8.2 Transformers, Voltage Regulators & Zener Diodes

8.2 Transformers, Voltage Regulators & Zener Diodes

Quick Answer: (V_p / V_s = N_p / N_s). Step-up needs more secondary turns than primary. Series primary capacitor can improve power factor. Linear regulators vary conduction of a pass element; switching regulators use on/off duty cycle. Zener diodes are the usual reference in linear regulators (ratings about 2.4 V to 200 V and above).

Power for radiotelephone gear almost always starts with a transformer (or offline switcher with a transformer) and ends with a regulated DC rail. Topic 3-C key topics 021–022 test both layers.

Transformer turns ratio and voltage

Ideal transformer relationships (sinusoidal steady state, neglecting losses):

[ \frac{V_p}{V_s} = \frac{N_p}{N_s} \quad \Rightarrow \quad V_s = V_p \frac{N_s}{N_p} ]

Step-up transformer: must have more turns of wire on its secondary than on its primary so (N_s > N_p) and (V_s > V_p).

Step-down: fewer secondary turns; used for low-voltage filament and logic supplies after rectification.

Worked example — pool style

Primary 2250 turns on 120 VAC, secondary 500 turns:

[ V_s = 120 \times \frac{500}{2250} = 26.67,\mathrm{V} \approx \mathbf{26.7,V} ]

Distractors like 2300 V reverse the ratio or invent wild multiplications. Always multiply primary voltage by Ns/Np, not Np/Ns when solving for secondary.

Second worked example

Primary 200 turns, secondary 500 turns, 20 V on primary:

[ V_s = 20 \times \frac{500}{200} = \mathbf{50,V} ]

Same wire size on both windings is assumed in the pool’s resistance comparison questions; voltage still follows turns.

Isolation transformers vs autotransformers

An isolation transformer’s secondary can float (within insulation ratings) relative to the primary earth reference. An autotransformer always has a conductive path between input and output through the shared winding—never treat it as a safety isolation barrier.

Power rating, wire, and winding resistance

Transformers are rated in VA (or kVA): product of secondary voltage and secondary current the unit can deliver continuously within temperature rise limits. Exceeding VA rating overheats the winding insulation.

Pool resistance insight: a power transformer has one primary and three secondaries of 5.0 V, 12.6 V, and 150 V. Assuming similar wire sizes, the 150 V winding has the highest measured DC resistance because it has many more turns of the same wire → longer conductor → more ohms. Low-voltage high-current secondaries use fewer turns (and often heavier wire in real designs).

Current roughly transforms inversely with turns (ideal): (I_s / I_p \approx N_p / N_s). High secondary voltage pairs with lower secondary current for the same power transfer.

Core materials and losses (exam-level)

Power-line transformers use laminated silicon steel. RF transformers and wideband baluns often use ferrite or powdered-iron cores. Laminations and ferrites both fight eddy losses at their intended frequencies.

Series capacitor on the primary — power factor

A capacitor is sometimes placed in series with the primary of a power transformer to improve the power factor. Inductive magnetizing current of the transformer is lagging; capacitance supplies leading vars that can improve power factor seen by the line. Pool distractors about “rectifying the primary” or “improving regulation” as the defining purpose are wrong for this stem.

Rectifier frequency link (appears under transformer group)

Ratio of output frequency to input frequency of a single-phase full-wave rectifier: 2:1.

With 60 Hz AC in, a full-wave bridge or full-wave center-tap delivers 120 Hz ripple frequency (two peaks per cycle). Half-wave would keep 1:1. Remember this when sizing filter capacitors: higher ripple frequency is easier to filter for a given C.

flowchart LR
  AC[AC line Vp] --> T[Transformer]
  T --> R[Rectifier]
  R --> F[Filter C]
  F --> REG[Regulator]
  REG --> LOAD[Radio DC load]

Voltage regulators and zener diodes (3-C-022)

Linear electronic voltage regulator

In a linear electronic voltage regulator, the conduction of a control element is varied in direct proportion to the line voltage or load current. A series pass transistor acts like a variable resistor, dropping the excess voltage as heat so the output stays constant. Efficiency falls when (Vin − Vout) and load current are both large.

Switching electronic voltage regulator

A switching electronic voltage regulator switches the control device on or off, with the duty cycle proportional to the line or load conditions. Energy is stored in inductors/capacitors; less average power is burned in the switch than in a linear pass element for large step-down ratios. Switching supplies need filtering to control RF hash—critical next to receivers.

Zener diode as reference

What device is usually used as a stable reference voltage in a linear voltage regulator? A zener diode.

In a regulated power supply, what component most likely establishes a reference voltage? Again, a zener diode (not a tunnel diode, SCR, varactor, bare battery, or the pass transistor itself as the reference element).

Zener voltage rating range (pool): about 2.4 volts to 200 volts and above. Distractors that compress the range to a few volts only are wrong.

Simple shunt zener regulator: series resistor from unregulated DC, zener to ground, load across the zener. The zener holds approximately Vz while excess current is absorbed in the zener and resistor. Series regulators use the zener as a reference into an error amplifier that drives a pass transistor for better load handling.

Three-terminal regulators

A three-terminal regulator (classic 78xx / 79xx / adjustable LM317-class idea) contains a voltage reference, error amplifier, sensing resistors and transistors, and a pass element in one package. It is not defined as “three separate output voltages” or “three error amplifiers only.” Pins are typically input, output, and ground (or adjust).

Load regulation concept

Load regulation describes how much the regulated output voltage changes as load current varies from light to full rated load (often expressed as a percentage or mV change). Good regulators hold Vout nearly constant. Line regulation is the companion idea for changing input voltage. Element 3 emphasizes how linear and switching controllers act and what provides the reference (zener), which is the foundation of regulation performance.

Worked intuition — series pass + zener reference

Unregulated 18 V DC after filter → zener 12 V reference → error amp compares sample of output to 12 V → drives series pass transistor so the load sees a stable 12 V until dropout (Vin too close to Vout) or current limit. If load current jumps, the control element conduction changes in proportion (linear case) to restore the set point.

Exam-day checklist for 021–022

1. (V_s = V_p \times N_s / N_p); step-up ⇒ more secondary turns.
2. Highest voltage secondary (same wire size) ⇒ highest DC resistance.
3. Series primary C ⇒ improve power factor.
4. Full-wave rectifier ripple frequency ratio ⇒ 2:1 vs line.
5. Isolation vs autotransformer ⇒ separate windings vs shared winding / isolation difference.
6. Linear ⇒ proportional conduction; switching ⇒ duty cycle on/off.
7. Reference ⇒ zener; range ~2.4 V to 200 V+; three-terminal IC packs reference + error amp + pass element.

With transformer and regulator literacy locked, you are ready for diodes, SCRs, and triacs that actually steer and switch the power.