- Regulated Power Supply Definition: A regulated power supply ensures a consistent DC output by converting fluctuating AC input.
- Component Overview: The primary components of a regulated power supply include a transformer, rectifier, filter, and regulator, each crucial for maintaining steady DC output.
- Rectification Explained: The process involves diodes converting AC to DC, typically using full wave rectification to enhance efficiency.
- Filter Function: Filters, such as capacitor and LC types, smooth the DC output to reduce ripple and provide a stable voltage.
- Regulation Mechanism: Regulators adjust and stabilize output voltage to protect against input changes or load variations, essential for reliable power supply.
What is a Regulated Power Supply?
A regulated power supply uses feedback or another control method to keep an output voltage or current within specified limits as input, load and temperature change. Its source may be Alternating Current, an unregulated DC bus, a battery or another supply. Regulation reduces variation in Direct Current output; it does not guarantee an exact value under every condition.
A regulated DC supply may use a linear regulator, a switching converter or both. Linear supplies are often simple and low-noise but dissipate the input-to-output voltage difference as heat. Switching supplies can be more efficient, especially across a large voltage ratio, but add switching ripple and electromagnetic-interference considerations.
The figure shows one traditional mains-fed linear supply. It accepts AC, provides isolation and voltage conversion, rectifies and filters the waveform, then regulates the DC output within the circuit’s ratings.
The building blocks in this illustrated linear supply are:
- An isolating step-down transformer
- A rectifier
- A reservoir and ripple filter
- A regulator
The related digital electronics MCQs include questions about these blocks.
Operation of Regulated Power Supply
Step Down Transformer
A mains-frequency step down transformer provides galvanic isolation and reduces the mains voltage. Its turns ratio and current rating must allow for mains tolerance, load, rectifier loss, ripple and the regulator’s required input headroom. The secondary of the transformer feeds the rectifier. Not every regulated supply uses a mains transformer; an isolated switching supply performs conversion at high frequency.
Rectification
A rectifier uses diodes or controlled switches to make the load current unidirectional. An unfiltered diode rectifier produces pulsating DC, not a smooth regulated voltage.
A half wave rectifier uses only one half-cycle and produces ripple at the line frequency. A full wave rectifier uses both half-cycles, so its ripple frequency is twice the line frequency. A bridge rectifier provides full-wave operation without a centre-tapped secondary, but current passes through two diodes on each half-cycle. The figure shows a full-wave bridge.
A bridge rectifier contains four p-n junction diodes. For the secondary polarity shown in the figure, the electrical transformer makes one bridge input positive. Diodes D3 and D2 are reverse biased, while D1 and D4 conduct. A practical diode has leakage and reverse-voltage limits, so D3 and D2 are only approximately open. The conducting path also has a forward voltage drop across D1 and D4. When the transformer polarity reverses, D3 and D2 conduct instead. Both paths send current through the load in the same direction, so a positive pulsating voltage reaches the filter.
DC Filtration
A filter reduces the rectifier’s ripple before regulation. Options include a reservoir capacitor, an LC section, a choke-input filter or a pi filter. The figure shows a capacitor-input filter and its approximate waveform.
Near each rectified peak, the source supplies a short charging pulse and the capacitor charges. Between peaks, the capacitor supplies load current and its voltage falls. Ripple therefore depends on load current, capacitance, line frequency, source impedance and rectifier conduction. Larger capacitance reduces ripple but can increase peak diode and transformer current.
Regulation
The regulator compares the output with a reference and changes its control element to reduce error. Line regulation describes response to input change, and load regulation describes response to output-current change. Regulation remains valid only within input, dropout, current, power, temperature and stability limits. A series-pass circuit, fixed or adjustable linear IC, switching regulator or a basic zener diode shunt circuit may be suitable, depending on the load. Positive 78XX and negative 79XX families provide fixed nominal voltages; the IC 7805 is a positive 5 V example that needs adequate input headroom and heat dissipation.
The LM317 is an adjustable positive linear voltage regulator. Two resistances, R1 and R2, set its nominal output. The complete equation includes the regulator’s reference voltage and adjustment-pin current. Input, output and adjustment-pin capacitors are selected from the exact device datasheet and layout rather than one universal 0.01 µF to 10 µF rule. Input headroom, load current, line and load error, ripple rejection, transient response and thermal dissipation determine the achieved output. The simplified nominal relation is shown below.
The complete figure shows one regulated +5 V DC supply, not the only regulated-supply architecture.
