Section 2.3: Power Supply Fundamentals #

Now that we’ve explored how transistors and tubes amplify signals, let’s turn our attention to what powers them. Every piece of electronic equipment in your shack needs clean, stable DC power to operate properly. Your household outlets provide AC power, so we need a way to convert that alternating current into the direct current our radios require.

Understanding power supplies isn’t just about passing your exam—it’s about knowing how to troubleshoot problems, select the right power source for your equipment, and even build or repair your own supplies. Whether you’re setting up a new station or trying to track down that annoying hum in your transmitted audio, these fundamentals will serve you well.

The Power Supply Journey: From AC to DC #

Converting AC from your wall outlet to clean DC for your radio involves several stages. Think of it like refining crude oil into gasoline—each step removes unwanted components and brings us closer to the pure product we need. The basic stages are:

Transformation - Changing the voltage level (if needed)
Rectification - Converting AC to pulsating DC
Filtering - Smoothing the pulses into steady DC
Regulation - Maintaining constant voltage despite load changes

Let’s explore each of these stages in detail.

Rectification: Converting AC to DC #

The first step in converting AC to DC is rectification—changing alternating current that flows back and forth into direct current that flows in one direction. There are two basic approaches to rectification: half-wave and full-wave.

Key Information:
A half-wave rectifier converts 180 degrees of the AC cycle to DC.
A full-wave rectifier converts 360 degrees of the AC cycle to DC.

Comparison of AC input with half-wave and full-wave rectified outputs

As you can see in the waveform comparison:

The input AC signal alternates between positive and negative
Half-wave rectification preserves only the positive portions, creating gaps in the output
Full-wave rectification flips the negative portions to create a continuous series of positive pulses

Half-Wave Rectification #

Key Information: A half-wave rectifier is characterized by using only one diode.

Half-Wave Rectifier Circuit schematic showing an AC source, one diode, and a load in a circuit

The half-wave rectifier is the simplest design:

A single diode allows current to flow only during the positive half-cycle
The negative half-cycle is completely blocked
The output pulses occur at the same frequency as the input AC

Despite its inefficiency (using only half the available power), half-wave rectification is sometimes used in:

Simple battery chargers
Power indicators
Applications where cost and simplicity outweigh efficiency concerns
Circuits where only a small amount of DC power is needed

Full-Wave Rectification #

Full-wave rectifiers utilize the entire AC cycle, making them more efficient and easier to filter.

Key Information: An unfiltered full-wave rectifier connected to a resistive load produces a series of DC pulses at twice the frequency of the AC input.

The main advantage of full-wave rectification is its efficiency:

It utilizes both positive and negative portions of the AC cycle, effectively doubling the energy captured compared to half-wave rectification
By converting both halves of the cycle, it inherently produces pulses at twice the input frequency (120 Hz from a 60 Hz input)
This continuous flow with no complete gaps makes filtering easier and requires less capacitance to smooth the output

The exam covers two common full-wave rectifier designs: center-tapped transformer and bridge rectifier.

Center-Tapped Transformer Design #

Key Information: A full-wave rectifier circuit using a center-tapped transformer uses two diodes.

This design uses:

A transformer with a center tap on its secondary winding
Two diodes that alternately conduct during opposite half-cycles
The center tap serves as the common (often ground) connection

During operation:

When the top of the secondary is positive, the top diode conducts
When the bottom of the secondary is positive, the bottom diode conducts
Both scenarios create current flow in the same direction through the load

Bridge Rectifier Design #

The bridge rectifier uses four diodes arranged to:

Direct current through the load in the same direction regardless of input polarity
Eliminate the need for a center-tapped transformer
Provide full-wave rectification with a standard transformer

To illustrate the current path we’ll use compass points – N, E, S, W (going clockwise starting at the top of the diagram on the right). We’ll use the “positive to negative” convention of tracing current flow.

When N is positive, current flows from N -> D2 -> DC+ -> Load -> DC- -> D4 -> S
When N is negative, current flows from S -> D3 -> DC+ -> Load -> DC- -> D1 -> N

Center-tapped vs. Bridge Rectifier Designs #

Each design has advantages and disadvantages:

Center-tapped transformer design:

Uses fewer components (only two diodes)
Only one diode voltage drop in the current path
Requires a special center-tapped transformer
Less efficient use of the transformer (each half of the secondary winding conducts only 50% of the time)

Bridge rectifier design:

Works with any transformer (no center tap required)
More efficient use of transformer windings
Requires four diodes instead of two
Two diode voltage drops in series (higher loss)

Practical Considerations #

In practical circuits, the diode forward voltage drop affects the output. Each silicon diode typically drops about 0.7V when conducting. This means:

In half-wave rectifiers: output is reduced by about 0.7V
In center-tapped designs: output is reduced by about 0.7V
In bridge rectifiers: output is reduced by about 1.4V (two diodes in series)

For high-power applications, this voltage drop represents wasted power and heat generation in the diodes.

Most amateur radio power supplies use full-wave rectification because of these advantages, with bridge rectifiers being the most common in modern designs due to their flexibility and the low cost of diodes.

Filtering: Smoothing the Pulses #

Rectification alone produces pulsating DC—not the smooth, constant voltage our radio equipment needs. The next step is filtering, which smooths these pulses into steady DC.

Key Information: Capacitors and inductors are used in a power supply filter network.

The most common filter configuration uses large electrolytic capacitors that charge during voltage peaks and discharge during valleys, filling in the gaps to create smoother DC. Inductors can also be used in filter circuits, resisting current changes and further smoothing the output.

Think of filter capacitors like water towers in a municipal water system. During periods of high flow (voltage peaks), they fill up. During periods of low flow (voltage valleys), they release their stored energy to maintain pressure (voltage). The larger the capacitor, the more energy it can store and the smoother the output becomes.

Safety: Bleeder Resistors #

When you turn off a power supply, those large filter capacitors can retain dangerous charges for minutes or even hours. This creates a serious shock hazard for anyone working on the equipment.

Key Information: A power supply bleeder resistor discharges the filter capacitors when power is removed.

Bleeder resistors are high-value resistors connected across the filter capacitors. They provide a discharge path that safely drains stored energy when the power supply is turned off. While they do consume a small amount of power during operation, the safety benefit far outweighs this minor inefficiency.

Never assume a power supply is safe just because it’s unplugged! Those capacitors can deliver a painful or even dangerous shock. Always wait for bleeder resistors to do their job, or manually discharge capacitors through an appropriate resistor before working on equipment.

Modern Alternative: Switchmode Power Supplies #

Traditional linear power supplies work well but tend to be large and heavy due to their 60 Hz transformers and massive filter capacitors. Switchmode (switching) power supplies offer a more compact alternative.

Key Information: High-frequency operation allows switchmode power supplies to use smaller components compared to linear power supplies.

Instead of transforming 60 Hz AC directly, switchmode supplies:

Rectify the incoming AC to DC
Use high-speed switching transistors to create high-frequency AC (typically 20-200 kHz)
Transform this high-frequency AC to the desired voltage
Rectify and filter the output

Because transformers and filter components can be much smaller at higher frequencies, switchmode supplies achieve the same power output in a fraction of the size and weight. This is why modern transceivers use switchmode supplies despite their increased complexity.

The tradeoff? Switchmode supplies can generate RF interference due to their high-frequency switching. Good design and shielding minimize this issue, but it’s something to consider when choosing between linear and switching supplies for your station.

Understanding power supplies helps you make informed decisions about your station equipment. Whether you’re selecting a new supply, troubleshooting voltage problems, or building your own, these fundamentals provide the foundation you need. Next, we’ll explore digital circuits and see how modern radios use digital technology to enhance performance.