Monday, August 14, 2017

Battery Charger

As part of the house electrical update, I also rebuilt the car battery 12v charger. This has been improved to deliver more current, thus charging the car battery faster.

Circuit Description:
The circuit is built around the voltage regulator U1 - 7812. This regulator has been wired to deliver a higher voltage than 12v, enough to charge a car battery. Output from the regulator is limited to just 1 amp, definately not enough to charge a car battery in a reasonable amount of time. The current has been amplified by using transistors Q1, Q2 and Q3 wired in parallel. The output from the regulator is used as a base voltage/current for the power transistors. Each transistor can carry 4 amp (peaking at 15 amp), thus 12 amp can be easily delivered from this power supply. Obviously, these power transistors will need adequate cooling and therefore I have used a large heatsink. Each Transistor is fed through an 0.1 ohm 9W resistor. This is used to balance the input current. Also, the emitter output from each power transistor is about 30 cm long. This is also used as an emitter resistor to help out in current balancing as well.
BR1 is a 35 amp full wave rectifier. I just needed one diode and opted to use this rectifier since I didn't have a single +20 amp diode readily available.

Parts List:
SW1 - 15amp Toggle switch
F1 - 10amp
C1 - 100nF
C2 - 10uF
C3 - 1000uF
C4, C5, C6 - 4700uF 25v
U1 - 7812 voltage regulator
R1, R2, R3 - 9W 0.1 ohm wire-wound resistors
R4 - 4.7K
R5 - 1K Preset
BR1 - 35amp full rectifier
Q1, Q2, Q3 - 2N3055
Q4 - BC559

This photo below is showing the battery charger connections

The installed battery charger. I opted to install the power transistors outside of the connection box to improve heat dissipation.


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