Hello, I was wondering if it was possible to draw 1.2V and 800mA from a USB port to power an mp3 player that requires a AAA battery. How would I go about doing this? Thanks!
its possible, but it shouldn't be just tapped directly, you should ask the USB root hub for it. You will need a voltage regulator. but it would be better to tap it off the internal molex power connector.
There are some power conversion circuits that do what you want. Take a look at some of the Maxim chips.
That's the capacity. How many hours does it last with typical usage? A=Ah/h (Current draw is battery capacity divided by hours of usage.) An extremely simple charger can be made with a diode and a resistor. As long as the - terminal of the battery is connected to USB ground, connect the diode in series with the resistor and use that to connect USB Vcc to the + terminal of the battery. (The diode prevents charge leaking.) The value of the resistor should be such that the desired amount of current flows (C/10 or lower in most cases). For your application, it should be 40 ohms or a little higher. (47 ohms rounding up to next standard value.) Just don't leave it plugged into the USB port for too long (more than 15 hours or so) or the battery will overcharge and wear out faster. Dedicated battery charger chips can also be used if you want automatic charge control and fast charging. Also note that the limit for current draw on USB is 500mA (100mA for low power ports), which you should take into account with larger batteries. Don't forget that the device probably already uses some current, too. BTW, I have modded my TI83+ to have an integrated USB port that allows for communications and charging. I used a charge pump to get enough voltage to charge the batteries.
500mA at 5v. (2.5w) thats enough to get 800ma, which is more than you need anyway. Not to mention you shouldn't just blindly tap the USB port, as the bus has the abality to tell a device to consume a tiny <1mA current in sleep mode. Any power you taske from USB should be requested, and not taken until you've got permission.
It's not a real world issue as long as you don't use too much current. It's really no different than taking power from a molex, just not as much current. In fact, going over the limit may be fine (although not a good idea) in many real world situations. I have a 2.5" HDD enclosure that's powered from USB, and I'm sure that some 2.5" HDDs use more than 500mA at startup. So just make sure that the total current draw doesn't exceed 500mA, and then it should be fine.
I have a laptop i bought for £80 because the USB had died because of something like that (took about 1A as it was going into hybernation, part of the chipset is dead now) As such i wouldn't recomend this.
I just recently was running a fairly simple circuit off of USB power (it was not a usb device though - I was just too lazy to hook up my 5V supply). Anyways - it was behaving a bit eratically, and eventually I figured out that USB wasn't supplying enough current. It needed wayyyy under 2.5W - so it seems that the USB controller was waiting for the extra power to be requested.
That is very individual from motherboard to motherboard how the current is supplied, some can be overloaded a lot while some can not be loaded over max without breaking or switching off a safety feature. Doesn't most of those USB powered 2.5" drive cases use dual USB plugs to take power from 2 root hub jacks, to get a full 1A?
most mobile HDDs need full 1A for such a short time, the problem becomes a case of power management. let me try and explain this again, i'm a slightly obsessive compulsive person, who has a mind which uses a certain logic. Now this says to me, when the specifications of USB EXPLICITLY state the manner in which power can be tapped, you should obey it. There are reasons for this, i know i only understand less than a 3rd having only made USB devices using off the shelf sie-integrated PICs, never making my own route hub, i decide i should obey these specifications. Now my mobile HDD (laptop HDD) thats powered of the USB uses 1A at start but i've only ever see suck 300ma for a few seconds (if your REALLY intrested i can capture this on a scope some time next week!) at once running barley uses 250. Now it makes the assumption that you plug both cables onto the same root hub, and uses the fact this means the only time the power management would mess up, the USB controller for the HDD will already be going to "sleep" mode. Long story short. Its bad. Drugs are bad. If you want 5v, theres no shortage of other places (game port, molex, cheap 5v adaptors. i actually have a 12v,9v,7v,5v,3.3v ring on my desk (just cheap plugs n wire + braiding), which is used for charging phones, mice, walkmen, tourches even nimcid batteries.)
TheAnimus: I'm not saying it's the correct way of connecting it, because it's not, i'm just saying that is how it's done in the ones i have seen
