New uses for old phone chargers

Inside bunker light

New electronic devices usually come with a charger or power supply. But what happens to those power supplies when the appliance dies and you buy a new one? Lance Turner shows how to reuse them for some simple but resourceful lighting projects.

The power supplies that come with most small appliances, such as mobile phones, are generally very similar. They are usually rated somewhere between 5 volts and 12 volts and may have current outputs up to about 2 amps.

Power supplies of this type may not seem very useful, but you can in fact use them to drive LEDs for lighting with very little effort. What’s more, if you match the LED voltage to your power supply output voltage you can end up with a quite high overall system efficiency.

It should be mentioned here that there are two common types of power supplies. The first is the older type that uses a heavy iron core transformer. These are usually unregulated and their output voltage is dependent on the load placed on them. For instance, a 12 volt supply might actually produce around 18 volts with no load. This variable output makes them a little harder to use. What’s more, the efficiency of this type of supply can be quite low, so they are often not a good candidate for reuse in this manner. Fortunately, most manufacturers are changing over to switchmode power supplies.

Switchmode supplies are generally regulated so that their rated voltage is the voltage that you get out of them. Also, their efficiencies are usually better than 75% at their rated load. Like most plugpacks, switchmode power supplies have isolated outputs, which allows you to connect multiple supplies in series to get higher voltages.

The easiest way to tell which type of supply you have is by the size and weight. Iron core power supplies have a large and heavy transformer inside so they are usually bulky and heavy for their rated output. Switchmode supplies are much smaller and lighter in most cases.

Voltage matching

So how do you use your old power supplies for driving LEDs? Let’s look at an example to explain how to match the LED voltage to the power supply.

You might have a spare 12 volt, 500mA power supply. This is ideal for driving three 1 watt white LEDs connected in series. A single LED might have a forward voltage of around 3.5 volts. Three LEDs in series adds up to 10.5 volts, so any simple current limiting driver (such as a linear driver or resistor) only needs to drop around 1.5 volts (LEDs are current driven devices, so you must have some form of current limiting).

This means that the LEDs receive 85% of the power coming out of the power supply. If the plugpack has an efficiency of 80%, then the overall efficiency of the plugpack/LED driver setup is 0.85 x 0.8 = 68%. In the scheme of things, this doesn’t seem that high, but if you calculate the total efficiency of power input to light output, you will find that your home-made LED light can be more efficient than most domestic lighting systems.

For instance, if high efficiency LEDs such as Q5 bin Cree XR-Es, which have an efficacy of over 100 lumens per watt, are used, then the light fitting could have an efficacy of around 70 lumens per watt overall. This is better than almost all domestic lighting systems except strip fluoros.

LED driving options

So what are the options for driving LEDs in such applications? LED drivers fall into two categories: switchmode drivers and linear drivers.

Read the full article in ReNew 109

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