Power 5v device on 3.6v

[beowulf]

I am experimenting with building a small portable Hard Drive unit for a PDA. Everything I am reading tells me that the PDA does not have enough power to run the drive. I was thinking that maybe an old cell phone battery would work.. Now, this is where my math (and question) come into play...<p>The battery is 3.6V at .7A and the drive runs on 5V at .5A. When I pull out my old P=IV formula, the battery and the drive are very similar (2.5w and 2.54w). Does this mean I could use the battery to run the drive? Would a difference of this size require some sort of voltage regulator circuit? Would it even power the drive long enough to be worth it?<p>Thanks,
beowulf

[Externet]

Hi.<p>..."Would it even power the drive long enough to be worth it?"... <p>Depends on your definition of long.
A DC to DC converter could be used, but batteries have a discharge curve to be taken in account.
Two cell batteries in series and a 5V low dropout regulator would make more sense.
Miguel

[haklesup]

Are you trying to use a microdrive (compact flash type II) in the CF socket on your PDA but just don't have enough power in the PDA battery or is it more complicated.<p>You generally cannot run 5.0V logic with a 3.3V supply. IC devices recognize certain voltage levels as logical 1 and 0. These voltage levels Vil, Vih (for the inputs) and Vol, Voh (for the output) would most likly be shifted in ways so that the logic of your drive is either unreliable or incompatible with the other circuits.<p>Furthermore, without knowing the core technology of the 5V devices (only some kinds of CMOS can tolerate variable voltage), it is likly that they will not even function at all.<p>As for how long it will last. That is unknowable unless we know what the capacity of the battery (mAh) is. Wattage only tells us about the instantanious load capability (and probably only for a fully charged battery) or "how much power do I need right now, not how long can I provide that power.<p>The battery you described wouldn't run a standard HDD at all but it might power one of those micro drives. Do you have a link to some specs?<p>Get a 5V or greater battery and use that. Use the 5V regulator like extranet suggested and that will power up the drive. Depending on the electrical requirements of the I/O pins of this drive and PDA interface, you may also need to build a level shifting buffer to make the 5V logic compatible with the 3.3V logic of the PDA (if thats the case)<p>Can your PDA be powered/charged externally (like from a wall wort) maybe it will have enough power if you rig a battery to that input. A DC-DC converter would consume some of that power in itself and is probably more expensive than a bigger battery.<p>Good Luck, Chris