Simple Battery Charger Circuit using SCR

Battery chargers are critical components in electrical circuits because they replace the energy in rechargeable batteries.

Among the different approaches for building battery chargers, the use of Silicon Controlled Rectifiers SCRs offers a reliable and efficient solution.

SCRs, a type of semiconductor device, are known for their ability to precisely control and correct high voltages and currents.

SCR are a four layer, three junction semiconductor device that functions as both a switch and a rectifier.

It can be turned on with a tiny gate current and remains on until the current through it falls below a particular level.

This feature makes SCRs ideal for managing power in AC circuits, which is a significant advantage for battery charger applications.

SCR circuits can be easier to develop and implement than other methods.

This simplicity generally translates into lower costs and easier maintenance.

Circuit Working:

Parts List:

Here in this article a basic SCR based battery charger circuit is shown, the SCR converts the AC mains voltage into DC to charge the battery.

When the battery is discharged, its voltage drops, this drop prevents the forward bias voltage from reaching the base of the transistor T1 through VR1 and D2, turning the transistor off.

When the transistor is off, the SCRs gate gets a triggering voltage via R1 and D3, causing the SCR to conduct and rectify the AC input voltage.

The rectified voltage is then supplied to the battery through the resistor R5, starting the charging process.

When the battery is fully charged, the base of T1 receives a forward bias signal through a voltage divider circuit made of R3, VR1, R4 and D2 turning T1 on.

This action cuts off the trigger voltage at the SCRs gate, turning the SCR off.

In this state, only a small amount of current flows to the battery via R2 and D4 for trickle charging.

Since the charging voltage is half wave rectified, this charger is suitable only for slow charging.

For faster charging, a full wave rectified voltage is required.

The Pot VR1 allows you to specify the voltage of the battery at which charging should cease.

In this circuit design a 230V primary step down transformer with a secondary voltage of 15V / 3A transformer is used.

If required the battery can be linked to the charger circuit by crocodile clips.

You can experiment the battery charger circuit on a PCB or a common board.

Formulas:

Here are some crucial formulas for the simple battery charger circuit using an SCR:

1: Resistor Calculations

Power Rating of Resistors:

Resistors must be capable of handling the power they dissipate.

The power dissipated by a resistor can be computed using ohms Law.

P = I² × R

where:

• P is the power in watts W
• I is the current in amperes A
• R is the resistance in ohms Ω

You can also use:

P = V^2​ / R

where,

• V is the voltage across the resistor in volts

2: Triggering SCR Gates

Current flow is enabled by triggering the SCRs gate.

Both the gate trigger voltage VG and the gate resistor R1 have an impact on the gate trigger current IG.

Calculating Gate Trigger Current:

If the gate resistance G and gate trigger voltage VG are known, then:

I_G ​= ​V_G​​ / R_G

3: Current for Charging

The series resistor and the rectified output voltage determine the charging current to the battery:

Calculating Charging Current:

I_charge ​= ​V_rectified​ − V_{battery​​} / R_series

where,

• Vrectified is the DC voltage that has been rectified after filtering
• Vbattery is the battery voltage 12V
• Rseries is the series resistor 100Ω 5W resistor R5

The battery charger circuits design and analysis are based on these above formulas.

Changes and further thought may be required in light of particular needs and elements.

How to Build:

To build a Simple Battery Charger Circuit using SCR follow the below mentioned steps for connections:

• Assemble all the required components mentioned in the above circuit diagram.
• Connect transistor T1 collector pin to a diode D1, connect base pin to center leg of pot VR1 through a diode D2, and connect emitter to ground.
• Connect a resistor R3 from positive supply to first leg of pot VR1 and third leg of pot VR1 to ground.
• Connect SCR cathode leg to a diode D4 and resistor R2 in series, connect anode leg of SCR to a transformer primary of 230V, connect gate leg SCR to diode D3.
• Connect a resistor R5 and a fuse in series to a positive supply of battery from a SCR cathode leg, and connect a negative supply of battery to ground
• Connect transformer 230V one wire between resistor R1 and resistor R2 and second wire to ground.

Conclusion:

To conclude, battery charging demands can be effectively managed with SCR based battery charger circuits because they combine simplicity, efficiency and control.

These circuits offer a dependable way to preserve battery health and guarantee peak performance in a range of applications by utilizing the special qualities of SCRs.

References:

Battery Charger Circuit Using SCR