A Guide to USB Chargers for MP3 Players

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A Guide to USB Chargers


The USB port on a computer is commonly used to connect printers, MP3 players, external hard disks, cameras, etc, for the purpose of transferring files between the computer and the device. 
A USB port  not only provides a data connection, it also provides a DC power supply. Many devices now take advantage of the computer's DC power out connection on the USB port to power themselves or recharge their own battery. Examples of this are fans, lights and MP3 players which recharge their own batteries.


This has resulted in a demand for USB chargers which can operate away from the computer, such as USB mains chargers, USB car chargers and USB emergency battery sticks. 
This note provides some information to help you choose your USB charger.


Why is there a problem?


It would be reasonable to think that an external USB charger would replicate the power output of a computer's USB port.
Therefore, if a device could be powered from a computer, it could also be powered from the external USB charger.
There are two reasons why this is not always the case.


1) Power Output Characteristics
A computer produces a 5 volt output to the USB port. The computer uses a large and expensive AC/DC adaptor to convert the voltage and current from the mains supply to 5 volts at the USB port. An AC supply has a frequency and a modulation, that is to say that it is a wave which alternates from a positive to a negative value many times every second. This wave needs to be smoothed out  and reduced to a constant 5 volts. The "quality" of the 5 volts will depend on how good the AC/DC adaptor is. If you need to buy a replacement power supply for your computer, you will find that it is fairly expensive. An external USB charger needs to produce a quality of output which is as good as that produced by the computer.
In addition to the quality of the 5 volts, the external charger needs to supply current which is measured in amps. A computer allows each USB port to consume a maximum of around 400mAh of current. The computer monitors the power consumption of each port and will "shut down" any port which consumes too much and, with Windows, a pop-up warning message will appear. External chargers need to be able to supply at least 400mAh. This is not usually a problem with mains chargers, but it is an issue with battery, solar and wind-up chargers.  For example, weak batteries may not output enough current even if the voltage is correct. Solar chargers generate volts in dim light but require strong sunshine in order to generate amps.
It is necessary to consider the quality of the output of the USB charger. The label, often saying 5 volts max 400mAh, is not a guide to quality. Many devices are built with protective circuitry around the battery which looks at the quality of the electricity and will not allow the battery to be charged in some circumstances.

2) Data Conversations

The USB port specification appears to be fairly straightforward. There are 4 pins. Pins 1 and 4 are for power, with pin 1 being positive and pin 4 being ground. Pins 2 and 3 are for data signals. 
A device like a USB light or fan can connect to pins 1 and 4 and ignore pins 2 and 3 and it will receive 5 volts.
"Standard" USB mains chargers connect only pins 1 and 4. Most MP3 players will recharge from a standard USB mains charger. For example, the iPod Shuffle will charge in this way. We call this a Type 1 Charger.

Why do standard USB chargers not work on all devices?

The reason is because they use pins 2 and 3 also.  Because data is carried by electricity, a computer's USB port has a 5 volt circuit between pins 1 & 4, 1 & 2 and 1 & 3, with pin 1 being positive. An interesting example is the iPod Video, which will not charge if only pins 1 and 4 provide power, but will charge if 5 volts is present between pins 1 & 2, and 1 & 3. The iPod Video is doing nothing wrong in the sense that 5 volts is present between pins 1 & 4, 1 & 2 and 1 & 3 on a computer when a device is first connected to a USB port. The new iPod Nano also charges in the same way as the iPod Video. We will call this a Type 2 Charger.
However, it does become more complicated with devices like the old iPod Nano and iPod Mini and some others such as the Samsung YP-Z5. When the old iPod Nano connects it has 5 volts between pins 1 & 4, 1 & 2 and 1 & 3. It then sends a signal to the computer which results in a 5 volt circuit between pins 1 & 4, 2 & 4 and 3 & 4. Only when these circuits are present, will it allow the battery to be charged. I will refer to chargers which handle this as "Type 3" chargers.
It should be added for completeness that this problem could become more complicated. The USB 2.0 specification allows devices to be supplied with only 50mAh of power (ie, virtually nothing) when they first connect to the USB port. The device and the computer then "talk" to each other and increase the output on the power circuit in response to the needs of the device and what is available to the computer. This will make it more difficult to produce an external USB charger. We will call this a Type 4 Charger.

The Creative V and Creative V Plus, for example, do not charge from a Type 1,2 or 3 Charger.

It is more likely that the charger is not charging because the correct signals are not present on pins 2 and 3 than because of any other reason.

What sort of charger does my MP3 player need?

It is almost impossible to tell in any easy way. I have even seen some models where the charging requirements have changed according to the firmware installed and there are thousands of USB powered devices in the market. Therefore, it is not possible to give a complete list of models which require  a certain type of charger. 
Type 2 Chargers will usually charge Type 1 devices. Most Type 3 Chargers are intelligent and will switch between Type 2 mode and Type 3 mode, and are referred to as Type 2 / 3 Chargers. If you are not sure, and only want to buy one charger, we would recommend a good quality Type 2 / 3 Charger with a maximum current of at least 600mAh.

It is more likely that the charger is not charging because the correct signals are not present on pins 2 and 3 than because of any other reason.


Other things to remember


It is also worth pointing out that some external USB chargers have circuits to prevent over-charging. Your MP3 player should monitor the charging level in its battery and switch off charging when it is full. However, some external chargers will also do this.
Some also include a protection circuit to protect your device from power surges.

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