Power Supply Basics

A power supply converts electrical energy into the voltage and current required by an electronic circuit. A typical linear DC power supply may contain a transformer, rectifier, filter capacitor and voltage regulator.

AC mains
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Transformer
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Bridge Rectifier
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Filter Capacitor
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DC Output
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Electronic Load

The required power supply rating depends mainly on the output voltage, load current and operating conditions.

DC Power Calculator

For a DC load:

P = V × I

Therefore:

I = P / V

V = P / I

Example — 24 V at 5 A

A DC power supply must provide 24 V at 5 A.

P = V × I

P = 24 × 5

P = 120 W

The load therefore requires approximately 120 W of DC output power. The power supply should normally have some additional capacity rather than operating continuously at its absolute maximum rating.

Power Supply Current Calculator

Power Supply With Design Margin

It is often useful to allow additional capacity above the calculated load requirement.

Required Power =
Load Power × (1 + Margin / 100)

AC Transformer Requirement

For a transformer-fed linear power supply, the transformer must be large enough to handle the required apparent power.

For a simple single-phase AC load:

VA = V × I

A rectifier and capacitor-input filter can draw a non-sinusoidal current waveform, so transformer sizing for a DC supply should not be based solely on the final DC watts.

Transformer VA Calculator

Bridge Rectifier Output Voltage

For a conventional bridge rectifier supplied from a transformer secondary, the capacitor charges toward the peak voltage of the AC waveform.

For a sinusoidal RMS voltage:

Vpeak = Vrms × √2

A bridge rectifier has approximately two conducting diodes in the current path, so a simplified estimate is:

VDC ≈ Vrms × √2 - 2Vd

where Vd is the forward voltage of one diode.

Example — 25 VAC Transformer

Suppose a transformer provides 25 VAC RMS to a bridge rectifier. Assuming approximately 0.8 V across each conducting diode:

Vpeak = 25 × √2

Vpeak ≈ 35.36 V

VDC ≈ 35.36 - 1.6

VDC ≈ 33.76 V

This is an approximate unloaded value. The actual DC voltage will depend on transformer regulation, diode characteristics, load current, capacitor ripple and other circuit losses.

Filter Capacitor Calculator

A capacitor-input filter stores charge between rectifier peaks. The approximate ripple voltage is:

C = I / (f × Vripple)

For a full-wave bridge rectifier supplied from a 50 Hz mains system, the ripple frequency is approximately 100 Hz.

Therefore:

C = I / (100 × Vripple)

Example — 5 A Load

For a 5 A load, 100 Hz full-wave ripple and 2 V allowed ripple:

C = I / (f × Vripple)

C = 5 / (100 × 2)

C = 0.025 F

C = 25,000 µF

A practical design would select a suitable combination of standard capacitors with adequate voltage rating and ripple-current capability.

Capacitor Voltage Rating

The filter capacitor must have a voltage rating safely above the maximum voltage that can appear across it.

For a transformer secondary, the approximate peak voltage is:

Vpeak = VAC × √2

The capacitor voltage rating should provide appropriate margin above the actual maximum operating voltage.

Linear Regulator Power Dissipation

A linear regulator dissipates the difference between its input and output voltage as heat.

Pheat = (Vin - Vout) × I

Example — 18 V to 12 V at 2 A

A linear regulator receives 18 V and supplies 12 V at 2 A.

Pheat = (18 - 12) × 2

Pheat = 12 W

The regulator must therefore dissipate approximately 12 W of heat. This can require a substantial heatsink depending on the regulator package and allowable temperature rise.

Switching Power Supplies

Switch-mode power supplies operate differently from conventional linear supplies. A switching transistor rapidly switches the input energy, while inductors, transformers, capacitors and control circuits transfer and regulate the energy.

Switching supplies can achieve high efficiency and smaller physical size, but require careful design of switching frequency, magnetic components, MOSFETs, rectifiers, filtering, EMI and thermal management.

Power Supply Efficiency

Power supply efficiency is:

Efficiency (%) =
Pout
──── × 100
Pin

Power Supply Ripple

Ripple is the residual AC component that remains on a DC output after rectification and filtering.

For a capacitor-input full-wave rectifier, a simplified estimate is:

Vripple ≈ I / (f × C)

Increasing capacitance reduces ripple, while increasing load current increases ripple.

The ripple frequency is approximately twice the AC frequency for a full-wave rectifier.

Multiple Filter Capacitors

Multiple capacitors connected in parallel increase the total capacitance:

Ctotal = C1 + C2 + C3 + ...

For example:

4700 µF
+
4700 µF
+
4700 µF

= 14,100 µF

Parallel capacitors can also share ripple current, provided the capacitors are suitable for the application.

Power Supply Transformer Selection

When selecting a transformer for a linear DC supply, consider:

• Required DC output voltage
• Required DC output current
• Transformer secondary RMS voltage
• Transformer VA rating
• Rectifier losses
• Filter capacitor charging current
• Transformer voltage regulation
• Allowable temperature rise

The nominal transformer voltage should not be selected simply by subtracting the desired DC voltage from the mains voltage. The rectifier, capacitor filter and regulator all affect the final voltage.

Power Supply for an Audio Amplifier

Audio amplifiers can produce highly variable current demand because their output power changes with the audio waveform.

A power supply for an amplifier should therefore consider the amplifier's output power, efficiency, channel count, load impedance, crest factor, transformer capability and reservoir capacitor bank.

For capacitor-input supplies, the transformer and rectifier must be able to tolerate the charging pulses produced by the reservoir capacitors.

Power Supply Safety

Mains-powered power supplies can contain lethal voltages even after the equipment has been disconnected from the mains.

• Use appropriate insulation.
• Use correctly rated fuses.
• Provide protective earthing where required.
• Use suitable enclosure and creepage/clearance distances.
• Use capacitors with appropriate voltage ratings.
• Discharge high-voltage capacitors safely.
• Never assume a disconnected supply is automatically safe.

Common Mistakes

• Using DC output voltage directly as the transformer AC voltage.
• Ignoring the peak voltage after rectification.
• Using insufficient capacitor voltage rating.
• Ignoring capacitor ripple current.
• Using too little capacitance for the required ripple specification.
• Ignoring transformer regulation.
• Ignoring regulator heat dissipation.
• Selecting a transformer with no capacity margin.
• Assuming transformer VA is exactly equal to DC output watts in a capacitor-input supply.
• Ignoring mains safety requirements.

Key Points

• DC load power is P = V × I.
• Transformer apparent power is approximately VA = V × I.
• A bridge rectifier and capacitor filter produce a DC voltage related to the AC RMS peak.
• Full-wave rectification produces ripple at approximately twice the AC frequency.
• Filter capacitance can be estimated from C = I/(f × Vripple).
• A larger capacitor generally reduces ripple.
• Linear regulators dissipate voltage difference as heat.
• Switch-mode supplies require additional consideration of switching and magnetic components.
• Transformer sizing for capacitor-input supplies requires consideration beyond DC watts alone.