2.5 trigger voltage for cell phone charger???

Thread Starter


Joined Oct 13, 2010
I need to find out how to power/charge a Palm Centro "smart phone" ell phone from a USB "power" port ( a port that supplies 5.5 volts only and is not conected to a computer). I have the USB sync/charger combination cord. It powers the phone fine when conected to a computer.

But when I plug it into a USB "power" only port it's a no go. By power port I mean a usb plug that just supplies power. It is not hooked up to a computer or anything other then power. Like you find in some cars. And are on some power plugs to go into a lighter.

I am assuming I will need to modify the sync/charger cable or make an adapter. I'm guessing I need to either connect some of the wires to other wires (jump them), or send voltage down a data point to trigger the phone to "see" the power.

I have tested the USB cable when powered though a turned on computer and plugged into the Centro (phone). Black is Neg. Red is +5 volts. Wire next to red one (green on my cable) is 2.5 volts when tested by touching green and red wire with the voltage meter. I'm assuming the 2.5 volts triggers the phone to start to charge. In fact once the unit is charging off the computer though a powered port. I can turn the computer off, and as long as the hub it is plugged into is powered, and the phone stays connected it will keep charging.

Hopefully you get what I'm talking about.

I need all the help i can get. I am at the limit of my knowledge. I took electrice shop over 40 years ago, and for some strange reason can't recall how to figure this out. I mean it has only been 40 years of not thinking about this stuff.

I need this as we have different phones in the family so we put USB power plugs in all the cars, and use USB cables to power the phones and other toys. The only phone I can't get to work is the centro. I know i can get a car charger, and that is what I'm using, but getting a usb sync/power to work, would make like easier.

Thank for your help.


Joined Sep 9, 2010
Hopefully you get what I'm talking about.
I do. Google around for iPod charger and you'll see how others have solved this problem for iPods. Like your phone, they won't charge unless they see the right thing on the data pins, and cheapo adapters don't provide that.

The solution is a pull-up resistor on one pin and a pull-down on another. I don't recall the details but I built two following that advice and both work fine with iPods. I think it'd be easy to incorporate the resistors into a cable if you can't open up the adapter to modify it.