Replace Piano Halogen With LED Lamps

About: I'm a retired IT consultant. Besides answering questions on StackOverflow I play around wth Atmels in various home projects. Recently I played a lot of Minecraft but since the hardcore mode does no longer th...

Intro: Replace Piano Halogen With LED Lamps

Once you moved you know that things get lost. We had this nice halogen piano lamp with two 20 Watt bulbs. But the transformer was lost. I looked everywhere but all I could find was a switched power supply for 36 Watts (I guess it was from an old printer). This somehow lit the lamp but the light was not very bright. Furthermore it did not start but often needed several seconds to "warm up". A new transformer was too expensive (I thought without even looking, but still think, I'm right here). There are 2 Watt LED replacements with G4 socket at reasonable prices (4 to 8 € per piece at Amazon). But still this seemed too expensive and I also wanted to work with LEDs on my own.

A new project was born.

Step 1: Schematics

LEDs are quite picky when it comes to lighting them up with the right current. They need a specific current rather than a certain voltage. There are thousands of different LEDs and after some research I found one SMD munted type:

  • 14 cd (light intensity)
  • 5000K (neutral white)
  • 100 mA operating current
  • around 3V operating voltage

per piece. 4 of those should be a nice replacement for a 20 Watt halogen bulb. The simple variant would be to use a resistor to burn over-voltage so the current through the LED would be in the normal operating range. That's fairly okay with small signal lamps, but for higher performance LEDs this would result in tiny resistor heatings (and thus not much different to halogen bulbs). Instead I used a current limiting circuit which is very simple. Just Google for "current limit circuit mosfet" and you will find quite some explanations. Basically the 6.8 Ohm resistor will level at 0.6 Volt and open the transistor to let 86 mA (a bit less than the operating current for the LEDs) pass.

Somehow I calculated wrong and ordered a 2 Watt resistor, but actually a 1/4 Watt would have been sufficient (6.8 Ohm * 0.086 Ampere = 0,0516 Watt which is less than 1/4 Watt). The transistor is a BCV 47 SMD and the MOSFET an IRLML 2803 SMD. Honestly I'm not an electronics pro but these worked very well. Also I added an idiot-diode. The 12V power supply actually produced a stabilized voltage (just as said only up to 36 Watts). But since the G4 socket is symmetric one will not know where is plus and where minus. The G4 LED replacements have an internal bridge rectifier but I would save the money and space for it and just turn my LED to connect right. LEDs are a bit unforgiving when connected the wrong way any they go up in a puff of smoke. Thus the diode to prevent that.

I have also attached the LTSpice file (Lamp.asc) which I used to simulate the circuit.

Step 2: PCB

Etching and soldering the PCB was not as difficult as I thought. I did read some instructions about soldering SMDs and indeed this worked quite well for my first attempts here.

Fail: Of course Murphy is with every project. And here it was the 4th LED that was too much. Since the LEDs are in series, each of them eats around 3 Volts (as per spec it could go down to 2.8 V). My measure showed that the power supply delivered a bit more than 12 Volts. So I thought I was save. But then the idiot-diode ate another 0.7 Volts and that was too much. The 4th LED would not make it. I simply short cut it and only used 3 LEDs instead. Finally the light of the 2 times 3 LEDs was still bright enough :-)

Once I mounted and lit the lamp the LEDs got quite hot. I guess the temperature was safe but I wanted to be a even more save. So I folded a heat sink from a piece of aluminum sheet. Just 4 bores and 2 small copper wires soldered to the PCB gave a nice cooling. Even the wrongly ordered resistors turned out to be of good use. The remains from their thick legs were used for the socket connector.

Step 3: Let There Be Light

... and music :-)

As you can see the LEDs produce a bright light which is fine for reading the notes.

The picture above shows the lamp with my LED lamps measuring 5.6 Watt in operation. This is by far less than the 40 Watts which the halogen lamps would eat.

Lessons learned

Eventually I would add a mirror the next time to disperse the light a bit more since it is very directed. Possibly a longer stretched PCB would also be of advantage. The heat sink is fine but for the next time I will think of mount points during PCB design.

The cost for this amounts mainly to 2.15 € for the 8 LEDs (from which I only used 6 in the end) and the rest was just a few cents. I also had ordered tow cheap 2W G4 LED from my electronics provider at 3€ each. First they did not fit into the small casing of the lamp and second they were by far not as bright as my own construction.

So: very happy with the outcome :-)



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