I just started the design for a 12v to 5.6v converter for charging Axims and other PDAs from Firewire or a 12v battery pack. I'm considering the MAX724 as it's a high efficiency voltage regulator. http://pdfserv.maxim-ic.com/en/ds/MAX724-MAX726.pdf
There is already a starting point (the reference circuit).
Changes necessary include: a protection diode for reverse polarity protection, larger capacitors for better voltage regulation, and change the resistor values to slightly increase the voltage.
I was going to wait until I had a chance to do some current measurements.. but in case this will help
This is a 12V to 5V converter I built (one switching regulator, and one cap)... I also plan a 6V output (part is in the mail).
I didn't see a lot of parts btw 5.25 and 6V, tho I guess worst case you could take a 6V part and voltage-divide the output, wasting ~8% of the power.
Anyway, I have not tested at low battery charge levels, but with 5V and a sync/charge cable, it was charging and on at the same time, b/c this part can deliver 2A (== 2 billion watts using Kozak's math ;) )
Jake, thanks for clarifying (again) the limitations of drawing power from USB. Now my question is this: would the same limitations apply using a USB sync cable that is drawing power from a regular wall outlet? I have a sync cable adapter that allows this. I guess what I was wondering was if the cable itself had limits or just the actual USB port.
Jake, thanks for clarifying (again) the limitations of drawing power from USB. Now my question is this: would the same limitations apply using a USB sync cable that is drawing power from a regular wall outlet? I have a sync cable adapter that allows this. I guess what I was wondering was if the cable itself had limits or just the actual USB port.
The wiring in the sync/charge cable I bought on ebay is ~ 24 guauge, which if you look on
So the issue is the current source. Some/many USB ports won't be able to supply the current. Wall chargers should. In theory, a non-cheapo car charger (using a decent switching regulator) should also.
Actually I'll qualify this... if you use their voltage drop widget (6V source) you'll see that for 1.5A load, on a 3 ft cable, 24awg, there is a 0.3V drop... so its possible that on a 5V source (battery packs) the Vdrop could be enough to drop below the min charging voltage on the Axim. With a 6V source, it should not be an issue. I'd have to measure the output of the sync/usb (ebay) wall charger.. the Dell supplied on put out 5.69V.
So the issue is the current source. Some/many USB ports won't be able to supply the current. Wall chargers should. In theory, a non-cheapo car charger (using a decent switching regulator) should also.
Mike
Thanks, Mike. That makes a lot of sense. However, I have noticed screen flickering when wifi is running and screen brightness was at max. This occurred even when the main battery was almost completely charged. Does this imply there is still some bottleneck at the cable? (Sorry, but my understanding of power is nil.)
Gigi, see my post again, I was edittnig and added the qualify section.
If you have the ebay charger? I'll try to measure that this weekend. I need to hack up a USB cable so I can do power measurements (so I can insert my multimeter into the path).
I've seen the flickering issue too, but haven't researched it much (other posts I quickly skimmed seemed to suggest it might be the Axim switching between its 2 normal power sources, and/or between deciding if the supplied voltage/current is enough). Once I hack up a cable, I can measure from the ebay stuff to verify what it delivers, and then use a voltage supply to drop the voltage and current to see if I can induce the flickering and find some bounds on when it occurs.
Also need to verify they used in the construction of the wall chargers (ebay ones) as to how much current they can supply... which is partly why I just built my own portable source instead ;)
In direct answer to your question.. it could be the wiring, IF the power source was only giving a minimum voltage, and the high current draw caused enough voltage drop.. or it could be the source itself is limited. What are you using and do you have the means to do any voltage measurements on its USB output port?
Ebay.co.uk has a Sync'n'Charge cable for the Axim x50v. I ordered it a few days ago and its just come today. Works, but the screen flickers momentarily whilst switched on. It does charge the battery, though.
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Gigi, you and Mike are in a dialog and will sort it out, but basically the issue is the SOURCE, not the WIRES that convey the power. The USB Standard only requires 100, with an option for 500 mA to be provided. That means that no "well behaved" USB Device should demand more than 500 mA at a maximum. Wall units designed to use the USB connector can provide as much power as the designer wants to engineer in, so it could be possible to provide sufficient power for the Axim X50 in that way. As Mike says, it COULD be the wiring, but it's much more likely to be the source.
That makes a lot of sense, Jake. From my experience, it definitely appears to be more related to the source; however, I am still puzzled as to why my screen flickers even when charging from a regular power source (i.e., the wall outlet). I'll wait for Mike's testing to see what he finds out. :)
Jake is correct. The only thing I find curious is that Gigi can induce flickering using a (non-OEM) wall source (which one would expect to supply plenty o juice)... which implies to me that either they really chose a limited AC/DC converter, or chose to supply only a minimum level voltage, in which case the Vdrop across the cable might come into play (since it sounds like Gigi is trying to enable every possible option at the same time ;) and thus is drawing a relatively high amt of current).
I guess one interesting experiement would be to drive the USB and AC power lines separately, and reduce the voltage/current on the AC to see when it chooses to switch over to USB (as a power source, not a charging source).. not sure I want to mess with my connector that much, but maybe.
I suspect it uses the AC voltage until it drops below 5, in which case it swaps to the USB until it drops below 5, at which point the battery kicks in. In Gigi's case, she has the brightness set at one level for battery, a different one for powered, so as it uses/drops the external power, the screen brightness changes--flickering.
This is a 12V to 5V converter I built (one switching regulator, and one cap)... I also plan a 6V output (part is in the mail).
I didn't see a lot of parts btw 5.25 and 6V, tho I guess worst case you could take a 6V part and voltage-divide the output, wasting ~8% of the power.
Anyway, I have not tested at low battery charge levels, but with 5V and a sync/charge cable, it was charging and on at the same time, b/c this part can deliver 2A (== 2 billion watts using Kozak's math )
It's great that some chicks just love to play with technology. However, the "two billion watts" statement is *very* incorrect. Every chick who has even a little knowledge of electronics will know that (for DC circuits anyways) W=V*A. It will be 10w.
My design is 90% complete, and I just need to stress test it with a large load. I'm thinking of a 2.5A load (for the Axim, worst case is 2.41A).
I'll probably use some of those large resistors left over from an old project.
So far, the only changes from the reference design are:
1. Change the input and output capacitors to 1000uF.
2. Add a diode in series with the input.
3. Use a 1k resistor for the lower resistor of the voltage divider and 2.2k in parallel with 4.7k for the upper resistor.
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Originally Posted by A friend of mine who has a Linux kernel named after his girlfriend.
If I was VirtualBox, I could load my virtualization module into Hannah and boot up another kernel in the same address space.
Gigi, the mystery is easily solved... upon inspection of the "ebay" power converter, it says "12V 400mA".... using unknown's model AC08 transformer (but Google came up empty on any datasheets for exact specs)... anyway, even if they did a 100% efficient power conversion, this wall transformer should max out under 1A of current, and as demonstrated in the other threads, the Axim can demand much more (the Dell brick provides 2.41A).
If I could find their spec sheet, it might be easy to find a way to upgrade the device, but prob not someone most people want to do (I find the form factor appealing, so I'd prob do this one day).
LOL, I wonder how bad the car adaptor is (will investigate later).
I guess I'd say you might have a "charger", but not a "during use" charger, at best (same for me).
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