Power Supplies

Competency 10.0 is the power supply: how equipment turns the mains or a battery into the steady DC its circuits need, and how to service a supply without getting hurt. This page follows ETA's list in order of the signal path: the hazards, transformer and line-connected supplies, rectifiers and their DC voltages, filters, regulation, switch-mode and multiple-output supplies, the integrated high-voltage transformer, batteries and charging, and fuses. Section 12 is job practice. Try the numbers in the Power Supply calculator, the Rectifier Lab and the Battery tool.

Shock hazards in power supplies

Competency 10.1 asks for the shock hazards of servicing power supplies. The supply is where the dangerous energy in most equipment lives, and it stays dangerous after the power switch is off. Treat each of these as live until you have measured it:

Transformer-powered vs line-connected

Competency 10.2 asks how transformer-powered supplies differ from line-connected ones. In a transformer-powered supply the mains feeds the primary winding of a power transformer, and the circuits run from a separate secondary winding. The two windings share only a magnetic field, so the secondary circuits, and the chassis they are tied to, are isolated from the mains. A line-connected supply has no transformer between the mains and the rectifier: the AC goes straight to the rectifier (or a voltage doubler), so the circuit's common is tied to the power line. Leaving out the 60 Hz power transformer saves its size, weight and cost, which is why many CRT televisions were built this way, and every off-line switch-mode supply is line-connected on its primary side.

Why an isolation transformer matters on the bench. A bench oscilloscope's signal common is tied to the safety ground through its power cord, so every probe's ground lead is an earth connection. Clip that lead to a hot chassis and you short the line to earth through the lead; Tektronix warns that the current can heat the lead until it melts open like a fuse. A 1:1 isolation transformer feeds the unit under test from a winding that has no connection to earth, so touching one point of the circuit while you are grounded no longer completes a path, and the scope's ground lead no longer shorts the line. It does not make the circuit safe: a path from one side of its secondary to the other still shocks you. Never lift the scope's own safety ground to "float" it; isolate the unit under test, or use a differential probe.

Rectifier circuits

Competency 10.10 asks you to identify and analyze rectifier circuits. A rectifier is a set of diodes that lets current through the load in one direction only, turning AC into pulsating DC. Each diode must withstand the reverse voltage across it when it is off, its peak inverse voltage (PIV), so the rating has to exceed the figure below with margin.

Full-wave rectifiers deliver two pulses per AC cycle, so their ripple is at twice the line frequency. Diode faults show up clearly. If one diode of a bridge opens, the bridge works as a half-wave rectifier: the DC drops and the ripple rises and falls to 60 Hz. If a diode shorts, the next half-cycle puts the secondary across a dead short and the fuse blows. Watch both cases in the Rectifier Lab.

Calculating DC voltages

Competency 10.9 asks how to calculate the DC voltages in a power supply. Every calculation starts from the transformer's secondary voltage, which a meter reads as RMS. Use the factors on ETA's formula sheet, and allow 0.7 V for each conducting silicon diode (see Semiconductor Basics).

ETA's sheet gives 1.414 (peak from RMS) and 0.637 (the average of one half-cycle). The half-wave figure, 0.318, is half of 0.637 because half the cycles are missing; it is not on the sheet. The 0.637 and 0.318 figures treat the diodes as ideal; real ones take a little off. A filter capacitor charges toward the peak, so the more current the load draws, the further the DC sags below that light-load figure (section 5).

Filtering, hum and the L, T and pi filters

Competency 10.4 asks why supplies are filtered, what hum is, and which filters are common. A rectifier's output is DC with a large ripple riding on it. The filter's job is to pass the DC and block the ripple, the AC left over from rectification. Ripple that reaches an audio stage is heard as hum: 120 Hz from a full-wave supply, 60 Hz from a half-wave one. (A 60 Hz hum in full-wave equipment points elsewhere, such as a ground loop or pickup.) A filter capacitor that has dried out, with its ESR risen and its capacitance fallen, is the classic cause of rising hum.

For a capacitor filter, the ripple follows from the I = C(dV/dt) line on ETA's sheet: between peaks the capacitor alone feeds the load for about one ripple period, so

This is an approximation, and it is not printed on ETA's sheet. It shows the levers: more capacitance, less load current, or a higher ripple frequency all mean less ripple.

The L, T and pi filters (competencies 10.4.1–10.4.3). Adding an inductor (a choke) in series helps because its reactance opposes the ripple while the DC passes, dropped only by the choke's small winding resistance. Which element comes first decides how the filter behaves. A filter that starts with a capacitor across the rectifier is capacitor-input: the capacitor charges toward the peak. The L-section (choke-input) filter puts the choke first, so the output sits near the average of the rectified wave instead of the peak. Hammond's transformer guide gives about 0.90 × the secondary's RMS voltage for a choke-input bridge against up to 1.41 × (the peak) for a capacitor-input one at light load. The 0.90 is on ETA's sheet too: the average over a half period is 0.9 × the RMS value.

Choose the L-section when the load varies and regulation matters; choose the pi when you need the highest, smoothest DC at a modest, steady current. Size the filter capacitor's voltage rating for the no-load peak, and its ripple-current rating for the job: Hammond notes the RMS ripple current in a filter capacitor can be 2 to 3 times the DC load current.

Regulation

Competency 10.5 asks why supplies are regulated and which parts do it. Every source has internal resistance, so its output falls as the load draws more current. ETA's sheet states it for a battery, and the same idea applies to a transformer and rectifier:

An unregulated supply's output also moves with the line voltage. A regulator holds the output steady despite both; NEETS notes that regulators holding within ±0.1% are quite common. How well a supply holds up under load is its percent regulation, which compares the change from no load to full load with the full-load voltage:

Lower is better: 0% is ideal. ETA's 2025 formula sheet does not print a percent-regulation formula; this is the textbook form, the one NEETS describes and the Power Supply calculator uses.

Switch-mode supplies

Competency 10.11 asks how a switch-mode power supply (SMPS) works. Start with what it replaces. A linear regulator drops the difference between input and output across its pass transistor and turns it into heat:

A 7805 making 5 V from 12 V at 0.5 A delivers 2.5 W and wastes (12 − 5) × 0.5 = 3.5 W: about 42% efficient, because a linear regulator's efficiency is roughly V_out ÷ V_in.

A switching supply instead drives its transistor either fully on, where it drops almost nothing, or fully off, where it passes no current, so it wastes little either way. It switches at high frequency, and an inductor and capacitor (a low-pass filter) smooth the pulses to their average. The output is the pulse height times the duty cycle, the fraction of each period the switch is on. For a step-down (buck) converter:

A feedback loop measures the output and adjusts the switch's on-time to hold it steady: that is pulse-width modulation. Other arrangements step the voltage up (boost) or produce several outputs. An off-line supply, such as a PC or phone-charger supply, runs this chain from the mains:

1. Rectify the line. A bridge and a large bulk capacitor make high-voltage DC straight from the mains (line-connected).
2. Chop it. The switching transistor turns that DC on and off at high frequency.
3. Transform it. A small high-frequency transformer isolates the output from the line, and its turns ratio sets each winding's voltage. Working at high frequency is what keeps the magnetics small and light.
4. Rectify and filter again. Diodes and capacitors on the secondary make the DC outputs; the capacitors' ESR sets much of the output ripple.
5. Feed back. An optocoupler carries the output-error signal back across the isolation barrier to the controller.

The trade-offs: switching supplies are smaller, lighter and cooler (TI's buck example runs 80–89% efficient), but the fast switching makes electrical noise (EMI) that must be filtered, and there are more parts to fail. The parts that wear out are mostly the electrolytic capacitors, whose ESR climbs as they dry out.

Multiple-output supplies

Competency 10.7 asks how one supply can deliver more than one voltage. There are three common ways:

A lower rail can also be made from a higher one with its own regulator, for example 5 V logic run from a 12 V rail. When you troubleshoot, check every output: one dead rail with the others present points at that rail's own rectifier, filter or regulator, not at the shared front end. And remember the standby rail: a PC supply that is "off" still has line voltage inside and 5 V on its standby output.

The integrated high-voltage transformer (IHVT)

Competency 10.6 names the "integrated high voltage transformer" supply and asks how it differs from other supplies. It is the high-voltage supply of a CRT (picture-tube) television or monitor. It is legacy technology: Panasonic ended its tube-TV production in 2009, manufacturers stopped making CRT sets during the 2010s, and today you meet one only on older equipment. ETA still lists it, so learn how it works.

A CRT needs 20–30 kV on its anode, far more than any line transformer would sensibly make. The flyback, or line output transformer (LOPT), produces it by working with the horizontal output transistor, which switches the flyback's primary at the horizontal scan rate; the pulses at its collector reach about 1,500 V. A high-turns secondary, or a lower-voltage winding feeding a diode and capacitor multiplier, raises them to the anode voltage. The set's B+ of about 120 V goes in; tens of kilovolts come out. In an integrated high-voltage transformer the high-voltage rectifier diodes (often a diode and capacitor multiplier) and the resistive divider for the focus and screen (G2) voltages are built into the flyback's sealed housing. Its outputs are the CRT's anode voltage and usually its focus and screen voltages, and other windings feed lower supplies for the rest of the set. The CRT's own capacitance filters the high voltage.

Batteries and recharging

Competency 10.3 asks about battery supplies and their common uses. Primary cells are not rechargeable. Primary batteries run remote controls, toys, flashlights, wristwatches, tire-pressure gauges and pacemakers: 1.5 V alkaline cells in the everyday devices, and lithium primary cells in most implanted pacemakers. Secondary cells are rechargeable. Cells in series add their voltages: three 3.6 V lithium-ion cells make a 10.8 V pack. Capacity is rated in ampere-hours: divide it by the load current for a rough runtime, as the Battery tool does.

Recharging principles (competency 10.3.1). Charging drives current back into a secondary cell to reverse the reaction that discharged it. Three rules hold for every chemistry: use a charger made for that chemistry and cell count, limit the current, and stop (or drop to a safe maintenance level) when the cell is full. The chemistries differ in how "full" is detected and what is safe afterwards. Lead-acid tolerates a continuous float, and its charge voltage should drop about 3 mV per cell for each degree above 25 °C. Nickel cells signal full charge with a small voltage dip or a temperature rise. Lithium-ion cannot absorb overcharge at all: prolonged charging above 4.30 V on a 4.20 V cell plates metallic lithium on the anode, and so would a continuous trickle charge, which compromises safety.

Fire classes are a separate question: Class D covers combustible metals such as lithium metal, and a lithium-ion pack is a different hazard (see the Safety guide). Don't try to recharge a primary cell.

Fuses and circuit breakers

Competency 10.8 asks where fuses and breakers go, roughly what size they are, and what to watch for in a house service box. The ratings themselves (current, voltage, interrupting rating, fast-acting vs time-delay) are in the Safety guide, section 8.

Where they go in equipment. The main fuse is normally the first part on the primary side, right after the line cord, so it protects everything behind it, including the transformer or the switch-mode front end. It belongs in the hot (ungrounded) line. A fuse that opened in the neutral alone would leave the whole circuit still connected to line voltage. The NEC states the same principle for building wiring: no overcurrent device in series with a grounded conductor unless it opens every conductor of the circuit (240.22). Some equipment adds fuses or fusible resistors elsewhere, such as on individual secondary rails.

How equipment fuses are sized. Pick a current rating so the normal current is no more than 75% of it; Littelfuse notes a 10 A fuse is not usually recommended above 7.5 A at 25 °C. The voltage rating must be at least the circuit voltage. A capacitor-input supply draws a large inrush at switch-on while its filter capacitor charges, and Littelfuse names that inrush as a common cause of nuisance blowing, which is why such supplies use time-delay fuses.

In a house service box. Each branch-circuit breaker protects its wiring, so its rating is set by the wire size. Under NEC 240.4(D) (2017 edition), 14 AWG copper is protected at no more than 15 A, 12 AWG at 20 A, and 10 AWG at 30 A. Breakers sit in the hot conductors, never in the neutral alone. A 120 V branch circuit uses a single-pole breaker on its one hot leg. A 240 V load runs between the two hot legs, and its breaker must open all of the circuit's ungrounded conductors (NEC 240.15(B)): a two-pole, common-trip breaker. Two single-pole breakers count only with an identified handle tie, and a handle tie does not link their internal trip mechanisms. Before you work near a panel:

1. The main breaker does not kill the whole box. The service conductors and the line side of the main breaker stay live even with the main off; only the utility can disconnect them. Leave the service-side barrier in place.
2. Treat the panel as live until you have tested it with a CAT III meter, live-dead-live (Safety guide, section 11).
3. Replace like with like. A replacement breaker must be compatible with the panel and listed for it, and its rating must match the wire. Never fit a larger breaker to stop nuisance trips.
4. A breaker that trips again is telling you something. Find the overload or fault before you reset it, and never keep resetting it.

On the job: a dead supply

Job practice, not exam content. This walk-through applies the sections above to a linear supply on the bench: 120 V in, a 12.6 V secondary, a bridge, a 4700 µF filter capacitor and a 7812 regulator delivering 12 V at 1 A. Your employer's procedures and the equipment's service manual come first.

1. Make it safe. Unplug it, discharge the filter capacitor (below), and confirm with a meter that it reads near zero.
2. Look and smell. Burnt resistors, bulging or leaking capacitors, cracked solder joints and scorched boards often point straight at the fault.
3. Fuse. Check it out of circuit with an ohmmeter. If it is open, don't replace it yet: measure across the rectifier output and the filter capacitor for a short, and test each bridge diode out of circuit. A diode that reads near zero ohms both ways is shorted.
4. Power up gently. Once nothing reads shorted, fit the correct fuse and bring the supply up with its current limited, through a series lamp or a variac (fed from an isolation transformer), so a remaining fault cannot destroy more parts.
5. Measure the checkpoints in the table below, from the line toward the output. The first wrong reading puts the fault between it and the last good one.
6. Check the capacitors' ESR. A filter capacitor can still read close to its marked capacitance while its ESR has climbed. An ESR meter can test it in circuit (Test Equipment guide).
7. Find the root cause and verify. Ask why the part failed (an overloaded output, a failing capacitor that overheated the regulator) and fix that too. Then run the supply at full load and confirm every checkpoint.

Why a blown fuse is a symptom. A fuse opens because something drew too much current. In Goldwasser's switch-mode repair notes, a fuse that blows instantly means a short on the primary side: a rectifier diode, the filter capacitor, a surge suppressor, or the switching transistor, which can take other parts with it. Replacing the fuse without finding that short only blows the new fuse, or worse. Check everything, even after you find one bad part, because one failure often causes another.

Discharging filter capacitors safely. Unplug the unit and connect a resistor across the capacitor with insulated clip leads, so your hands stay clear. The resistor limits the current: Goldwasser suggests a high-wattage resistor, for example 2 kΩ at 10 W for a 200 V, 400 µF capacitor. Never short the terminals with a screwdriver: the resistor is what prevents the arc welding that a dead short causes. Hold the resistor on for at least five time constants, τ = R × C, then confirm with a meter before you touch anything.