The numbers on the back of a standard transformer can be baffling to a user unfamiliar with the properties to which they refer and can therefore be a source of great confusion to those who wish to purchase a replacement charger for an appliance. It is important, though, to gain an understanding of the key figures shown on a charger, since getting the values wrong can mean either a charger which provides insufficient power to fully charge the device’s battery (thereby resulting in long term degradation of the battery, and likely causing damage to the charger), or a charger which supplies power in staggering excess to that required by the device, thus damaging its internal components.
How a Charger Works
For a charger to pass charge to a small battery, the mains supply of 240 volts AC must be stepped down to a more manageable value, and be converted to DC current. The components used to mediate both of these changes are described in the following sections.
A transformer is a device which exchanges voltage for current, while keeping the overall power of the supply the same. The relationship between current, voltage, and power follows an equation of the form: power is equal to the product of current and voltage. Therefore, since power must remain the same, any increase in voltage results in a subsequent reduction in current. The transformer is composed of a laminated iron core with two copper wires coiled around it. The core is laminated to prevent loss of energy through heat-generating electric currents, known as Eddy currents, induced in the solid metal core when a changing magnetic field is allowed to pass through it. When current is passed through the input coil, a magnetic field is generated, which then pulls the electrons in the second coil, thus inducing a current in it. The proportion of turns in each coil determines the change in voltage and current when current is passed through the transformer. For example, if the input and output coils each have ten turns, an input voltage of 240 volts will give a 240 volt output, while if the input coil has ten turns, and the output coil has five turns, an input voltage of 240 volts will give a 120 volt output, but a current double that of the input. Because a transformer requires current to be in constant flux, DC current (which provides a constant current, i.e. one which has a flux of zero) will not induce a current in the second coil. Therefore, the AC current must be stepped down before its conversion to DC current.
Converting AC to DC
A small circuit known as a bridge rectifier is designed to convert AC current to DC current. This circuit is composed of an array of four specifically positioned diodes (semiconductors which are able to promote the flow of current in one direction, and impede the flow of current in the other direction) in order to encourage the current flow to be constant; a DC current.
With the basic understanding of the operation of the electrical components contained within an electrical charger, it is now possible to investigate the most important figures when selecting the appropriate charger for an appliance.
Input Voltage and Frequency
The input voltage refers to the electrical push which can be supplied to the device, while the frequency refers to the number of times the voltage alternates from 240V to -240V every second. Frequency is measured in Hertz (Hz), a value corresponding to the number of cycles per second. Most European countries supply their grid with electricity standardised at 240V and 50Hz. Therefore, buying chargers from UK sites should yield chargers capable of handling voltages of this magnitude. The USA and Canada both use a 115V 60Hz supply, meaning that appliances purchased from these countries will often require a converter to allow them to operate from the European grid.
The output voltage is the voltage provided by the transformer to the device. The importance of selecting a charger with the correct output voltage cannot be overstated, since a voltage which is fractionally too high will trigger a protective circuit within the device and prevent charging of the battery. An output voltage greatly above that which the device is designed to handle will result in damage to this protective circuit as well as irreversible damage to the battery itself. Supplying a device with too high a voltage can also result in personal injury caused by explosion of the battery or by the overheating of components.
The output current is typically not of the greatest importance when selecting a charger for a device since, unlike output voltage, this value refers to the maximum current the charger is able to supply and not the constant supply rate of the transformer. Although each device will typically draw only what current it needs, making the output current only an afterthought for many applications, an extra layer of protection can be conferred upon the device by limiting the maximum amount of current provided to it. The main consideration when selecting a charger with a particular output current is to ensure that the maximum current the charger is capable of providing is greater than or equal to the current the appliance is likely to draw. This can be determined by investigating the output current provided by the device’s current charger or by performing a quick search online. Some chargers are designed to output DC current (represented by a solid line with a dashed line parallel to and below it), while some are designed to provide an AC output (represented by a tilde). Even if all other properties are equal, getting the type of current wrong can cause an appliance not to function and can even cause damage to the appliance itself.
The polarity of an output is important only with DC devices, which have both a positive and a negative connection. The majority of device manufacturers are very much aware of the perils of connecting a power supply in reverse polarity and include a protective diode to prevent sensitive components from being damaged if the voltage is applied in the wrong orientation. That said, the main problem with a charger of incorrect polarity is that it simply will not be capable of powering the device. The majority of transformers have a diagram depicting the polarity of the centre and surround of the circular DC plug attached to them. Many mobile phone chargers will use standard configurations such as mini-USB ports, or brand specific connections, which often have signalling channels for detecting critical information about the transformer during charging. Laptop chargers often have a similar signal channel, allowing the computer to determine when the charger is plugged in and supplying current and also to detect possible problems with its operation.
Buying Mobile Phone Chargers and Docks on eBay
By shopping around on online sites, such as eBay, a wealth of chargers to suit any application may be found. A generic search directly from the eBay homepage is often largely unhelpful due to the huge range of irrelevant results which often bury the specific item required. Becoming familiar with eBay’s category system is the best way to glean the most relevant items from the system, and select the most appropriate charger for the appliance at hand. Beginning on the eBay homepage, select Mobile Phones & Communication under Electronics & Technology. From here, choose Mobile Phone Accessories, then Chargers & Docks. Now within a category designed to filter for phone chargers and docks, searching for 3.5V charger, 5V charger, or AC charger provides a specific range of results pertaining to this category. The results of these searches can be further refined by brand or by price using the check-boxes on the left of the page.
Finding a charger or dock with sufficient power to supply a device, while making sure that the charger is not so powerful as to destroy the device which it must power, is one of the greatest challenges of working electrical equipment. The skill of finding the correct charger is of the utmost importance for any device, but with the information provided in this guide, the process of finding the correct one needn’t be so daunting.