumps between two levels: HIGH and LOW. Its main numbers:

Multivibrators make pulses with two transistors or a timer IC:

A Schmitt trigger cleans a slow or noisy signal into sharp pulses. Changing the duty cycle (PWM) changes the average power: 25% duty gives a quarter of the power.

Modulation and demodulation circuits

A voice signal is slow (about 300 Hz to 3 kHz) and cannot travel far on its own. So we put it on a fast carrier wave (for example 1 MHz). This is modulation.

Demodulation takes the message back. A simple AM detector is a diode, a capacitor and a resistor: the diode removes one half, the capacitor follows the peaks (the envelope), and a final filter blocks the carrier. The receiver also has a tuning circuit to choose one station.

Try it: see the waves

In the 3D, make the capacitor smallest and note the ripple, then make it biggest. Predict: which one is better for a radio? Set the pulse duty to 50% and then 20% and say which gives a dimmer lamp. In modulation, raise the depth above the middle and look at where the envelope nearly touches zero: that is the limit before the message gets distorted.

Key formulas and definitions

Worked examples

1. What is the output ripple frequency of a full-wave rectifier on 50 Hz mains?

2 × 50 = 100 Hz.

2. A power supply gives 100 mA. The ripple frequency is 100 Hz and C = 1000 µF. Find the ripple voltage.

V = I/(fC) = 0.1 / (100 × 0.001) = 1 V.

3. An LC oscillator has L = 1 mH and C = 0.1 µF. Find f.

f = 1/(2π√(10⁻³ × 10⁻⁷)) = 1/(2π × 10⁻⁵) ≈ 15.9 kHz.

4. A pulse is HIGH for 2 ms in a period of 8 ms. Find its frequency and duty cycle.

f = 1/0.008 = 125 Hz. Duty = 2/8 × 100 = 25%.

5. A 555 astable has R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µF. Find f.

f = 1.44 / ((1000 + 20000) × 10×10⁻⁶) = 1.44 / 0.21 ≈ 6.9 Hz.

6. An AM wave has a message peak of 3 V and a carrier peak of 5 V. Find m and say if it is fine.

m = 3/5 = 0.6, which is below 1, so there is no distortion.

Common mistakes

Practice quiz

1. What does a rectifier do?
2. What does the filter capacitor reduce?
3. An oscillator needs:
4. A pulse is HIGH 3 ms in a 12 ms period. Duty cycle:
5. In AM, the message changes the carrier's:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

How does a phone charger make DC from AC?

It lowers the voltage, rectifies it with diodes, smooths it with a capacitor and regulates it to a fixed voltage.

What is duty cycle?

The share of each cycle for which a pulse is HIGH, shown as a percentage. 50% means ON half the time.

What is the difference between AM and FM?

In AM the message changes the carrier's height; in FM it changes the carrier's frequency. FM is less noisy.

Where this is taught

Japan高校（専門学科）1〜3年Electronic Circuits

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