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The problem
Most electrical students study uninterruptible power supplies only as block diagrams: battery, inverter, charger, changeover switch. The part they never get to see working is the actual mains-fail event — the sensing, the relay throw, the load surviving the transition. This project turns that event into a demonstrable bench rig. A hand-built SG3525 PWM inverter board drives AC lamp loads through a toroidal step-up transformer, backed by two 12 V 7 Ah batteries. A 2-channel relay changeover module, driven by a mains-sensing comparator stage, moves the loads between mains and inverter automatically when the mains is disconnected and restored. A digital multimeter on the board monitors output voltage throughout, so the student can verify the transition live.
Two 12 V 7 Ah sealed lead-acid batteries that feed the inverter board.
Follow the working sequence
How it works
- Two 12 V 7 Ah batteries feed the SG3525 inverter board, which generates a PWM drive for the MOSFET stage and the toroidal transformer.
- The transformer steps the inverter output up to mains-level AC (design target approximately 230 V), feeding one side of the 2-channel relay changeover module.
- A mains-sensing comparator stage watches the incoming mains supply; while mains is healthy, the relays connect the lamp loads to mains.
- When mains fails (demonstrated by unplugging the mains lead), the sensing stage drops out and the relays transfer the loads to the inverter output automatically.
- On mains restoration, the relays transfer the loads back to mains; the inverter idles back on battery standby.
- The digital multimeter stays connected to the changeover output throughout, so output voltage is readable in every state.
Tech stack:
- SG3525 PWM inverter board with LM324 feedback
- Toroidal step-up transformer
- 12 V 7 Ah sealed lead-acid batteries (x2)
- 2-channel relay changeover module
- Mains-sensing comparator stage
- Digital multimeter for output monitoring
- AC lamp test loads
- Wooden base board with labeled wiring
| Parameter | Value |
|---|---|
| Inverter control | SG3525 PWM controller (datasheet part) |
| Output voltage | Approximately 230 V AC (design target, buyer-verified during calibration) |
| Output frequency | Approximately 50 Hz (design target, set by PWM timing network) |
| Battery bank | 2 x 12 V, 7 Ah sealed lead-acid (datasheet ratings) |
| Changeover | 2-channel relay module, mains-sense driven |
| Test load | 2 x AC lamps (design target approximately 40 W combined) |
| Backup time | Varies with load; measured by the buyer during the test procedure |
| Monitoring | Digital multimeter on changeover output |
Project features
- [SG3525 PWM inverter stage] A hand-built inverter board with SG3525 PWM controller, LM324-based feedback and MOSFET output stage drives the toroidal transformer, converting 12 V DC battery power to AC for the loads.
- [Automatic mains-fail changeover] A mains-sensing comparator stage detects mains loss and restoration, and the 2-channel relay module throws the AC lamp loads between mains and inverter without manual intervention.
- [Battery backup bank] Two 12 V 7 Ah sealed lead-acid batteries provide the inverter's DC source, with series/parallel wiring documented for the build.
- [Live output monitoring] A digital multimeter is wired to the changeover output so the student can read output voltage continuously during normal operation, transfer and backup modes.
- [Demonstrable load transition] Two AC lamp loads make the changeover visible: a mains-fail simulation (unplugging the mains lead) shows the lamps stay lit on inverter power.
- [Buyer-run calibration procedure] A documented step-by-step procedure for verifying changeover threshold, inverter output and battery wiring on the student's own build.
- [Complete viva kit] Report covering UPS topologies, inverter PWM and changeover logic, plus PPT and a viva Q&A document.
What is included
- Hand-built SG3525 inverter board with toroidal transformer
- 2-channel relay changeover module with mains-sensing stage
- Battery bank (2 x 12 V 7 Ah) with wiring
- AC lamp test loads and digital multimeter
- Base board with labeled, documented wiring
- Calibration and test procedure manual
- Project report PDF (UPS topologies, inverter design, changeover logic)
- PPT presentation for final review
- Viva Q&A preparation document
Limitations & prerequisites
- The inverter produces a stepped (modified) waveform, not a pure sine wave — stated honestly in the report.
- Output voltage regulation is approximate; exact figures are measured by the buyer on their own build, not claimed upfront.
- Backup duration depends on load and battery state of charge; no runtime is promised as a measured figure.
- The demo operates at safe bench-test power levels and is not a substitute for a certified commercial UPS.
- Relay transfer is fast but not truly break-before-make verified at the sub-cycle level; it is a demonstration-grade changeover.
Frequently Asked Questions
How does the changeover happen?
A mains-sensing comparator stage watches the incoming mains. When it drops below the set threshold, the relay coils de-energize and the contacts fall back to the inverter side; on restoration, the coils re-energize and the loads return to mains.
What makes it an "online UPS" demo?
The rig demonstrates the online-UPS idea that the load is always fed through a conditioned path with automatic source transfer. The inverter stage runs continuously on the battery path so the transfer concept is demonstrable at bench scale.
Is the output a pure sine wave?
No — the SG3525-based inverter produces a modified/stepped waveform. The report states this honestly and explains the difference from pure-sine commercial units, which makes a good viva topic.
How do I verify it myself?
The included procedure has you measure inverter output voltage and frequency, test the mains-fail transfer by unplugging the mains lead, and record battery and transfer behavior on your own build.
Can the backup time be stated?
Only by measurement: it depends on lamp load and battery charge, so the procedure has the student time it themselves rather than the page claiming a figure.
Is this project suitable for a final-year project?
Yes — for Electrical and Electronics programs. It covers power electronics (PWM inverter), transformers, sensing and relay logic, and the demonstration is live and repeatable for the viva. Suitable for B.E./B.Tech final-year projects in Electrical and Electronics engineering.
Components & software requirements
- SG3525 PWM inverter board with LM324 feedback
- Toroidal step-up transformer
- 12 V 7 Ah sealed lead-acid batteries (x2)
- 2-channel relay changeover module
- Mains-sensing comparator stage
- Digital multimeter for output monitoring
- AC lamp test loads
- Wooden base board with labeled wiring
Delivery information
Built-to-order project. Delivery timeline is shared after order confirmation based on current queue.
Support terms
Complete documentation, setup guide, and viva preparation included. Support for setup and explanation provided.
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