Railfan Technology > Wall Chargers for USB Devices

We see these power taps sold all over the place now, even on some AC wall outlets. My concern is that while they substitute for the wall wart plug your Smartphone came with, the mA's may differ.

Are they safe for your device ?

My LG cellphone's wall adapter is for Output 5.1v, 0.7A
My Moto Smartphone wall adapter is Output 5.1v 850mA

Also, the data transfer/charging cable, micro-USB on one end and regular USB on the other, while seemingly identical, are not interchangeable. If I use my LG cable to charge my Smartphone from my PC, it will turn the phone on, but will not go into charging mode. I have to use the Moto one for that. But it wil charge my LG phone.

What is different internally about the two cables ?

Can't describe them, but there are two "official" type cables: Micro USB and Mini USB. One appears slightly fatter than the other. I think it was rivalrie between corporations, such as Verizon, that originally caused this, but that is speculation. I really don't know that the reason is technical.

The proper voltage (5v) is defined in the USB standard. Looking at the current rating (amps), you want an adapter can provide (source) as much or more than your device draws. A device which requires 1.5A is fine with a powersource that provided more. The device will draw what it needs provided the source is capable of that. Sounds like your device is drawing more than the source can provide causing the voltage to drop and prevent the device from charging.

joemvcnj Wrote:
-------------------------------------------------------
> We see these power taps sold all over the place
> now, even on some AC wall outlets. My concern is
> that while they substitute for the wall wart plug
> your Smartphone came with, the mA's may differ.
>
> Are they safe for your device ?
>
> My LG cellphone's wall adapter is for Output 5.1v,
> 0.7A
> My Moto Smartphone wall adapter is Output 5.1v
> 850mA
>
> Also, the data transfer/charging cable, micro-USB
> on one end and regular USB on the other, while
> seemingly identical, are not interchangeable. If I
> use my LG cable to charge my Smartphone from my
> PC, it will turn the phone on, but will not go
> into charging mode. I have to use the Moto one for
> that. But it wil charge my LG phone.
>
> What is different internally about the two cables
> ?

It depends on the current capability of the USB port and the impedance that the phone senses through the cable and/or what charging the device is going to allow. A cable with thicker wires or better connectors can be enough that the phone senses the capability to charge at a higher current, which is easier for the battery circuit, if the phone thinks it is attached to a "low power" port it won't charge.


http://en.wikipedia.org/wiki/USB_Battery_Charging_Specification#BCS



Edited 1 time(s). Last edit at 07/23/14 20:44 by nicknack.

joemvcnj Wrote:
-------------------------------------------------------
> We see these power taps sold all over the place
> now, even on some AC wall outlets. My concern is
> that while they substitute for the wall wart plug
> your Smartphone came with, the mA's may differ.
>
> Are they safe for your device ?
>
>

Avoid off-brand, knockoff, or no-name USB power converters, because the quality control and choice of components may be poor. As long as the rated current equals or exceeds that required by the smartphone or other USB powered device, the converter should work.

I assume 0.7A means 700mA ?

Then it looks like the Moto Smartphone wants more current: 850mA. Is that the difference between the 2 adapters ?
Some AC wall outlets have USB slots as well. You see them attached to the bases of lamps in hotels. But since I don not know their mA output, I am afraid to try them. Be nice not to have to carry around wall warts.

The small end on each cable are exactly the same: Micro USB. Yet the LG cellphone one won't "charge" the Moto Smartphone, yet the PC detects when it has been plugged in since it appears on Windows Explorer. That I don't get at all.

As mentioned above, quality is important. I had a cheap one with two ports for use in my car. Plugged the GPS in and the Cell Phone. It ran the GPS OK but wouldn't charge my phone at the same time. If I unplugged the GPS the phone would charge. Bought a new one and it works fine. The current rating isn't important unless it's less than what your device requires. Think of it like this. The battery in your car can supply over 100 Amps but if you have a security system in your car it might only require 0.1 Amps or less when in standby. The capacity is there but the need isn't. On the other hand, don't put 8 D-cells in series (12 volts) and try to start your car with them. I usually look for 10 to 25% capacity above my needs when I use power supplies. You never know when you might need some additional capacity.

I worked at the phone company. We had a 48V, 10,000 Amp power plant for the switch I worked in. Normally it ran about 6000 amps during the busy part of the day. One day after a tornado went through the area and calls peaked, we were drawing 9600 Amps. The plant was fused at 12,500 Amps. It was a good thing we were not supplied with a 7500 Amp plant.

John

joemvcnj Wrote:
-------------------------------------------------------
> I assume 0.7A means 700mA ?
>
Yes; mA means milliamperes.

> Then it looks like the Moto Smartphone wants more
> current: 850mA. Is that the difference between the
> 2 adapters ?

Yes.

>
>
> The small end on each cable are exactly the same:
> Micro USB. Yet the LG cellphone one won't "charge"
> the Moto Smartphone, yet the PC detects when it
> has been plugged in since it appears on Windows
> Explorer. That I don't get at all.

I'm not sure why it won't charge, but the cable has the additional purpose of carrying data (e.g., downloading pictures).

Is there any harm is using the 850mA adapter on a device that only requires 700mA ?

No...you can always go higher on the the power supply end. The device will only draw what it needs...no more than 700ma.

joemvcnj Wrote:
-------------------------------------------------------
>
> The small end on each cable are exactly the same:
> Micro USB. Yet the LG cellphone one won't "charge"
> the Moto Smartphone, yet the PC detects when it
> has been plugged in since it appears on Windows
> Explorer. That I don't get at all.

The cable quality could do this. But also Motorola might be doing the Apple thing and only detecting Moto components and fully charging with them.

http://blog.curioussystem.com/2010/08/the-dirty-truth-about-usb-device-charging/

This may be helpful.

http://www.extremetech.com/computing/115251-how-usb-charging-works-or-how-to-avoid-blowing-up-your-smartphone

Be Well,

Tim Colbert K3HX

Celebrating 50 years in Amateur Radio.

Did someone mention stay away from cheap no-name junk? There is no transformer in these new USB chargers. They direct regulate 120VAC to 5VDC. If a cheap one fails, not only does not work, but it can fry your device. If fact there have been several deaths in China from these cheapo chargers. The UL rating helps weed out the worst of them here in the US.

Scott Schifer
Torrance, CA
TinyTrains Website