The guy in FB group asked how to charge his 5VDC phone with a 12V solar panel. Several others said to use a 7805 V regulator to drop the 12VDC down to 5VDC. This will do the job, but I told them that the 7805 wastes most of the power as heat, and only a small amount will go to the phone.
The others then gave other ways, but those all wasted most of the solar panel’s power in the regulator. One of them said instead of using the 7805, use a Zener diode and transistor. The problem is he doesn’t understand that the solar panel’s power is precious, and it cannot be wasted. So a very efficient DC to DC converter must be used.
Here is my reply.
“That won’t help. The problem is the output from the solar panels will not be enough to charge the phone because 2/3 of the solar panel’s power is being wasted by the V regulator. The only way to get most of the solar panel’s power to charge the phone is to use a SMPS (switch mode power supply). This converts 85% or more of the 12V to 5V.
A cell phone needs 1 or more amps at 5V to charge. If the 7805 puts out only 1/2 amp or 2.5 watts. The phone may use 2.5W or more just to keep the phone service, WiFi and CPU running even when it’s not being used. With 2.5 W, the phone may never get fully charged.”
Update Jul 10 – Rajesh still doesn’t understand, because he didn’t read my earlier replies. So here is my reply to him.
“Rajesh Shankhalpura
You are missing the point. I posted this earlier but you refuse to read it. If you had, you would understand why you are wrong.
A few more details this time.
The solar panel puts out, for example, 12V at 1/2 amp, or 6 watts. This can be converted with a switching DC to DC converter to 5 volts at about 1 amp. That’s just enough to charge the phone.
Instead you say to save money, use a 7805. So the 12V at 1/2 amp from the solar panel comes into the 7805, and the regulator drops 7 volts, and 5 volts at less than 1/2 amp comes out to the phone. A small amount of current is used by the 7805.
5 volts at 0.48 amp is 2.4 watts. That much current may be used by the phone to keep the phone service, WiFi, Bluetooth and CPU running while it’s trying to charge. Instead of charging in an hour or so, the phone may take all day to charge, or may never get fully charged.
And this is because your 7805 is wasting more than half of the solar panel’s power as heat! So now you have saved money with the 7805, and you have to buy a much larger solar panel to fully charge the phone. You have saved a little and spent much more.
Now that you understand, do you still believe the 7805 is the best choice?”
