Successfully replacing a halogen bulb with an LED

While clearing out an old cupboard prior to painting a room we recently uncovered an old desk lamp that had stood the test of time and was sitting all forlorn for want of a new bulb. Do we simply replace the bulb with another 12V halogen - or do we try one of the new LED bulbs and see how they go?

Well, like many others, we tried the LED route - but it was not quite as simple as we thought it would be....

Choosing a replacement LED
A typical desk lamp is fitted with a 12V 10W or 20W halogen bulb that consists of a glass envelope with two pins coming out of one end.  The whole assembly is known as a G4 bulb.  Therefore, to choose an appropriate LED replacement we need to look for LED G4 bulbs.  But there are lots of different types, designs and styles...

On top of the different styles you've also got to be a little smart when choosing the LED bulb as manufacturers will overplay their light output so a little research and perhaps not choosing the cheapest replacement will often yield better results.

The tiny LED chips on these units - the yellow-and-white blocks in the picture above - come in one of three types known as SMD 5050, SMD 3528 or SMD 3020.  These numbers refer to the size of the chips - for example the SMD 5050 chip is 5.0mm x 5.0mm and the SMD 3528 is 3.5mm x 2.8mm.  One other point to note is that the 5050 chip is capable of three times as much light as the other two simply because they are fitted with three LED chips whereas the others are only fitted with one.

Armed with this I chose a G4 LED bulb from Amazon. It cost me a couple of UK pounds to buy.

Here it is pictured with the bulb it is replacing:

Fitting  the replacement bulb was easy - just undo the three screws holding the protective glass shield, remove the glass shield, pull out the halogen bulb and push in the LED replacement.  Testing the bulb proved troublesome though - it flickered.  And the light levels were much less than I was expecting.

Why is this?

Simple.  The LED is a diode. A diode only conducts electricity one way.  In our simple desk lamp, where a 12V transformer fed the old halogen bulb directly, the bulb was lit with 12V AC.  For a halogen bulb this doesn't matter - it just gets hot and glows.  However an LED replacement will only light up for half of the AC waveform. It will be off for the other half - and this on/off every half cycle will be noticed as flicker.

A note to readers: Opening the base of these lamps as I describe below could expose potentially lethal voltages that can easily kill. If you are not confident about working with mains voltages then please do not attempt the modifications below.  If in any doubt, ask a qualified electrician.

Improving the light quality
To get round this, I built a simple bridge rectifier and smoothing capacitor on a bit of stripboard with spare parts I had around. This should provide a decent 12V DC supply capable of lighting the LEDs properly.  Here is the circuit diagram:
And here is a top-and-bottom view of the finished circuit.  The rectifier is a simple 1A 50V bridge rectifier, the capacitor is a 47uF 25V Electrolytic.  Carefully placing the two components and four wires on a stripboard meant I did not even need to cut any strips.The yellow leads are to be connected to the transformer and the red and black leads to the LED in the lamp housing.

Taking the base of the lamp apart shows how the transformer is connected - the two black leads with eyelets were connected to the metalwork connected to the lamp housing.
First of all I tested the rectifier before installing it in the base of the lamp.  That light was much better - no flicker and plenty bright enough.
Next, I threaded an insulating boot round the rectifier so that it would not touch anything and cause a short.  Then I cut off the eyelets on the transformer's black wires and, using a simple screw terminal block, connected the yellow wires from my rectifier to the black cables from the transformer.  I then crimped fresh eyelets on the red and black wires from my rectifier and screwed these to the base of the frame that supported the lamp.  You can see all this from the next photograph.

Finally, I tucked the rectifier and leads into the base and re-assembled the base making sure no wires were crimped or exposed.  After fully re-assembling the lamp I tested it again and this time success.  Even with the camera flash, the LEDs show up bright and clear - without any flicker.
After adding a rectifier and simple smoothing capacitor I have successfully replaced a 10W halogen bulb with an LED bulb that will last a long time and gives out a very pleasant light to work under.  Not only that, the head of the lamp will no longer get hot, as the LED only consumes a fraction of the power of the halogen bulb.

If you read through the comments below, I've had a question asked by one reader regarding making a rectifier for a larger project involving 10x 2W LEDs in a chandelier.  In this case you may find that the best route here is to get a proper LED Driver to complete the job as this will provide better safety against heat, short circuits and installation concerns.  These are also designed to be installed above the ceilings where chandeliers such as the one my reader is adapting are usually installed.