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Posted: Fri Mar 17, 2006 6:06 pm Post subject: split usb charging, have i got the right idea?
I've got an MDA Vario, and will be getting a Holux GPSlim 236 reciever at the end of the month, so i can run TTN5.
They both charge from a mini USB type plug, As i only have one fag lighter socket in my car for power, so i need to sort out a suitable method of charging both devices at once.
I've been having a look at those car fag lighter plug USB converters, where you plug them into your car, and it will drop the 14.4v from your car down to a 5v USB plug.
What i want to know is: I've got a (really) small 4 port usb hub lay arround, could i just plug this into the car fag lighter plug USB converter thingy, and run USB leads from the mini hub to the PDA and reciever, so that both devices will get charged / powered at the same time?
The reason I'm hoping this would work is that if i go away on the motorbike over summer and stop at freinds or family for a day or two, i can run the sat nav from battery, and use the same setup to charge the PDA and reciever from a normal PC or laptop whilst away, this may not seem much, but as i usually just use a small tank bag (that gets filled to the brim with a couple of t-shitrs, and a toothbrush in it,) space is at a premium and would save me a lot of hastles, by not having to take 2 full sized mains chargers...
I really like traveling light, and this would be a perfect solution if my thinking is right....
I look forward to someone pointing out that my ideas will not work because of the laws of physics, or some other insignificant thing like that, once again! _________________ TT 730
Joined: 11/07/2002 14:36:40 Posts: 23848 Location: Hampshire, UK
Posted: Fri Mar 17, 2006 6:25 pm Post subject:
This is unlikely to work for a number of reasons. Firstly the cigarette lighter USB plug only puts out a few amps and may not have enough oomph to power two devices and secondly the mini hubs don't always carry power.
The easiest option is a 2 IN 1 cigarettle lighter adapter to give you two sockets. _________________ Darren Griffin
thanks for the tip, its just those cigar lighter socket extenders, are getting to the point where on the bike, i may as well take the mains chargers.
As for the mini hub not carrying power, mine does (i'm currently charging my vario through its USB lead plugged into the mini 4 port hub) The hub even has a blue LED that illuminates when plugged into a usb socket.
I thought that the usb standards called for there to be a pair of data wires, and a pair of 0.5a dc power wires (the ones that do the charging) _________________ TT 730
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
Posted: Fri Mar 17, 2006 9:04 pm Post subject:
EvilTwin wrote:
I thought that the usb standards called for there to be a pair of data wires, and a pair of 0.5a dc power wires (the ones that do the charging)
Yes it does, the problem is the voltage, the PDA/ GPS if using a mini USB charging lead require to be fed a constant 5V, your car/ Bike will be typically be 13,8V with the engine running, this will fry the PDA/ GPS.
Using a single cig socket charger and feeding multiple devices from its output is not a good idea, as this will overload the charger leading to a failure of the unit earlier than expected - Mike
Joined: 26/10/2002 10:38:36 Posts: 789 Location: Hong Kong
Posted: Sat Mar 18, 2006 4:23 am Post subject:
I disagree with Darren.
It should work as long as you have both devices off, assuming each individual cable works that way.
It may take a long time to charge both but it should be fully charged overnight. _________________ Pc-Mobile
http://pc-mobile.net/gps.htm
I thought that the usb standards called for there to be a pair of data wires, and a pair of 0.5a dc power wires (the ones that do the charging)
Yes it does, the problem is the voltage, the PDA/ GPS if using a mini USB charging lead require to be fed a constant 5V, your car/ Bike will be typically be 13,8V with the engine running, this will fry the PDA/ GPS.
The device I'm describing drops the voltage down from the typical car voltage of 13.8v - 14.4v down to 5v for the USB outlet, presumably they use a zenner diode, or other similar device. So there is no worry of frying anything, the main concern is the availability of current to power such a setup.
My PDA shows itself to use 100 mA, from the hub (if device properties in windows device manager are correct). so assuming the GPS reciever uses a similar amount of current, then my idea may well work, this seems to be borne out by my vario charging via the USB hub. _________________ TT 730
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
Posted: Sun Mar 19, 2006 11:32 pm Post subject:
EvilTwin wrote:
The device I'm describing drops the voltage down from the typical car voltage of 13.8v - 14.4v down to 5v for the USB outlet.........
My PDA shows itself to use 100 mA, from the hub (if device properties in windows device manager are correct). so assuming the GPS reciever uses a similar amount of current, then my idea may well work, this seems to be borne out by my vario charging via the USB hub.
The idea is sound in some respects, the voltage will be OK, but I have serious concerns about the current.
IF all your batteries in the PDA/ GPS etc are fully charged then it might just work.
IF the batteries need charging this current level will increase and overload your regulator (in this case the USB hub).
I would not rely on what the Windows OS informs you the power that device is pulling, you really need to measure it, IMHO a figure of 500mA is the minimum I would expect from a PDA when powered up, this will increase as the devices radios are used (typically phone, Bluetooth and WIFI), but certain programs that burden the processor can also increase the power drawn by the device - not an easy one to call, sorry to be less than optomistic - Mike
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