Sorting things out with a small circuit

[alkenerly]

Hi, as a beginner into the electronic creation world, I'm working on designing a portable USB charger to charge my iDevices and such on the go. I need help ironing out the details. I am going to use 5 rechargeable D cell battery's in series totaling 6V with each battery being 10,000 mah each to go through a 5v linear voltage regulator then to a female USB. What I was wondering is; will the iPhone and other USB devices draw way to much current because there is nothing restricting the current of the D cells?

Thanks for any reply, I will probably have a few follow up questions for each answer thanks!

 

[KrisBlueNZ]

No, the iPhone and other USB devices include a charge current regulator that limits the current drawn from the USB port.

Find the charger that was supplied with the iPhone and other USB device and see what current it's rated at. Often they're about 1A. Your 5V regulator only needs to be able to provide that much current. (Add them together if you want to be able to charge two or more things at once.)

If you're concerned, you can get regulators with current limiting built in.

Dropping 6V down to 5V at significant current will require a "low dropout" regulator, also called LDO. Typical regulators like the 7805 need their input voltage to be at least 2~3V higher than the output voltage, otherwise they "drop out" of regulation.

Check Digikey or Mouser for low dropout regulators with current limit.

Good luck!

 

[alkenerly]

Thanks for the reply, could you recommend a voltage regulator model #? and I have read something vague about iDevices wont recognize a "dumb" USB port, and that I need a small series of resistors for the 2 data leads of the female USB for it to recognize and charge. Can anyone help me jump that hurdle, thanks so much!

 

[davenn]

do a google search on LDO regulators as kris suggested

also do a google search on the Ipod USB resistors, there's info for that available as well

Dave

 

[alkenerly]

Change of plans

I've looked at the regulators and prices, and decided to propose a different plan. I'll add another D cell making 5 cells and 7.5volts and a more normal and cheaper 7805 regulator.
I've attached a circuit I made using "iCircuit" on my iPad. Please look it over to see if I made any flaws. So currently my list is 5 D cells and holders for them; 7805 voltage regulator; 1k resistor; wire; female USB. So what do y'all think? Will I need a heat sync for the regulator?

 

[KrisBlueNZ]

Nearly all regulators need decoupling capacitors across the input and output, for stability. Look at the 7805 data sheet.

I would stick with a 6V battery and an LDO. Less heat, less power wasted, more standard battery, smaller. Suitable LDOs aren't THAT expensive. Definitely cheaper than an extra cell!

Here are three that look pretty good.

Texas Instruments LP3965ET-ADJ
http://www.digikey.com/product-detail/en/LP3965ET-ADJ/NOPB/LP3965ET-ADJ/NOPB-ND/364747
Adjustable; 7V max input; 1.5A rated output; 0.38V typical dropout at 1.5A; 5-pin TO-220; USD 3.48.

Micrel MIC29312WT
http://www.digikey.com/product-detail/en/MIC29312WT/576-2231-ND/1029145
Adjustable; 3A rated output; 0.6V typical dropout at 3A; 5-pin TO-220; USD 2.50.

Exar SPX29302U5-L
http://www.digikey.com/product-detail/en/SPX29302U5-L/1016-1244-5-ND/2411176
Adjustable; 3A rated output; 0.6V typical dropout at 3A; 5-pin TO-220; USD 2.31.

Be aware that regulators draw current from the input source all the time; this is called the quiescent current or the ground current. The amount of current is specified in the data sheet, which you should DEFINITELY download and examine for each of these devices. You will probably need an on/off switch in series with the battery, otherwise it will discharge constantly due to the regulator's ground current.

 

[alkenerly]

You are amazing! Something I'm not familiar with its the 4th and 5th pins on these regulators. As I only have very basic knowledge of electricity some things confuse me. Can I complete this circuit without the capacitors? Do 3 pin regulators exist with close to the same specs as the ones you listed. What would I use the other pins for? Thanks for giving me so much of your time!

 

[KrisBlueNZ]

All three of those regulators are adjustable. One of the pins is probably used to adjust the output voltage. Download the datasheets. They will have a sample circuit diagram for a typical application.

I said that almost all regulators NEED bypass capacitors, so no, you can't complete the circuit without them.

Three-pin LDOs do exist. Those ones I linked to are the three cheapest suitable options I could find at Digikey. They are all adjustable, and they all have five pins.