E7D: Power Supplies

Every radio station depends on a stable power supply. Understanding how voltage regulators work — and why they sometimes fail to regulate — is essential for building and troubleshooting amateur radio equipment. This lesson also covers battery calculations, switching power supply advantages, solar panel interfaces, and safety features like step-start circuits.

Figure E7-2 shows a linear voltage regulator circuit. Three exam questions ask about the function of specific components in that circuit.

Linear Voltage Regulators

A linear electronic voltage regulator works by varying the conduction of a control element (the pass transistor) to maintain a constant output voltage. As the input voltage rises or the load decreases, the pass element conducts less; as the input drops or load increases, it conducts more. This continuous adjustment keeps the output steady.

Three-terminal voltage regulators (such as the 7805 series) are series regulators — the control element is in series between the input and output. They are the most common type and easy to use.

A shunt regulator works differently: it loads the unregulated voltage source directly by drawing more or less current to ground, holding the output constant. It operates by shunting excess current rather than blocking it in series.

Switching Regulators

A switchmode voltage regulator works by varying the duty cycle of pulses fed into a low-pass filter. The switch drives energy into an inductor and filter capacitor at a high frequency; the duty cycle (on-time fraction) determines the average output voltage. Higher duty cycle produces higher output.

Switching power supplies are less expensive and lighter than equivalent linear supplies because the high frequency inverter design uses much smaller transformers and filter components for equivalent power output. At higher frequencies, inductors and capacitors can be physically smaller for the same performance.

Zener Voltage Reference

A Zener diode is used as a stable voltage reference in regulator circuits. When reverse biased beyond its Zener voltage, it maintains a nearly constant voltage drop regardless of current variations. This stable reference voltage is compared to the output voltage to control the pass element or duty cycle.

Dropout Voltage and Power Dissipation

The dropout voltage of a linear regulator is the minimum input-to-output voltage difference required to maintain regulation. If the input voltage falls below (output voltage + dropout voltage), the regulator loses control and the output drops. Low-dropout (LDO) regulators reduce this requirement to as little as a few tenths of a volt.

Power dissipated by a series linear regulator is calculated as: P = (Vin − Vout) × Iout. All of the voltage difference between input and output is dropped across the pass element and converted to heat. This is why linear regulators are inefficient when the input is much higher than the output.

Filter Capacitors and Step-Start

When multiple high-voltage filter capacitors are connected in series to share voltage, equal-value resistors connected across each capacitor serve multiple purposes: they equalize the voltage across each capacitor, discharge the capacitors safely when voltage is removed, and provide a minimum load on the supply.

A step-start circuit in a high-voltage power supply uses a current-limiting resistor (or other element) in series with the AC input, then bypasses it after the filter capacitors have charged. Its purpose is to allow the filter capacitors to charge gradually, preventing the surge current that would otherwise flow when a large capacitor bank is connected to a charged transformer.

Battery Runtime and Solar Inverters

Battery operating time is calculated as: Time = Capacity (amp-hours) ÷ Average current (amps). A 100 Ah battery powering a radio drawing 5 A average will last approximately 20 hours.

When a solar panel is connected to a grid-tied system, an inverter is used to convert the panel's DC output to AC at the grid frequency and voltage. The inverter is not a charge controller — its role is specifically the DC-to-AC conversion.

Figure E7-2: Linear Regulator Circuit

Figure E7-2 shows a linear voltage regulator circuit with labeled components. Three exam questions ask about the function of Q1, C2, and the overall circuit type.

• Q1: The pass transistor. Its purpose is to control the current to keep the output voltage constant. The base of Q1 is driven by a comparison circuit that senses the output and adjusts Q1's conduction to maintain the set output voltage.
• C2: Its purpose is to bypass rectifier output ripple around D1 (the Zener reference diode). By bypassing AC ripple around the Zener, C2 keeps the reference voltage stable and prevents ripple from modulating the regulated output.
• Circuit type: The overall circuit is a linear voltage regulator — it uses a series pass transistor (Q1) controlled by a feedback circuit to maintain constant output voltage.

E7D Practice Questions