Voltage regulator

[FishTankX]

I'm intrigued in doing a project where 3 AAA batteries would be used to charge a device over USB, but i'm curious as to how difficult it would be to create a voltage regulator to maintain the voltage at 5V. Or using more to maintain a certain voltage for charging laptops with C cells and what not. Can anyone point me int he right direction?

 

[jmcoreymv]

I thought AAA batteries are 1.5V each so thats only 4.5V. If you want to use a standard voltage regulator youll need >5V input.

 

[dowxp]

voltage regulators need a lot of current to be useful. i don't think batteries have a lot of current.

 

[shekondar]

Originally posted by: jmcoreymv
I thought AAA batteries are 1.5V each so thats only 4.5V. If you want to use a standard voltage regulator youll need >5V input.

You could do it with a step-up regulator (for example, Maxim MAX1674) - I used one of these recently to produce 5V from two AA batteries in parallel (1.5 V). If you want to use a linear regulator, you would need >5V.

 

[jmcoreymv]

Originally posted by: shekondar

Originally posted by: jmcoreymv
I thought AAA batteries are 1.5V each so thats only 4.5V. If you want to use a standard voltage regulator youll need >5V input.

You could do it with a step-up regulator (for example, Maxim MAX1674) - I used one of these recently to produce 5V from two AA batteries in parallel (1.5 V). If you want to use a linear regulator, you would need >5V.

You can do that but it will increase the part cost a bit over a linear regulator. Its not a big deal if you just want to make a few, but if you want to mass market a low cost device it might be too much.

 

[bobsmith1492]

Switching power supplies are your friend. Look for ltspice switcher cad by Linear Technologies; you can give desired input and output voltages and currents and they'll recommend a chip and circuit setup. http://www.linear.com/designtools/softwareRegistration.jsp

 

[bobsmith1492]

Originally posted by: jmcoreymv

Originally posted by: shekondar

Originally posted by: jmcoreymv
I thought AAA batteries are 1.5V each so thats only 4.5V. If you want to use a standard voltage regulator youll need >5V input.

You could do it with a step-up regulator (for example, Maxim MAX1674) - I used one of these recently to produce 5V from two AA batteries in parallel (1.5 V). If you want to use a linear regulator, you would need >5V.

You can do that but it will increase the part cost a bit over a linear regulator. Its not a big deal if you just want to make a few, but if you want to mass market a low cost device it might be too much.

Switching supplies are dirt cheap anymore. We got a 5 to 12V 100mA switching supply for like 2 dollars (unregulated of course) and it was tiny.

 

[FishTankX]

So wait, the power supply will take any voltage between 5 and 12v and transform it into a steady voltage of my choice?

 

[bobsmith1492]

Basically, yes; you have a few options, but you can regulate it back down from there; not the most efficient way (switch up and regulate down) but it will work.

It would help if you explained your thought better; are you trying to recharge a laptop with standard batteries?

 

[FishTankX]

Basically, I have several devices in mind.

A device that takes 2 or 3 AA's and provides power to a device that charges off USB (iPod, maybe PSP but I don't know if that's a USB charger)
Laptop charger that runs off several D cells and provides a certain voltage to the jack (I think my lappy is either 11.6 or 14.5)

Also, would it be possible to take 10 or so Ni-MH D cells (which are suposed to be 12v stable) and regulate them to keep them from dropping in voltage and run those through one of those 12vDC-120VAC adapters?

 

[bobsmith1492]

If you took 3 rechargeable AAs, you could easily output a 5V supply using a small switching supply. Do you know how much current the thing takes? For a 500mA maximum output, you could use a LT1377 switching regulator and a couple of resistors and capacitors; that would give a nice 5V supply. Try this:

http://i4.photobucket.com/albums/y125/bob_smith1492/power%20supply/3.jpg

About the laptop charger, sure, you could use it; unfortunately, laptops suck up so much power, I doubt you could get much use out of even D batteries....

Some of the best Ni-MH AA batteries are around 2.4 amp-hours; if a D was twice that, say 5 amp-hours, and you used 8 of them for ~12V nominal, that would give you 5 amp-hours at 12V. If your laptop used 12V and about 2.5 amps (most supplies would be AT LEAST that), that only gives you two hours of use - with 12 D batteries. I suppose you could do it, but I don't see how it could be practical.

You could also use them for an inverter (12VDC -> 120VAC), but again, you have the whole power consumption issue. Usually when you have to step up to 120VAC from a battery and back down to DC to power something, it would be much more efficient to convert it directly DC to DC (through maybe a switching power supply).

Well, thar; enough to think about?

Oh, that regulator is in stock at Digi-Key for about 8 bucks. http://www.digikey.com/scripts/DkSearch...l?Detail?Ref=487030&Row=367424&Site=US

 

[pm]

How about something cheaper and easier like an LM7805? <$1 each. Not quite as efficient as the more expensive alternatives, but a heck of a lot easier to use - and cheaper.

Buy one:
http://www.sparkfun.com/shop/index.php?...&match_criteria=all&rec=&keywords=317&

Tutorial on using it:
http://www.iguanalabs.com/7805kit.htm

Datasheet:
http://www.fairchildsemi.com/ds/LM/LM7805.pdf

 

[FishTankX]

Most rechargable D cells are 9AH. So I could get quite a bit of use out of my latitude D400, which's powersupply consumes somewhere along the lines of 40 watts max. Probably about 4 hours of use. That's why this has interested me. Less costly than buying a new battery.

 

[bobsmith1492]

Originally posted by: pm
How about something cheaper and easier like an LM7805? <$1 each. Not quite as efficient as the more expensive alternatives, but a heck of a lot easier to use - and cheaper.

No good for this case, if he wanted to use just a couple of batteries. They have a dropout voltage of at least 1.25V; for a 5V output, you'd need at least 6.3, more like 6.5 volts input; that's like 5 batteries...

 

[pm]

Originally posted by: bobsmith1492
No good for this case, if he wanted to use just a couple of batteries. They have a dropout voltage of at least 1.25V; for a 5V output, you'd need at least 6.3, more like 6.5 volts input; that's like 5 batteries...

Good point.

 

[Chesebert]

3 AAA is only 4.5V and it has an natual log decay so to maintain 5V over a long period of time is difficult and you are looking into some opamp design where you would have one battery as the source and use the somthing on the order of 8 batteries as Vcc Vdd for the opamp. but with that you need to creat a virtual ground..and calaculate some feedback loop and with variable gain to keep your output peggedat 5V...not too hard...with some sodering and some case modding you can create a 5V charger...and it will stay at 5V until the opamp runs out of power......wow that's really complicated..