Micro Inverter – Battery Charger Circuit

Need a quick power boost? This simple circuit converts 12V DC to 230V AC. Perfect for relaxing with a night lamp or charging your phone. Want to power more stuff? Simply add some extra MOSFETs and watch it go!

Battery Charger Circuit

This circuit has a two stage-battery charger with cut-off and battery level indicator and an inverter circuit. Charging circuit is built around IC1 (LM317) as shown below. When mains 230V AC is available, IC1 provides the gate voltage to SCR1 (TYN616) through diode D3 (1N4007). SCR1 starts charging the battery. For output voltage setting preset VR1 may be used.

The battery level indicator & inverter circuit is shown below. The battery level checking system is built around transistors T1 and T2 (both BC547) along with some discrete components.

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When the battery is charged (say, to more than 10.50V), LED1 glows and piezo-buzzer PZ1 does not sound. On the other hand, when battery voltage goes down (say, below 10.50V), LED1 stops glowing and piezo-buzzer sounds, indicating that the battery has been discharged and needs recharging for further use.

Inverter Circuit

The inverter is built around IC2 (CD4047), which is wired as an astable multivibrator operating at a frequency of around 50 Hz. The Q and Q outputs of IC2 directly drive power MOSFETs (T3 and T4). The two MOSFETs (IRFZ44) are used in the push-pull configuration. The inverter output is filtered by capacitor C1.

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Construction and Testing

Assemble the circuit on a general-purpose PCB and enclose it in a suitable metal box.

Refer the pin configurations before mounting the components on the PCB. Mount the transformer on the chassis and the battery in the box using supporting clamps. Use suitable heat-sinks for MOSFETs.

The circuit can be used for other applications as well by delivering higher power with the help of a higher current rating transformer and additional MOSFETs.

Safety and Protection Measures

1. Fuse Protection: Use fuses on the input and output to prevent short circuits and overcurrent damage.
2. Snubber Circuit: To prevent voltage spikes, connect a snubber across MOSFETs.
3. Heat Management: To reduce overheating, attach heat sinks to MOSFETs and consider using a fan with higher power.
4. Voltage Protection: Install cutoff circuits to disconnect the battery when the voltage falls below 10.5V or during voltage surges.
5. Insulation: To avoid shocks, insulate high-voltage components and utilize a grounded metal casing.

Testing and Troubleshooting

1. Initial Check: Inspect all connections and component orientations to ensure there are no short circuits.
2. Voltage Testing: Measure the output without a load for 230V AC and confirm the CD4047’s 50Hz frequency.
3. Load Testing: Begin with a modest load and gradually increase, checking for temperature increases in components.
4. Common Fixes: Check the frequency of oscillations for low output; enhance ventilation or utilize a lower load for overheating; and modify output filtering for flickering.

Application and Practical Case Studies

1. Backup Power: During power outages, used as backup power to power small equipment such as lights and fans.
2. Portable Outdoor electricity: Use automobile battery electricity to run camping lights or small AC equipment.
3. DIY Solar Projects: Pair with a solar panel to create a low-cost, off-grid AC power source.
4. Education and prototyping: Ideal for studying the fundamentals of DC-AC conversion as well as practical electronics.

The article was first published on 5 February 2018 and has recently been updated on 28 October 2024.