Charging 3.6v battery from 12v battery is just a module of the large circuit which i am trying to build. So here is the scenario of the specific module which i want to discuss.
I am using BD139 NPN as a switch. Input base current to the transistor is given by OP-AMP. What i am trying to do is; when the output current (provided by OP-AMP) reaches the base of transistor(BD139), the 12v battery should somehow reduce its voltage (via voltage divider or whatever) to charge my 3.7v AAA batteries (2800mA). So what I've done is; I have configured the transistor to operate in 'Saturation mode' and used 'Voltage division method' to ensure ~5v/~500mA charge to the battery (shown in the screenshot below).
The problem with this method is; my power dissipation is very high across the resistors (approx 6watts total). I want to somehow save my power (using non-active devices) to ensure conserving energy and i don't want to use Buck converter (costly) or linear regulator (which will result in the same power loss).
Kindly suggest me any good approach or transistor configuration scenario, I will be grateful.
Thank you
I am using BD139 NPN as a switch. Input base current to the transistor is given by OP-AMP. What i am trying to do is; when the output current (provided by OP-AMP) reaches the base of transistor(BD139), the 12v battery should somehow reduce its voltage (via voltage divider or whatever) to charge my 3.7v AAA batteries (2800mA). So what I've done is; I have configured the transistor to operate in 'Saturation mode' and used 'Voltage division method' to ensure ~5v/~500mA charge to the battery (shown in the screenshot below).
The problem with this method is; my power dissipation is very high across the resistors (approx 6watts total). I want to somehow save my power (using non-active devices) to ensure conserving energy and i don't want to use Buck converter (costly) or linear regulator (which will result in the same power loss).
Kindly suggest me any good approach or transistor configuration scenario, I will be grateful.
Thank you