Manan Dua

Summary

An AC-to-DC converter transforms alternating current (AC) from a source into direct current (DC), required by most electronic devices. This project’s objective was to design and build a DC power supply capable of delivering 10 mA at 3 V ± 0.1 V from a 120 V (rms), 1 kHz source. Core stages include a transformer (or direct AD3 source), a center-tapped full-wave rectifier, a filter capacitor, and a load resistor.

Components of an AC-to-DC Converter (Figure 1)

The major design requirement: output 3 V ± 0.1 V at 10 mA. Secondary requirements include maintaining ripple under 0.2 V p-p and ensuring component availability (e.g., 330 Ω resistor vs. ideal 300 Ω).

Design

A center-tapped full-wave rectifier was chosen over half-wave or bridge topologies to reduce ripple and utilize both halves of the AC cycle. The initial circuit model (Figure 2) guided part value calculations:

Initial circuit model (Figure 2)

Components:

  • Load Resistor (RL): Defines output current at 10 mA for 3 V.
  • Filter Capacitor (Cf): Smooths output to maintain ±0.1 V ripple.
  • Rectifier Diodes (D₁, D₂): Two 1N4148 diodes for center-tapped rectification.
  • Transformer: Center-tapped, step-down from 120 V (rms) to required Vpeak. (Not physically used; AD3 waveform generator substituted.)
  • Regulator: Omitted since filter maintains output within tolerance; could improve stability if added.

Calculations

Load Resistor (RL)

Given:

  • Vout = 3 V ± 0.1 V
  • Iout = 10 mA

Using Ohm’s law:
Vout = Iout × RL
RL = 3 V / 10 mA = 300 Ω
PL = 3 V × 10 mA = 0.03 W

Nearest available resistor: 330 Ω (0.25 W rating) was chosen—within acceptable tolerance.

Filter Capacitor (Cf)

Desired ripple: Vpp ≤ 0.2 V; output frequency (full-wave) fripple = 2 × 1000 Hz = 2000 Hz.

For a parallel RC filter:
Vpp = Iout / (fripple × C)
Solve for C:
C = Iout / (Vpp × fripple) = 10 mA / (0.2 V × 2000 Hz) = 25 µF

Available capacitors: three 10 µF units in parallel = 30 µF total—greater than 25 µF to ensure ripple ≤ 0.2 V.

Rectifier & Diode Drop

Chosen topology: center-tapped full-wave rectifier with two 1N4148 diodes. Diode forward drop (typical): 0.72 V (max ≈1 V).

To maintain VC ≈ 3.1 V at capacitor peak:
Vpeak = VC + VD = 3.1 V + 0.72 V ≈ 3.82 V

Thus, each secondary half-cycle sinusoid (V₁, V₂) must reach ±3.82 V peak. Expressed as:
V₁ = 3.82 sin(ωt) V, V₂ = –3.82 sin(ωt) V

No regulator was used, as calculated filter and diode arrangement keep output within ±0.1 V.

Transformer Turns Ratio

The AD3 waveform generator provided 3.82 Vpp on each half-cycle (in lieu of a transformer). If a center-tapped transformer were used, to find turns ratio:

  • Input RMS (120 Vrms) at 1 kHz.
  • Desired secondary RMS: Vout,peak × 2 / √2 = (3.82 V × 2) / √2 ≈ 5.40 Vrms.
  • Turns ratio Nprimary / Nsecondary ≈ 120 V / 5.40 V ≈ 22:1.

Since the AD3 provided the required 3.82 Vpp signals in place of the transformer’s secondaries, no physical transformer was implemented.

Circuit Schematic

The completed schematic, with calculated component values inserted, is shown below. Note: transformer omitted; AD3 waveform generator substituted at the center-tap inputs.

Circuit schematic with specific component values (Figure 5)

Measurement & Analysis

Physical Build

The circuit was assembled on a breadboard using:

  • AD3 waveform generator outputs as the two center-tapped secondary signals, each 3.82 Vpp, 1 kHz, 180° out of phase.
  • Two 1N4148 diodes in center-tapped configuration.
  • Filter: 30 µF total (three 10 µF capacitors in parallel).
  • Load resistor: 330 Ω (instead of ideal 300 Ω).
Physical circuit using components (Figure 4)

Initial Waveform Settings

AD3 set to produce two sine waves, 3.82 Vpp each, 180° apart—intended to yield ~3 V DC after rectification and filtering.

Initial waveform generator settings (Figure 5)
Initial circuit output with cursors (Figure 6)

Initial Output Observation

Measured DC output: ~2.80 V–2.94 V ripple (below desired 2.9 V–3.1 V range). Resulting output current: 8.7 mA–9.3 mA (expected due to 330 Ω vs. 300 Ω).

Adjusted Waveform Settings

Increased AD3 amplitude to 3.95 Vpp per half-cycle to compensate for higher diode drop (~0.9 V measured) and resistor tolerance.

Modified waveform generator settings (Figure 7)
Output with modified input values (Figure 8)

Corrected Output Observation

Measured DC output: ~2.92 V–3.07 V ripple, within specification. Output current: ~8.7 mA–9.3 mA, consistent with resistor choice.

Simulation

LTSpice Simulation Setup

A transient simulation (0 ms–4 ms) was run in LTSpice, using:

  • Two voltage sources: V1 N002 0 SINE(0 –3.82 1000) and V2 N001 0 SINE(0 3.82 1000), 180° out of phase.
  • Diodes: 1N4148 (LTSpice default model).
  • Filter capacitor: 30 µF.
  • Load resistor: 330 Ω.
Simulation schematic (Figure 9)

Simulation Results

Output voltage waveform: peak ~3.03 V, ripple ~0.1 V (within spec). Output current waveform: ~8.8 mA–9.2 mA.

Voltage output from simulation (Figure 10)
Current output from simulation (Figure 11)

Waveform Analysis

Voltage peaks occur when one diode is forward-biased; during valleys, both diodes are off and capacitor discharges. Peak ~3.03 V vs. calculated 3.1 V (acceptable). Current ripple reflects voltage ripple through 330 Ω load.

Discussion

Comparing calculations, simulation, and measurements reveals discrepancies due to component tolerances and diode forward voltage variations. Simulation closely matched calculations (peak ~3.03 V vs. 3.1 V). Physical measurements required raising input to 3.95 Vpp to compensate for actual diode drop (~0.9 V vs. assumed 0.72 V) and resistor tolerance.

Output current ripples (8.7 mA–9.3 mA measured, 8.8 mA–9.2 mA simulated) aligned closely, given 330 Ω load. RC filter choice proved adequate, though an LC filter would reduce losses but increase complexity and cost.

Limitations:

  • AD3 waveform generator max ±5 V (800 mA); limits maximum load to ~4 W with this rectifier topology.
  • Tolerance stack-up: diode forward voltage ±5%, resistor ±5%, capacitor ±10%—combined tolerance impacts output voltage accuracy.
  • RC filter: dissipates ripple energy in resistor; LC filter would be more efficient but bulkier and costlier for this application.
  • No dedicated voltage regulator: stability relies on filter and input adjustment; spikes could occur if load varies rapidly.

Overall, the DC power supply met specifications after adjusting input amplitude. Ensuring accurate component values (e.g., measuring diode drops) and tight tolerances would further improve performance.

References

  1. A. S. Sedra, K. C. Smith, T. C. Carusone, and V. Gaudet, Microelectronic Circuits, 8th ed. New York, NY: Oxford University Press, 2019.
  2. “Components—Part 1: The Capacitor is the Simplest Noise Filter,” Learn about Technology with TDK, 2024. https://www.tdk.com/en/tech-mag/noise/04.
  3. Electrical Technology, “What is a Rectifier? Types of Rectifiers and their Operation,” Electrical Technology, Jan 15, 2019. https://www.electricaltechnology.org/2019/01/what-is-rectifier-types-of-rectifiers-their-operation.html.
  4. J. Colvin, “Analog Discovery 3 Reference Manual,” Digilent, 2023. https://digilent.com/reference/test-and-measurement/analog-discovery-3/reference-manual.
  5. “1N4148 / 1N4448” Datasheet, Diodes Inc., 2008. https://www.diodes.com/assets/Datasheets/ds12019.pdf.