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Some advices on "self switcher circuit between battery & AC-DC ADAPTER power sources"

To put it shortly, I have small 12V 10A solar system (panels, car battery,  PWM charger, LEDs, car radio, car lighter (used for usb phone chargers etc) backed up with 12V 3A AC-DC adapter. They share common "ground" and outs connected in parallel but I dont want it to draw any power from AC adapter unless PWM controller shuts down power when battery gets empty. Anything below 500mah is neglible.
I dont understand much about relays or microchips perhaps a relay design would be simpler and more efficient...

So I tried to design a circuit with TIP33 transistor(because i found that it supports 10A continuous draw). but apparently it needs like 2V (max3) as Base-emitter voltage. if so, i need to add something between R1 and Adapter +12V and R4 and battery12V to reduce voltage to 2 volts...

I have 3 questions
1st: is there any more efficient and relatively simple way to drop voltage withoutt leds? otherwise i will just use them as both indicators and voltage droppers since current will be lower than 20mah.

2nd: i am not sure if R2 and zener diode would be enough to let T2 draw all the current to keep TIP33 off while battery is on. perhaps i should remove R2 as well?

3rd: Since PWM and adapter both have reverse voltage and overload protection im planning to put a single diode to deny unnecessary current flow on R3 from battery.(i already removed the second one from design just before i write this here). i am thinking about putting resistor values to let only 30ma run there. Would that diode overheat while running on AC or drop voltage from adapter too much?

R3=440
R1+R2=160
R4=anything depends on the T2(sth simple to handle 15-20mah C-E)

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sailorondatea (author) 3 years ago

i think i have to put that second diode back because it loops back through T2...