Battery Circuit Diagrams

# Battery Charger Circuit Using SCR

To charge the battery, a minimal AC or DC voltage is employed in the Battery Charger circuit. Therefore, when using an AC power source for charging, it’s necessary to take several steps: initially, restrict the high AC voltage, then employ filtering to eliminate any noise from the AC voltage. Next, regulate and stabilize the voltage to achieve a consistent output. Finally, this adjusted voltage is applied to the battery for charging purposes. Importantly, the circuit should include an automatic cutoff mechanism to halt the charging process once it’s complete.

### Outline

• Block Diagram of Battery Charger Using SCR:
• Circuit Diagram of Battery Charger Using SCR
• Battery Charger Circuit using SCR and LM 311
• Principle Behind this Circuit
• Circuit Diagram of Battery Charger Circuit using SCR and LM311
• Circuit Design of Battery Charger using SCR and LM311:
• How to Operate Battery Charger Circuit?
• Applications of Battery Charger Circuit using SCR and LM311:
• Limitations of Battery Charger Circuit:

## Block Diagram of Battery Charger Using SCR:

The AC source is given to the step down transformer which converts the large AC source into limited AC source, filter the AC voltage and remove the noise and then give that voltage to the SCR where it will rectify the AC and give the resulting voltage to the battery for charging.

## Circuit Diagram of Battery Charger Using SCR

Diagram of the Battery Charger Circuit using SCR can be seen below:

## Circuit Diagram Explanation

• The AC mains voltage is directed through a step-down transformer, designed to reduce the voltage to approximately 20V. Subsequently, this stepped-down voltage is introduced to the SCR (Silicon Controlled Rectifier) for rectification, effectively converting the AC mains voltage into a rectified form. This rectified voltage is then harnessed for the battery charging process.

• When the battery is connected to the charging circuit, it typically has some residual charge, and it’s not completely discharged. This residual charge provides a forward bias voltage to the transistor via diode D2 and resistor R7, which in turn triggers the transistor to turn on. Consequently, when the transistor is in the “on” state, the SCR switches off.

• As the battery voltage gradually decreases during the charging process, the forward bias applied to the transistor diminishes, eventually causing the transistor to turn off automatically. At this point, current flows through diode D1 and resistor R3, ultimately reaching the SCR’s gate. This current activates the SCR, causing it to conduct and rectify the AC input voltage, which is then directed to the battery via resistor R6.

• Once the battery voltage reaches an appropriate level, the forward bias current to the transistor increases. Upon the battery’s complete charge, Transistor Q1 will switch on once again, resulting in the SCR being turned off.

## Battery Charger Circuit using SCR and LM 311

Here is another circuit controlled battery charger using an SCR and LM311 . The AC signal is rectified using a SCR and a comparator is used to detect the battery charge voltage with respect to a reference voltage so as to control the switching of the SCR.

## Principle Behind this Circuit

The principle behind the circuit lies in controlling the switching of an SCR based on the charging and discharging of the battery. Here the SCR acts as a rectifier as well as a switch to allow the rectified DC voltage to be fed to charge the battery. In case the battery gets fully charged, this situation is detected using a comparator circuit and SCR is turned off.

When the battery charge drops below a threshold level, the comparator output is so as to turn the SCR on and the battery gets charging again. Here the comparator compares the voltage across the battery with a reference voltage.

## Circuit Design of Battery Charger using SCR and LM311:

Designing the whole circuit depends on the kind of battery used to be recharged.  Suppose we are using a 6 cell, 9V Ni-Cd battery with an ampere hour rating of 20Ah and a single cell voltage of 1.5V. This would set the required optimum battery voltage to be around 9V.

For a voltage of 9V across the potential divider. The voltage across the pot and resistor should be above 5.2V (reference voltage level). For this purpose we select a potential divider arrangement consisting of 22K resistor, 40K resistor and a 20k pot.

The output current of the LM311 is around 50mA, and since we’re using a transistor with a low base current like the BC547, we’ll need a resistance of around 150 ohms. A 230/12V transformer was employed in this project. The transformer’s primary is connected to a 230V AC power source, while the secondary is connected to a rectifier.

## How to Operate Battery Charger Circuit?

When the circuit is first turned on and the battery level is below the threshold voltage, the circuit begins charging the battery. Through the resistor R1 and diode D1, the SCR is triggered by a voltage at its Gate terminal. It then begins to rectify the AC voltage, but just for half a cycle. The battery is charged as the DC current flows to the battery through the resistor R2. The voltage across the pot RV1 and resistor R4 in the potential divider is determined by the voltage across the battery. This voltage is applied to the OPAMP LM311’s inverting terminal.

A Zener diode is used to provide a 5.2V reference voltage to the non inverting terminal. This reference value is more than the voltage across the potential divider during normal charging, and the comparator output is less than the threshold voltage necessary to turn on the NPN transistor. The transistor and the diode D3 thus remains off and the SCR gate gets triggering voltage through R1 and D1.

The voltage across the potential divider now exceeds the reference voltage when the battery starts charging and at a specific point when it is fully charged. This means that the voltage at the inverting terminal is less than the voltage at the non-inverting terminal, and the comparator output is greater than the transistor’s threshold base emitter voltage.

The transistor conducts and is turned on as a result. When the diode D3 is forward biassed, it begins to conduct, which prevents the SCR gate voltage from being triggered because it is now connected to low potential or ground. As a result, the SCR is turned off, and the charging process is halted or stalled. The charging operation resumes in the same manner as before when the battery charge falls below the threshold level. In the event that the SCR is turned off, the resistor R7 and diode D4 ensure that a tiny amount of trickle charging occurs.

## Applications of Battery Charger Circuit using SCR and LM311:

1. It can be used to charge batteries used for toys.
2. It is a portable circuit and can be carried anywhere.
3. It can be used as an automatic battery charger, used specially during driving.

## Limitations of Battery Charger Circuit:

1. The AC to DC conversion here uses only the rectifier and may contain AC ripples as there is no filter.
2. The half wave rectifier makes the charging and discharging quite slow.
3. This circuit cannot be used for batteries with higher Ampere-hour rating.
4. The battery charging may take longer time.
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