Combined USB/Power Cable for TomTom One

Leblanc · Occasional Visitor Joined: May 05, 2006 Posts: 17

I have just bought a Combined USB/Power Cable for TomTom One on E-bay for £4.99 inc postage.
The USB part is fine but I am afraid to use the power cable part.
Do you think the power from my computer via the USB port would damage the TomTom One?
Any technical experts?
I do not have a power meter.

Eldar · Posted: Wed May 10, 2006 3:48 pm Post subject:

It might damage your computer if you attempt to charge the ONE when the battery is nearly empty.

Leblanc · Occasional Visitor Joined: May 05, 2006 Posts: 17

I used the combined retractable USB/Power cable and it worked well.
I am very happy with it.

Eldar · Posted: Thu May 11, 2006 4:31 pm Post subject:

If you are absolutely sure that your USB port can provide up to 2A peak charging current, then go ahead.

steveJ · Posted: Thu May 11, 2006 6:55 pm Post subject:

Hi Leblanc

You should be perfectly ok using the aforementioned adaptor, and it will not damage your GO.

The reason for the TomTom adaptor for the GO being rated at 2A is so that the GO can be charged at a quicker rate because the GO can handle current this high.

If your GO is connected to the adaptor you bought it will simply "trickle charge " at the lower current that the USB socket outputs.

The downside to this is that your Go will take considerably longer to charge from flat, but doing it this way at least you can keep it "topped up" while its connected to your computer.

Steve

Eldar · Posted: Thu May 11, 2006 7:50 pm Post subject:

Steve, are you sure about that? My understanding is that GO will take as much current up to 2A as it can sink and if the USB port is not properly protected against that, it might damage the computer (I must admit I'm not very up-to-date on the current USB schematics)

steveJ · Posted: Thu May 11, 2006 11:11 pm Post subject:

Hi Eldar,

Before I got my new 910, I had been using just such an adaptor with my old TT300 for quite a few months, and not run into any PC problems with USB. Several others have also done the same.

Take the case of a battery charger used to charge a nicad battery, if your charger has a variable output (and a lot of them do), and you wish to charge a 400 mAh nicad, you would typically set the output to 40 mA and the battery would take approximately 10-14 hours to charge

If you were to reduce the charge current to 20mA, the battery would take twice as long to charge. Note that the battery will not / cannot increase the charge current beyond the max, it will only use the maximum available.

It should also be noted that a typical self powered USB hub will typically only deliver about 100mA per port, unlike the 500mA of a normal USB port (you'll just need to trickle for longer).

It may be prudent as you say to avoid charging a completely flat battery this way to avoid any sudden surges, but not entirely sure about this.

Steve

steveJ · Posted: Fri May 12, 2006 12:09 am Post subject:

To follow up on the above, the quote below is from an article by Len Sherman, Maxim Integrated Products, Sunnyvale, CA

"The popular USB interface can charge a portable device while transferring data. But for high-capacity batteries, the 500-mA output current of USB hosts and powered hubs greatly extends the charging time. (Unpowered USB hubs supply no more than 100 mA.)"

So I think no harm should come.

Steve

Andy_P · Posted: Fri May 12, 2006 1:31 am Post subject:

I'm with you Eldar...
Unless the USB socket has some current limiting circuitry (more cost, so I doubt it) I don't understand how the laws of physics can change so a load will only "ask for" as much current as the source can safely provide.
Isn't it like saying that if I put a six-inch nail (ie a dead short) across the mains it won't "ask for" more current than the 13Amp fuse in the plug?

Eldar · Posted: Fri May 12, 2006 10:48 am Post subject:

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