Car digital camera charging adapter

The attached picture shows the digital camera charging adapter circuit. The circuit uses a pair of transistor differential amplifiers and a first-level current amplifier. Output constant voltage and required current respectively. Transistors T1 and T2 form a pair of differential amplifiers. The base voltage of T1 is stabilized at 3V by the Zener diode ZD1. The T2 base voltage is obtained by dividing the power supply through R3 and R4. T1 and T2 compare their base voltages with each other and adjust the base current of current amplifier T3 accordingly.

Transistor T3 is connected as a current amplifier to provide sufficient current for charging. T3 adopts TIP127. It is a medium power Darlington transistor with a current amplification factor of 1000. Therefore, if the base current of T3 is 1mA, the maximum collector current will be 1A. T3 is connected to the inverting amplifier working mode. Therefore, when positive voltage is applied to the base of T1, T1 will output a negative pulse to drive T3 after it is turned on. R2 provides a current of about 1mA to the base of T3. So T3 collector will output 1A.

R3 of 100Ω plays an important role in the process of regulating the output voltage and current of the differential amplifier. The current through R3 is the sum of the emitter currents of T1 and T2. Since the T2 collector is directly connected to the 6V power supply, its collector current is fixed.

Therefore, the T3 base current changes with the current flowing through R2 and R3. This design can stabilize the charging voltage and current. Any change in the output voltage is detected by transistors T1 and T2, and the base current of T3 is adjusted accordingly.

The power supply of the adapter is obtained from the 12v car battery through the cigarette lighter socket. The voltage regulator chip 7806 (IC1) reduces the 12V input voltage to 6V and limits the output current to 1A.

Bypass capacitors C1 and C2 should be close to the pins of Ic1 when welding. IC1 must use a suitable heat sink to dissipate heat.

LED1 and LED2 are used to indicate "charging" and "charged full" respectively. When the digital camera is connected to the adapter, LED1 lights up to indicate "charging", while LED2 remains off. The 4.7V Zener diode ZD2 functions as a diode switch. It only drives T4 into the conductive state when the battery terminal voltage exceeds 5.3v (=4.7v+0.6v), causing LED2 to light up to indicate "full charge". Whether the rechargeable battery reaches 5.3v can be adjusted by VR1. D1 and D2 in the picture are used for polarity protection.

The circuit can be installed on an ordinary PCB board. and packed in appropriate boxes. Plug the plug of a car mobile phone charger into the cigarette lighter socket in the car.