Project Specification — Symmetric ±15V Power Supply
Project Overview
Status: Draft. Linear symmetric power supply that generates regulated +15 V, GND, and -15 V rails from a 24+24 Vac center-tapped transformer.
Intended Use
Bench/prototype supply for analog circuits that require symmetric power rails.
What the Device Should Do
- Accept an isolated 24-0-24 Vac input from an external transformer.
- Rectify, filter, and regulate the rails to +15 V and -15 V.
- Provide output on a 3-position terminal block: +15 V, GND, -15 V.
Main Features
- L7815CV and L7915CV TO-220 linear regulators.
- 1N4002 diodes for rectification and regulator protection.
- 1 A fuse on the transformer secondary input.
- Bulk, decoupling, and stability capacitors placed around the regulator stages.
System Architecture
24-0-24 Vac transformer → fuse/rectification → unregulated DC rails → 7815/7915 regulators → output filters → ±15 V terminal block.
Hardware Subsystems
- AC input: 24 Vac, center tap, and 24 Vac terminal block.
- Protection: 1 A fuse on one transformer secondary lead.
- Rectification: diode rectification referenced to the transformer center tap.
- Filtering: electrolytic capacitors on the unregulated rails and regulated outputs.
- Regulation: L7815CV for +15 V and L7915CV for -15 V with datasheet-required capacitors.
Interfaces and Connections
- J1: 24-0-24 Vac transformer input.
- J2: +15 V / GND / -15 V regulated output.
Power and Runtime Expectations
Bench linear supply; no battery. Practical continuous output current depends on the transformer rating, heat sinks, regulator dropout, and thermal dissipation.
Power Tree and Power Budget
- Transformer specified by user: 24+24 Vac, 1 A.
- Regulators: 78xx/79xx family, up to approximately 1–1.5 A with adequate heat sinking.
- Thermal warning: with a 24 Vac secondary, the rectified unregulated rail can exceed 30 Vdc at light load. At high output current, regulator dissipation can be substantial and requires robust heat sinks.
- Safety decision: unregulated rail bulk capacitors should be rated at ≥50 V; the original 25 V BOM rating is undersized for rectified 24 Vac.
Manufacturing and Assembly Expectations
Through-hole and TO-220 parts are suitable for manual assembly. Regulators require heat sinks.
Firmware-Relevant Hardware Requirements
Not applicable.
Physical Design Expectations
Separate the AC/rectification area from the DC output area. Use wide traces for current paths and leave thermal/mechanical clearance around the heat sinks.
Important Design Decisions
- Use L7815CV and L7915CV as direct equivalents for the requested positive and negative 15 V regulators.
- Correct unsafe original BOM ratings: use ≥50 V bulk capacitors on unregulated rails and ≥25 V output capacitors.
- Keep the design fixed-output ±15 V rather than adjustable unless the user requests the adjustable version.
Assumptions
- The transformer is external and connects to the PCB through a 3-position terminal block.
- The intended output is fixed ±15 V.
- Continuous output current must be validated thermally once the actual heat sink and enclosure are chosen.
Change Notes
- Created from the EasyEDA reference image and BOM provided by the user on 2026-05-26.
- Translated project metadata and project specification to English per user request.