Ensuring 5v out for USB charging project

[ericspittle]

Okay, I'm a complete noob, so if this is a dumb question I apologize in advance for wasting your time. I spent all day lost yesterday because my cell phone (which also happens to be my GPS) died, and the lighter socket in my car is broke so I couldn't use a car charger (simple solution: fix the lighter socket, my solution: build a AA powered USB charger). Now I've looked over the instructions for the MintyBoost and decided it would be way too complicated for me, it seems much easier to have 4 AA batteries supplying 6v rather than having to use a boost converter to get the 3v from two AAs to the 5v needed for USB.

I haven't done too much with electronics before, but from my understanding I could search for a transformer with 6v input and 5v output, wire it to my battery pack, and a hacked apart USB plug, and have just about what I want correct? My issue is that I know the batteries will lose voltage, and eventually 4 AAs will provide less than 6v. So my first question is: will this cause the transformer to output less than 5v? Is there some sort of (very cheap) transformer I can buy that will, as long as it has at least 5v input, provide a constant 5v output? I'd imagine once it dropped below 5v it wouldn't provide the necessary voltage anyways, and I'm fine with that, I can take those batteries out of my charger and put them in a remote control or something.

Again I'm sorry if this is a stupid question, or if I could have answered it myself much easier, I didn't really know where to look, and searching google for a forum filled with experts seemed like my easiest course of action. I know there are a million ways to make this out of a 9v battery but from what I've read they don't work with any sort of efficiency, nor will they provide a very long charge for most devices, and from the reading I did AAs are much better suited to this task. Any help would be appreciated.

Thank you,
Eric

 

[DerStrom8]

Okay, I'm a complete noob, so if this is a dumb question I apologize in advance for wasting your time.
.....
Again I'm sorry if this is a stupid question, or if I could have answered it myself much easier, I didn't really know where to look, and searching google for a forum filled with experts seemed like my easiest course of action. I know there are a million ways to make this out of a 9v battery but from what I've read they don't work with any sort of efficiency, nor will they provide a very long charge for most devices, and from the reading I did AAs are much better suited to this task. Any help would be appreciated.

Thank you,
Eric

Welcome to Electro-Tech, Eric!
The first thing you need to know is that there's no such thing as a stupid question
Instead of using a transformer, I would recommend using an LM7805 5-volt voltage regulator. However, as you mentioned, when the 6 volts from your AA batteries drains to a certain voltage, the 7805 will no longer be sufficient. Perhaps you could use a rechargeable 9v battery? I'm not sure how efficient those are...
Good luck!
Der Strom

 

[ericspittle]

Or, am I a complete idiot, and do I need to buy +5V Fixed-Voltage Regulator 7805 - RadioShack.com and hook it up to whatever voltage is above 5v I want?

 

[audioguru]

A rechargeable 9V battery voltage drops like a rock when loaded because it is small and its capacity is very low.

 

[DerStrom8]

A rechargeable 9V battery voltage drops like a rock when loaded because it is small and its capacity is very low.

Ok, sorry... bad idea
I still think that a 7805 voltage regulator is the way to go, though, in terms of simplicity and effectiveness.

 

[Reloadron]

Ok, sorry... bad idea
I still think that a 7805 voltage regulator is the way to go, though, in terms of simplicity and effectiveness.

Won't a 7805 require a minimum of 7 volts input to get 5 volts out? Maybe a 5 volt low dropout regulator but I don't see the 7805 working with a 6 volt input.

Ron

 

[peufeu]

AAs have the best power/weight/cost ratio of readily available batteries, so use that.

7805 is not suitable at all, because the voltage drop between input and output is too high.

You need to find a suitable LDO (low dropout regulator) with enough output current (short circuit protection is a bonus), check the search feature at digikey (category is PMIC / Voltage Regulator (linear)) and sort by price !..

 

[DerStrom8]

Won't a 7805 require a minimum of 7 volts input to get 5 volts out? Maybe a 5 volt low dropout regulator but I don't see the 7805 working with a 6 volt input.

Ron

That's why I suggested a different supply. Since a 9 volt battery is not a good idea, you may be able to use 5 or 6 AAs (?) instead. On the other hand, is there some other sort of regulator that would work better than the 7805? If there is, I'd certainly like to know about it!

 

[KeepItSimpleStupid]

The USB charging port isn't quite as simple as you might expect. See **broken link removed**

Motorola Cell phones use a specific cable to up the allowable charging current. Apple does too. The Motorola cable has two resistors inside the cable. I have not yet taken apart a "5V charger" to see what makes it tick.

You might find that OEM chargers can charge a phone faster because of this.

 

[audioguru]

is there some other sort of regulator that would work better than the 7805? If there is, I'd certainly like to know about it!

The 7805 needs an input voltage that is 2V higher than its output voltage so its dropout voltage is 2.0V.
Every semiconductor manufacturer makes low-dropout regulators. They work perfectly when their input voltage is only 0.2V to 1.0V higher than their output voltage so their dropout voltage is 0.2V to 1.0V. I have used the LM2931-5 in many of my 9V battery powered projects.

 

[ericspittle]

sorry it has taken me so long to reply, got a little busy after my first post, but i have been reading the thread on my cell phone. Because i was worried that with only 6v it wouldn't work correctly, especially without brand new fully charged batteries I went instead with 8 AA batteries and the 7805, and actually have it wired up and working now. From some previous reading on the subject i knew that for iPhones (didn't know about it for Motorola, but since my next phone will likely be a droid i'm glad i do now) that this charger wouldn't work, but with my LG enV Touch it seems to be working fine.

Thank you guys for all the help, i'm considering making a couple with a 9V battery to save space and weight and will look into purchasing some low-dropout regulators for that, but since i'm running ~12v in i should be fine on this one if my understanding of what the dropout is is correct, right?

I apologize for any grammar/spelling mistakes in this post, i'm typing on my phone because i'm not at home.

 

[audioguru]

8 AA alkaline cells are 12V when new and are 8V when fairly old. A 7805 works fine when the input is 8V to 12V.

 

[peufeu]

But given how easy it is to use single-chip switching regulator these days, why would anyone do that ?