# SparkFun Forums

By Bartowskio
#191264
Hello,
Everyone in here! I am a sophomore.
I want to build a mobile charger that is battery operated and found this diagram. At first glance, this diagram looks kinda simple and I thought that this won't work. I'm a newbie on making this kind of projects and its look like a simple one so I want to know how and why this diagram works. If this diagram really works I really need help in converting it
into a schematic diagram.

and also what is the diffference between the two in charging power?...
I'm an electronics hobbyist, not an EE. I am using the chip from http://www.kynix.com/Detail/601390/7805%201.5A.html I've done enough reading that I would feel comfortable building a regulated power supply using a linear regulator. It's not that hard, and well documented. (I'd rely heavily on research and build to somebody else's specs.) Linear regulators waste a lot of power however. They basically use a transistor in linear mode to control voltage, which makes it into a variable resistor. That burns all the excess voltage as heat. A buck converter is much more efficient, but a lot trickier.
My mobile's battery is a Li-Ion Battery, 1200mAh (4.44Wh), Nominal Voltage is 3.7 and Chargeable Voltage is 4.7V. Well I really want to build a portable mobile charger that is powered by DC so i could charge on emergency instances (power breakages, middle of the journey, etc.)

:dance: :dance:
My mobile device has an internal charger that (purportedly, at least) automatically maintains the battery if the phone is connected to an external 5V power source. So... if I use that first drawing (minus that resistor), and if I leave the battery (cell) in the phone & don't try to charge it directly from that circuit but instead connect that circuit to my phone& let the phone's internal charger handle all the details, then it'd probably work fine for me. The second circuit can't deliver enough current to do the job.
Best regards!
Bartowskio

By jperch
#192247
Bartowskio,

Given your admitted lack of experience in electronics, I would advise against trying to charge your battery directly. There are a number of devices out there that can be used to charge a single lithium cell with very few components, I think it would be better to just use the charging circuit in your mobile device and provide it with the 5V it expects.

This would actually be the way I, myself, would do it as it is that much easier to just provide a basic 5V to a charging cable and let the internal circuitry take care of the hard stuff. This is essentially what a USB port does with a charging cable.

The circuit you show below is very much a simple circuit that would provide 5V to your mobile device. You are also correct that it is a linear regulator and that linear regulators are not that efficient. Even so, they have their uses. Also, as you indicated, if you use that circuit, I would get rid of the 5ohm resistor in the output.

If you want to use a switch mode regulator, I would use google to find some switch mode regulators. There should be tons of them out there. A basic buck converter would be only slightly more complicated than what you are showing and there are a number of devices that have "application circuits" in their datasheets that you can simply copy directly (or make slight modifications to) to get what you want. I recall National Semiconductor had a web tool called simple switcher. I just googled it, It looks like TI has continued that tool online. You can check it out. Just type "simple switcher" into google and it was the first link to come up for me.

You could also try Linear Technologies (www.linear.com). They have a HUGE selection of both linear and switch mode regulators. They even have a parametric search tool where you can enter your input votlage range, your desired output voltage and your desired output current and they will list potential devices. As I recall they are also really good at having basic circuits in their datasheets. You can probably find a circuit there that will suit your needs.

Another option if you want to keep it simple is something like this: https://www.adafruit.com/product/1385
(I hate to point you to an Adafruit product on a Sparkfun forum but I couldn't quickly find an equivalent on SF). The UBEC is a buck converter that puts out 5V from a 6V to 16V input. I have used this exact product in one of my projects when I determined that the internal regulator on an Arduino ProMini (a linear regulator) was just getting too hot for my comfort. I simply added this product and it just worked. I have also used various brands of these BEC devices when I was flying RC aircraft. They work really well.

I hope all that helps,
Joe