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Electronics => Projects, Designs, and Technical Stuff => Topic started by: BRetonDP on May 13, 2015, 01:26:05 am

Title: (Beginner) Making a circuit to charge a power bank
Post by: BRetonDP on May 13, 2015, 01:26:05 am

Hello, EEVBlog members!

I am making a circuit which charges a cell phone power bank and have stumbled upon an issue.

I am using a very simple center tapped full wave rectifier and a 7805 IC to get the 5V output that the power bank takes as input. Of course, two things come to mind:

• My power bank says it takes a maximum of 800mA as input. Since I don't really know what's inside the power bank, I think limiting the current in the circuit is a good idea, but how would I go around doing that? I opened my textbook and found a circuit, which I added to my build, but I'm pretty sure it is not working.
• Even if I do manage to limit the output current, would that be all I need to have a functioning circuit? Is the bank ready to be plugged? Do i need the current limit in first place? Am I missing something

Every little observation and criticism is appreciated. Thanks guys!

Title: Re: (Beginner) Making a circuit to charge a power bank
Post by: moffy on May 13, 2015, 01:43:00 am

Can't work out what the TIP32C is doing except act as a series diode? The LM7805CT already has a 1 to 1.5 Amp current limit, as well as overtemp, do you really need more protection?

Title: Re: (Beginner) Making a circuit to charge a power bank
Post by: BRetonDP on May 13, 2015, 01:47:41 am

That's the issue, I'm not really sure if I need it  :P
I would like to try and limit the current that actually goes into the bank to around 700mA, even if its not really necessary to have it. The transistor is there because I was trying to built a circuit that I saw in my textbook but it didn't work and I just left it there.

Title: Re: (Beginner) Making a circuit to charge a power bank
Post by: moffy on May 13, 2015, 02:15:32 am

The following circuit is a rough guide. Hope it helps.

Title: Re: (Beginner) Making a circuit to charge a power bank
Post by: BRetonDP on May 13, 2015, 02:26:37 am

Hey, that's pretty neat, thanks!
Will Vin be the output of my regulator? Also, just to be sure, R1 is a 1ohm resistor?

Title: Re: (Beginner) Making a circuit to charge a power bank
Post by: moffy on May 13, 2015, 11:42:23 pm

Vin is the output of the bridge rectifier, you don't want the current limit voltage drop in normal operation to mess with your regulator output voltage, that's why it's on the input side. Yes, 1 ohm/1watt will do the trick. The PNP power transistor needs to have enough gain for the 470 ohm resistor to saturate it in normal operation, so a Hfe/Beta of about 100 or better should be fine.