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.


  1. Thanks for your information. It's really useful.I got lot of information from your blog. I was really looking for some info about successfully replacing a halogen bulb with an LED.Click here for more information.G4 Halogen Bulbs

  2. You were lucky to have the space in the lamp head. When I tried a similar conversion I found I could not get a bright enough LED in the space allowed for the halogen. Same applied when I tried to convert bathroom overheads from tungsten to LED. And when I converted the whole house to LED versions of 50watt Gz10's I found I needed the 5 or even 7 watt versions to get the light hue/brightness and so they all now poke through the ceiling by an inch and I cannot refit the old covers. Seems odd to see LED lamps with huge heat sinks, but I gather they just don't work properly if allowed to heat up even a little.

  3. Cool stuff! I like the detail of this operation 'replacement'. This post will serve as a guide whenever I'll deal with halogen bulbs.

  4. Nice work! Thanks for sharing it.

  5. LED light bulbs are available at affordable prices. Therefore, you should take a step forward to buy this element from market.
    LED Light bulbs

    1. Yes they are, I agree. But that's not the point of this post.

      The whole point of the post was to show you how to select a 'bright' replacement LED lamp and how to make sure you get the best output from it.

      I note there are no G4 compatible lamps in your web site?

    2. Hi Martin I have posted a comment at the bottom of this blog as in need of some advice regarding LED drivers. Any would be appreciated

  6. Nice article and more information posting! We have always maintained a modest and prudent attitude in the LED market and making constant development, the overall quality of staff have made great progress and improvement.We are a Manufacture for LED vintage light bulb, LED Lighting products,Include LED tube,LED bulb,LED Downlight,2835 flexible led strips ,LED Floodlight,LED ceiling light etc.

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  7. Very nice article. I suppose something change if I change the led light (I mean, since the consumed current changes) and if I need to feed a chandelier with 10 lights.
    I mean, I purchased G4 26 LED SMD 1210 Warm white light bulb lamp, DC 12V - power 2W. It means that each led bulb consumes about 330 mA, isn't it? The chandelier has alogen bulbs, 12V AC - 20W max. So, it means that the lights on the chandelier are connected in series.
    Therefore, it means that I need about 3.30 A output from the rectifier. Let's consider 5 A output from the rectifier. How to dimension the electrolytic capacitor, please?

    1. You're on the right track, but there is some logic missing from your post.

      First of all, are you sure that the bulbs are connected in series? I would find this unlikely - and probably unsafe. Please be very, very sure of the exact connection of the replacement bulb - is it a G4 12V bulb or is it a G9 mains bulb? If you are not sure, then please ask an electrician, because if you get it wrong you could cause a fire - or worse, electrocute yourself.

      Come back to me when you can confirm these points and we'll work the rest out between us.


  8. Dear Martin, bulbs are not connected in series. I was definitely wrong. So sorry, maybe I was tired. I summed up he consumed currents; how the bulbs could be connected in series?!? I wrote a nonsense. So sorry again. Surely, 10 bulbs connected in series, with 12V-20W, it is definitely unsafe.
    Let me resume. Each 12V-2W LED bulb consumes 330 mA, and the whole 10-bulbs chandelier would consume 3.30 A with those LED bulbs. Before the rectifier I have 12V AC. When it is converted to DC, I have (12*√2-2)V DC. This is because we consider 12V Veff in AC, but we have Vmax in DC, and Vmax=√2*Veff. Minus the voltage drop on the bridge rectifier. Am I right? If I am, since I need 12V DC, I need a resistor in series with the bridge, and the series between the bridge and resistor should be in parallel with the capacitor, isn't it? Finally, capacitor should be 25V but what about the uF? Thanks a lot.

    1. Hi.

      You're on the right track, but you've made a couple of errors:
      - A 2W LED will consume approx. 160mA from a 12V supply.
      - 10x LEDs would therefore consume approx. 1,600mA

      At this level a typical smoothing capacitor would need to be about 8000uF and would be quite large. For something like this I would recommend that you steer away from a home-made circuit and get a proper commercial LED driver. I've updated my blog to show a suitable LED driver that may be better in this case. These are not expensive and are designed to work with loads as large as this.

    2. Dear Martin, thanks for your suggestion. I apologize, you are right: I applied the formula for AC electric power to DC. You are right about the consumed current, but I disagree about the capacity of the capacitor.
      Since electrolytic capacitors have 20% of tolerance, I consider to dimension the capacitor according to the following formula:
      Since I=1.6, f=100Hz and Vp=10mV (the ripple voltage) -> C~=2.04F
      So I think to assume 2.5F/25V electrolytic capacitor. But I think it is still too large, am I right?

    3. Hi Matteo

      That's a very very small ripple voltage you are trying to get to there. No wonder the capacitor is so large! I would have thought you only need a ripple voltage small enough so you don't see any flicker in the LEDs - and in the case of most LEDs I would suggest the LEDs would tolerate a ripple voltage maybe as high as 2 volts - hence my calculations of about 8000uF. Mind you, that's still a big capacitor!

      One further point - you should not need a resistor to drop the voltage from the rectified 12VAC as any decent LED lamp would run with its internal driver circuitry on usually between 10 and 30V DC.

  9. You can also get very good LEDs that are now suitable for 10-24v. Great prices, last much longer and save on your energy. They are ideal for boats and camper vans.

  10. g4 halogen bulb

    Great article.

    Easy to read and understand

    but good tips in development.

    Thank you for this


  11. Nice modification, I would probably add a 250ma fuse to protect the transformer in the event of the bridge rectifier developing a short.

    1. Thanks for your suggestion, but the original lamp was fitted with a 3-pin UK plug. In the UK there is a fuse fitted in all 3-pin plugs so I had already got fused protection in my design.

  12. Martin,

    I came across this post and have a related issue that you could possibly help me with or point me in the right direction. Would you mind sharing your email address with me ( so that I could send you the document where I describe my problem.


  13. Hi there
    I have a chandelier that usually has 10x10w max halogen G4 bulbs. Can you tell me what LED driver I need to purchase to stop my LED G4 cool white bulbs from flickering please?
    Also can you tell me where the LED driver gets connected. Don't worry I'm getting my dad to fit who used to be an electrician before LEDs came about ��

    1. Hi Michelle.

      Thanks for contacting me. You've asked lots of questions, but I'll try to deal with each in turn.

      For the 10x10W halogen chandelier I would first get your dad to check that all the G4 lamps are connected together in parallel and that there is a mains to 12V transformer already installed. Then, if you've chosen the LED G4 replacements already, check what their power rating is. Let's say each is rated at 2.5W. Then, ten of these lamps would consume 25W and therefore your driver will need to be at least a 12V 25W LED driver. I suggest getting a 12V 30W driver to make sure you have some headroom - possibly something like this:

      I am having to make a big assumption here - I assume that you or your father can safely remove the old halogen transformer and can then wire the new LED driver in place of the old transformer. Hopefully you can get to the ceiling void above the chandelier and can hide the LED driver there.

      I hope that this helps. It's difficult to be 100% accurate without actually seeing the chandelier and seeing how it is installed.

      Good luck!

  14. Hi Martin
    I have a chandelier that takes 10x19w max halogen G4 12v bulbs. I have purchased 10x LED 12v but they flicker. Can you tell me what LED driver to purchase please?
    My dad used to be an electrician but has just retired before the common use of LED bulbs so he will fit this but assume it is straightforward?
    I also have a DAR lighting chandelier that takes 5x 20w max G4 bulbs which I want to convert to LED but the bulbs don't work in that when I tried would I get the same LED driver for this fitting too? And the matching wall lights with 3x20w bulbs?
    Thanks in advance for your advice

    1. Sorry first chandelier is meant to state 10x10w max

    2. This comment has been removed by the author.

    3. Hi again. I hope the first part of your post is the same as your first post above. Sorry if this confused you but I vet every comment that is submitted so I can be sure I don't have any spam on my blog.

      Anyway. Back to your comment. I posted a link to a suitable LED driver for the 10x10W chandelier, assuming your LED G4 lamps are 2.5W each. When you put the lamps in you may find one or more doesn't light. If this is the case just take the lamp out of the socket, rotate it 180° and put it back in again. Your LED should now light up.

      With the DAR chandelier I would suggest getting slightly more powerful lamps - say 3 or 4W G4 LEDs - and I would choose carefully so that the esthetics of the chandelier is maintained with the new LED lamps.

      Again, if the DAR fitting has 12V G4 halogens then there should be a mains to 12V transformer in the wiring above the chandelier. As before, you should look to change the old transformer for a newer 12V LED driver that has sufficient power (the wattage or W figure) to run all the lamps together. The driver I suggested above will drive these five lamps even if they are higher power.

      For the wall lamps you can use the same approach, but you may need to think carefully how you install the LED driver so that it is not visible. I'm sure your dad can help you there.

      I'd also be very grateful if you would use the Amazon search bar on this blog to look up any Amazon items
      you want. You pay no more for the goods - but I get a little something for helping you.

      Good luck - and please let me know how you get on.

  15. I have two chandeliers with Toroidal transformers running 10 X 10w G4 halogen bulbs each. I wish to replace these with led G4 which give a brighter light. I have found it difficult to find out if a Toroidal transformer can run LED bulbs or if I have to somehow replace these transformers. Have you any idea?
    David B

    1. The best solution is to change the toroidal transformer to a suitably rated LED driver - but you'll need to choose a driver that will fit safely in the housing of the chandelier and be able to wire the new driver in safely. If you're competent enough to do this (I've no way of knowing) then please look at the questions below:

      Q1 - what wattage LED lamps are you looking to use?
      Q2 - how much room is there in the base of the chandelier?
      Q3 - do you want the chandeliers to be dimmable?

      Let me know and we can start to see what works best for you.