7.1 AC, DC, Transformers, and Low-Voltage Power Supplies

Why the power supply is the next basic-electricity topic

Chapter 6 treated voltage, current, resistance, and power on circuits that were already DC. The New York Department of State (DOS) Security or Fire Alarm Installer written exam still expects you to know where that DC came from. Module 1.III is 10 of Module 1’s 15 hours. Independent OpenExamPrep teaching in this chapter applies those same quantities to utility AC, transformers, rectifiers, filter capacitors, and the dedicated branch circuit that feeds a fire alarm control unit (FACU), also called a fire alarm control panel (FACP). The same license covers security control panels, so 12 VDC outputs belong here next to 24 VDC fire outputs.

The sitting is closed-book. You will not be handed a transformer nameplate or an NFPA 72 excerpt. You must picture the power path, name primary versus secondary, and keep line-voltage landing (typically electrician work under 19 NYCRR 195.2) separate from the low-voltage alarm work this license covers.

Two electrical worlds on one job

Alternating current (AC) from the serving utility reverses polarity many times per second. In the United States that frequency is 60 hertz (Hz). Typical premises voltages are 120 V line-to-neutral and 240 V or 208 V line-to-line, depending on the service. Many listed FACUs accept 120 VAC as the primary supply; some listed power supplies are dual-rated 120/240 VAC. Read the listing on the cabinet door. Do not assume every panel is 120 V only, and do not land a 240 V two-pole feed on a 120 V-only cabinet.

Direct current (DC) does not reverse. Fire alarm notification appliance circuits (NACs), many initiating device circuits (IDCs), detector power, signaling line circuits (SLCs), and auxiliary outputs are commonly 24 VDC. Many security keypads, motion detectors, and auxiliary outputs are 12 VDC. Chapter 6 already used both voltages in Ohm’s law examples; this section is about how the panel makes them.

The primary of the panel power supply is the line-voltage side: the dedicated branch circuit, or the 120 V receptacle that feeds a listed plug-in transformer. The secondary is the low-voltage side: the transformer winding that feeds the rectifier, or the internal DC bus that leaves the power-supply board as 24 VDC or 12 VDC. Reverse those two words on an exam item and every later answer about lock-on breakers and battery float will be attached to the wrong half of the cabinet.

Transformers — step-down, VA, and isolation (conceptually)

A transformer transfers energy between two windings through a changing magnetic field. It works on AC, not on steady DC. That is why the utility feed and the plug-in transformer are AC, and why the battery (Section 7.2) is a DC source that does not go through the transformer.

Step-down is the usual alarm case: more turns on the primary than on the secondary, so voltage falls and current capability on the secondary rises. Ignoring losses, primary VA ≈ secondary VA (V × I on each side). A common listed security transformer is about 16.5 VAC at 40 VA. 40 VA ÷ 16.5 V is about 2.4 A of secondary current. Hang too many motions, glassbreaks, and a hungry communicator on that supply and the transformer overheats, the panel browns out, and the battery never finishes a charge. VA is the transformer’s power budget, not a decoration on the wall-wart.

Larger listed transformers (50 VA, 75 VA, and up) exist for bigger security panels. Match the listed transformer the panel instructions name. A physically similar 16.5 V part with a lower VA rating is not “close enough.”

Isolation is a second, conceptual benefit: the secondary is not a metallic continuation of the utility hot. The panel still needs an equipment grounding conductor on the line-voltage side. Isolation is not a substitute for grounding, and the FACU DC common is still not a grounding electrode (Chapter 6). Fire panels often hide the transformer inside the cabinet. Security panels often use a listed plug-in transformer on an unswitched receptacle so a wall switch cannot kill the system. Either way, do not land 120 VAC on a 24 VDC NAC terminal.

Rectifiers and filter capacitors

The transformer secondary is still AC. Panel electronics, NACs, and batteries need DC.

A rectifier (diodes, often a bridge of four) converts AC into pulsating DC — a waveform that keeps the same polarity but still has ripple. Filter capacitors store charge and release it between peaks, smoothing that ripple toward a usable DC bus. Polarized electrolytic capacitors are common; reverse them and they fail. They can hold a charge after AC is removed — treat the power-supply board as live until you have verified it is not.

After filtering, a regulator or charger circuit sets the DC bus that (1) runs the panel, (2) float-charges the batteries (Section 7.2), and (3) feeds 24 VDC or 12 VDC outputs. If AC fails, that same DC bus is supposed to ride through on the batteries without a gap that resets communicators or drops two-wire smoke-detector power.

The listing on the power supply is also what makes a fire-alarm secondary power-limited or not (Chapter 5, Article 760). A listed FACU with an integral power source, a listed circuit card, or a listed Class 3 supply is the usual fire path. A listed Class 2 plug-in transformer is the usual security path. The transformer does not become “low voltage” on the primary lugs just because the NAC is 24 VDC.

Dedicated branch circuit — typical NFPA 72 / NEC practice

The DOS exam bulletin bases code questions on the New York State Uniform Fire Prevention and Building Code and NFPA standards. The 2025 Uniform Code of NYS (effective 2025-12-31) references NFPA 70—23 and NFPA 72—22. What follows is typical taught practice, not a pasted code quotation. Confirm the adopted edition and any local amendment with the authority having jurisdiction (AHJ). New York City may be more restrictive; this statewide DOS exam is not an FDNY Certificate of Fitness sitting.

Typical fire-alarm primary power practice:

• A dedicated branch circuit that supplies fire alarm equipment and is not shared with lighting, receptacles, or HVAC.
• A circuit breaker (or other disconnect) that can be locked in the on position with an approved lock-on device, so a janitor cannot casually switch the system off.
• Red identification of the disconnecting means, with marking that does not obscure the breaker’s listing.
• Identification such as FIRE ALARM CIRCUIT or FIRE ALARM CIRCUIT CONTROL (NEC Article 760 and NFPA 72 use closely related wording — know the job, not a slogan fight).
• Access limited to qualified persons.
• Article 760 also commonly teaches that this branch circuit is not supplied through GFCI or AFCI devices.

NFPA 72 is what installers usually cite for the lock-on requirement; NEC Article 760 is what electricians usually cite for identification, dedicated use, and the GFCI/AFCI practice. On this exam, the expected picture is: dedicated circuit, lock-on breaker, red identification.

Security plug-in transformers are a different product. They often sit on a receptacle. That receptacle still should not be switched with the room lights. It is not automatically the same as a fire-alarm lock-on breaker. Do not copy a burglar-alarm transformer habit onto a FACU that the listing says must be hardwired to a dedicated circuit.

Field scenario: a restaurant FACU sharing a kitchen GFCI “because it only draws one amp” will lose primary power when the toaster trips the GFCI. The batteries then carry the system until they are exhausted. That is the opposite of a dedicated, lock-on, identified fire-alarm circuit.

New York license boundary — 195.1 and 195.2

19 NYCRR 195.1(a) defines a security or fire alarm system and states that direct line-voltage connections to an outlet, junction box, or power panel are not included in that definition.

19 NYCRR 195.2(c)(2) says a license is not required for line-voltage connections to an outlet, junction box, or electrical distribution panel.

So: running 120/240 VAC, landing it in a panelboard, and terminating the dedicated circuit at the FACU’s line-voltage lugs is typically electrician work. Connecting the low-voltage NAC, IDC, SLC, communicator, and batteries is alarm work. A statewide master electrician has a DOS path that can waive education/exam for this license (Chapter 2). A helper without credentials does not become an electrician by holding a transformer, and an electrician’s line-voltage exemption does not authorize unlicensed battery and NAC work.

Exam traps

• Calling 24 VDC “the primary.” Primary is the utility / transformer line side.
• Sharing the FACU with a lighting circuit “because it is only 1 A.”
• Treating a lock-on breaker as optional decoration rather than typical NFPA 72/NEC practice.
• Using a GFCI bathroom or kitchen receptacle as the fire-alarm feed.
• Landing 120 VAC yourself without the electrical credential the job actually requires, or claiming 195.2 lets you skip the alarm license for battery and NAC work.
• Feeding a 16.5 VAC security transformer from a switched split-receptacle that dies with the room lights.

Section 7.2 puts ampere-hours on the battery this supply keeps charged. Section 7.3 puts a meter on the DC outputs.