Upgrading an Ebay LED camping light to 12 volts

I bought a cheap battery-powered camping light on Ebay. It has a ring pattern and can be a handy task light. The problems I experienced are short battery life (only an hour or so of good light) and intermittent contacts (I think due to rusty battery terminals or the switch). It would have been pointless to buy another crappy light and so I took the challenge of modifying it to run on 12 volts. It’s rather a silly project, I know, I just wanted to see that it can be done.

 

Upgraded Ebay LED camping light with 12 V power lead.

Theory

I Googled LED circuit and found lots of ideas for modifying my LED camping light.

I hooked up some AA batteries and measured the voltage and current to a single LED with my multimeter. I used different combinations of 1.2 V NiMH and 1.5V Alkaline batteries to get different voltages. You do not have to measure a single LED. You can measure the total current for a parallel group and divide by the number of LEDs:

For LEDs in series, current is the same and voltage is additive.
For LEDs in parallel, current is additive and voltage is the same.

Here are the specifications for my light:

  • Number of LEDs: 48 (2 rings of 24 LEDs)
  • LED configuration: Parallel
  • Operating voltage: 4.5 V (3 AA 1.5 V batteries)
  • Single LED current at 4.8 V = 102 mA (measured)
  • Single LED current at 4.5 V = 80 mA (measured)
  • Single LED current at 4.0 V = 55 mA (measured)

The measured current is higher than 20 mA as quoted for white LEDs on many internet pages. If you don’t have the reference data then do not guess, measure the current. Also, beware that LED current rises exponentially with voltage.

Implementation

I rewired the LED circuit in my light as series-parallel: three groups in series and each group has 15 LEDs in parallel. Each group should have the same number of LEDs, of the same type, for equal currents through each individual LED. Here is my circuit diagram:

Series-parallel LED circuit diagram for 12 V. I have simplified the drawing with only three LEDs in each group (L=LED). For series groups: 4 V + 4 V + 4 V = 12 V. Each LED sees 4 V. For parallel LEDs: 55 mA + 55 mA + 55 mA + ... + 55 mA = 0.8 A, with 15 LEDs. As the groups are in series, the total current is also 0.8 A.

Usually the light will be used at 12 V, although 14.4 V is the nominal output of a car alternator (when the engine is running!). I know the LEDs can handle 4.8 V and a maximum voltage of 14.4 V (= 4.8 V + 4.8 V + 4.8 V) should be safe.

Don’t risk a fire: install an in-line fuse. I would suggest a 1.6 A or perhaps a 2 A fuse: 15 × 102 mA at 4.8 V = 1.5 A. A fuse also saves you in case of accidental short circuits. My light is fused in the power plug. Alternatively, there’s space inside the plastic light body for a fuse holder.

Upgraded Ebay LED camping light, internal view.

Some comments:

  • I have removed the battery terminals and switch. To turn off the light, I unplug it.
  • I made a mistake and broke an LED. Lesson learnt: do not try and remove LEDs from the boards! I had to cut off two more LEDs to get an equal number in each group. This is why I do not have 16 LEDs per group (48 ÷ 3).
  • Cut the wires between the segments first – it’s easier. Then cut the circuit board if needed, with a hacksaw. Leave adequate gaps to avoid short circuits.
  • I soldered jumper wires to complete the LED groups and to make the series connections. Solder lightly. The boards may lose insulation between tracks if overheated.
  • I made an extension lead from some spare cable. The current rating of the cable should be more than the total current of the light.
  • I drilled holes in the housing and soldered the power wires directly to the boards. The assembly is lightweight and it can hang from the power lead. The holes are symmetric so that it hangs level.
  • I connected a car cigarette lighter plug to the end of the power lead. To save money, you can modify any junk car-adapter plug, like an old mobile phone 12V charger.
  • I used grey RTV silicon to plug any holes in the case.

Update: July 2010
The LEDs were overheating in the original design. I reduced the voltage and rewired with four groups in series.

 

Series-parallel LED circuit diagram for 12 V with four groups.

The LEDs are dimmer with lower voltage, but I have used a new light with 48 LEDs (4 x 12). The light output is satisfactory but pretty lousy for the number of LEDs. My Princeton Tec Fuel headlamp has just three LEDs. Lesson learnt: there are millions of junk white LEDs manufactured in China.

Disclaimer: read this!
This help-file details a design for my LED camping light specifically. Operating parameters will change for different LEDs and for different wiring configurations. I do not use current limiting resistors. You could make a wrong design: too low voltage and your LEDs will not light, too high voltage and your LEDs may burn. Take your own risks. If you are not confident and capable, do not attempt modifying or building your own LED lights.

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