Simon | # 1 | 2006-01-18, 07:57 | Report

OK, I have been tweaking my little powered USB box. Added a red LED which flashes when the battery voltage gets low and a green LED to indicate it is charging. Can't add any more the bloody box is now full!

I made up an extension USB cable from the hacked remains of the Nokia one and a USB female socket. This allows me to plug things into my little box of tricks and measure how much current is being drawn. Here is what I found:

My USB stowaway keyboard draws bugger all. Not surprising since it had no LEDs and is designed to work with portable devices. I measured about 2mA. Still can't get it to work right though as every keypress just repeats. I think that's an OS thing not the KB which works fine on my real PCs.

My 128mB USB pendrive is more interesting. If you have the USB cable uplugged from the 770 and plug it in (i.e. into the box of tricks by itself) it draws a huge 71mA.

With the cable plugged into the 770 (in normal USB mode) if you plug the pendrive in it draws 37mA.

When you then enable host mode to detect the pendrive the current goes up to 41mA after a few seconds and the drive is detected. At this point the little LED on the drive comes on.

Mounting the pendrive the current jumps to 52mA momentarily. This seems to happen when the drive is being read. With the drive mounted I can the access it via the file manager. Playing an MP3 off it will cause the current to jump from 41mA to 52mA every few seconds as it is reading from the drive. I didn't seem to be able to copy anything to the pen drive though.

I don't really have anything else to try plugging into it that I know will work. I can't imagine ever really wanting to use the pen drive on it unless I wanted to use it as a way of transferring things across. I really wish my keyboard would work though.

Simon

Wooky | # 2 | 2006-01-18, 12:54 | Report

That's quite interesting. I think a 9V battery won't last much with that power draw, right? it would be interesting to build a device that can both use a wall mounted charger and a battery for portability.

Simon, could you provide the schematics for your box? I'd like to build one with a LED to indicate low voltage as well. Do you think your setup would also work with a 7805, non-switching regulator? Thanks.

Mike Cane | # 3 | 2006-01-18, 13:32 | Report

@ desktop:

Aha! I say the same thing. The key repeats (sporadic in my case) are driving me round the twist...

Hedgecore | # 4 | 2006-01-18, 14:55 | Report

I can't wait until Mike loses it and makes a mechanical hammer-style device to hit the onscreen KB like the oldschool impact typewriters

Simon | # 5 | 2006-01-18, 17:45 | Report

I think I fried my low power circuit when I was modding the box the last time. I need to rebuild it. Since I was pressed for space I wired it freestyle so I can't easily replace components in it. I found the circuit on the net here: http://www.techlib.com/electronics/flasher.html The low battery indicator circuit. I just used a small variable resistor in series with a 470k resistor for one of the 1Meg ones so I was able to adjust the actual voltage it starts flashing at.

I am going to get the parts I need for that today and will rebuild it so I can draw up a schematic then. I will post that on my site tonight. I can run mine when the charger is plugged in. But because I am only using 5 AA cells it means I am supplying power through the charging circuit which is current limited to only supply around 90mA. That should be enough though for small things. If I had used 6 cells instead of 5 I could have made it better using a diode so I could effectively supply the 5 volt regulator part and charge the batteries without the limit but because I only have 6 volts to play with fomr the battery the 0.7 volts dropped across the diode means the voltage is too low for my regulator. You can use a 7805 but then you need your inout voltage to be 2 or 3 volts higher than your output. I can't remember the exact amount. The low drop out regulators shouldn't be hard to find though. The one I used is a LM2940CT-5. With that you only need 5.5 -6 volt input to have it regulate to 5 volts on the output.

bhima | # 6 | 2006-01-18, 19:04 | Report

The design I'm working on will probably use two AA batteries. Two of them give you 2.4v at ~2000 mAh. Pretending your conversion to 5v is 100% efficient, that gives you around 960 mAh. That means you can draw 960 milliamps for one hour. Let's say our conversion is 70% efficient - it will likely be better, but that's a lower bound. That works out to be 672 mAh.

If we're powering a 100mA device, which is the most that low power USB devices draw, that's over 6 hours.

Modern switched-mode conversion can be done with one chip, one inductor, two capacitors, and maybe a diode and/or a resistor or two - a very small circuit. IMHO, it's really the direction to go if you're interested in portability. If you have a low power device and don't need too much battery life, you could even bring it down to a single cell.

Simon | # 7 | 2006-01-18, 19:45 | Report

And I thought my design was getting complicated But yeah, a little switched mode type converter running on two cells would be very cool. I did consider using AA cells instead of AAAs but then I figured huge capacity wasn't really necessary. Why have the USB power supply capable of lasting a lot longer then they 770 would last. When I plug in my 770 to recharge at night I can also plug in my USB battery pack. To tell the truth I don't even imagine using it much. It is just something interesting to play with.

The batteries I am using are 900mAH just so you know. One thing I have noticed though is when messing about with my USB drive they seemed to go flat way faster than they should. I need to do some more tests and measure the current at the battery rather that on the USB device. It shouldn't be much more of course but something funny is going on. There might be some bug in my design I think.

JMills | # 8 | 2006-01-18, 20:33 | Report

Tsk tsk Simon, not using bi-color or tri-color LEDs yet? :-)

BTW, I saw your post to Maemo-Developers@ ... the conversation hadn't come up before to my knowledge, hopefully one of the Nokia folks will be able to answer it definitively.

thoughtfix | # 9 | 2006-01-18, 20:34 | Report

I'm going to invest in a rechargable USB power supply soon. It's so much easier than this

JMills | # 10 | 2006-01-18, 20:39 | Report

"Standard sized" 9VDC Alkaline batteries (a battery of six AAAA cells typically) is rated for ~500mAh nominally from what I've been able to determine. 9VDC Lithium are probably in the ~800mAh range.

Rechargeables are in the ~120mAh to 250mAh range, unless you go to a larger form-factor.

The big problem is that as you crest over C/20 (sustained load at 1/20th of rated capacity) and head towards C/10 depletion rates, things like internal resistance and resulting voltage-drop / heat become painful.