building under cabinet LED lights -- Power supply design questions

[mattl]

I don’t know if anyone here is good a basic power supply design, I need a little help since it’s been many decades since I took any design classes.  what I'm doing is replacing all the florescent UC lights with leds.  I have 3  42" T6 tubes and one 64" T6 tube in addition to 2 small 18" tubes.  I've already updated the 2 18" units with 2 sets of 6 3w leds powered from a constant current driver from China.  I'm trying to avoid that and build my own with parts I have laying around or from RS.

 

Anyway, at this point I'm working on the replacement for the 64" unit, it will consist of 16 LEDs.  They are 3.6v in series so I need about 57v total to power them at 600 ma.  I have a 50v center tapped transformer I'm using, at this point I'm just using one side of it at 25v.

 

I'm using a design based on this circuit i found on line

 

<span style="font-size: large;"><span style="font-family: arial;"><span style="font-size: medium;"><span style="font-family: arial;"><span style="font-family: verdana;"><span style="font-family: verdana;"><span style="font-family: times new roman;"><span style="font-family: arial;">

The issue I'm having is the regulator is running quite hot. I'm only driving 8 of the LEDs with it rather than the 16. I'm wondering if I'd be better off using the full 50v feed from the transformer and running all 16 LEDs from it, the current stays constant at 600ma.

I'm open to any other circuit designs that are easy to make at home.
</span></span></span></span></span></span></span></span>

 

<span style="font-size: large;"><span style="font-family: arial;"><span style="font-size: medium;"><span style="font-family: arial;"><span style="font-family: verdana;"><span style="font-family: verdana;"><span style="font-family: times new roman;"><span style="font-family: arial;">Here are the spec's of the LEDs;</span></span></span></span></span></span></span></span>

 

<span style="font-family: Arial;"><span style="font-size: large;">Specifications: </span></span>

<span style="font-family: Arial;"><span style="font-size: large;"><span style="font-family: Verdana;"><span style="font-size: medium;">LED Emitter: 3W </span></span></span></span>

<span style="font-family: Arial;"><span style="font-size: large;"> <span style="font-family: arial;"><span style="font-family: verdana;"><span style="font-family: verdana;"><span style="font-family: arial;">- Output Lumens: 180-210 Lumens</span></span></span></span>
<span style="font-family: arial;"><span style="font-family: verdana;"><span style="font-family: verdana;"><span style="font-family: arial;"><span style="font-size: medium;">- DC Forward Voltage (VF) : 3.6-3.8Vdc</span></span></span></span></span>
<span style="font-family: arial;"><span style="font-family: verdana;"><span style="font-family: verdana;"><span style="font-family: Arial;"><span style="font-size: medium;">- DC Forward Currect (IF) : 700mA</span></span></span></span></span>
<span style="font-family: arial;"><span style="font-size: medium;"><span style="font-family: arial;"><span style="font-family: verdana;"><span style="font-family: verdana;">- Color Temp: 2700~3200K (Warm White</span></span></span><span style="font-family: arial;"><span style="font-family: verdana;"><span style="font-family: verdana;">)</span></span></span></span></span></span></span>

 

 

 

 

[rockland1]

Undercounter Flouresccent

Matt, you lost me after the first paragraph. LOL I just replaced a couple of 35 year old Progress undercounter fluorescent lights I have......with electronic
Progress flourescents. Love the warm white light.

 

[kb0nes]

Matt,

As you know LM-317 regulator is being used as a constant current source and it has to drop all the voltage that the string of LEDs doesn't. Since the forward junction voltage of each LED is about 3.7v, running eight of them in series totals ~30 volts. This leaves the regulator to drop 20 volts (50 minus 30) at .600 amps which equals 12 watts of regulator dissipation.

The 12 watt dissipation isn't too bad but it would require a heat sink. The 317 has internal thermal protection. You really can't harm the device but it will reduce output current to a safe level if you don't have a sufficient heat sink to keep the temperature down.

Adding more LED's in the series chain to drop more of the supply voltage and will reduce the heat dissipated in the regulator. A total of 12 LED's would drop about 45 volts across the string leaving the regulator to only have to drop 5 volts. This reduces the thermal dissipation to 3 watts in the 317.

The rub will lie with the 50v supply since a string of 16 LED's is about 60 volts total and you need about 2v minimum drop across the regulator to allow it to not drop out of regulation. You can't run a series string of more then 12 without increasing the supply voltage.

Are you heat sinking the LED's also? A 3w LED will run pretty warm if there is any significant duty cycle!

 

[mattl]

Thanks for the input.  In thinking more about it I came to similar conclusions as to the number of LEDs - was not aware of the 2V needed for the regulator.  I need to get an accurate measure of the loaded voltage since if I recall with a full wave rectifier and a filter capacitor the effective voltage rises, seems I recall a number like 1.4 x AC input voltage.

 

As for the LEDs, I epoxied them onto a large U shaped piece of aluminum.  Spacing is about 3.5".  With the 6 LED units I did last spring they do run warm but no issues- yet.  I had thought about building a dimmable source but I have no reason to dim, I like lots of light over the counter for working.

 

All totaled I plan on having 58 3w LEDs around the kitchen.  I did a practice run with 2 18" bars with 6 LEDs on each last spring.  I just bought a cheap constant current unit to power those and it works well.

 

[kb0nes]

Just curious, what are the LED's you are using for this project?

The regulator will adjust the current output to always drop 1.25v across the resistor tied between the Output and Adjust pins. You can use Ohms law to choose the resistor size.

So the resistor value would equal 1.25v divided the desired current in amps (R=E/I). In the case of the circuit you shared, a 2.7 ohm resistor should only give a string current of ~450ma a bit short of the 600ma you are shooting for. Component tolerances will cause variation, its easy to measure with a DMM though.

You could easily make that sense resistance out of a parallel pair of resistors and switch one in and out to have dual level brightness. Might be cool to have a low current "nightlight" setting at the flip of a switch.

I hope you post photos when you get it all complete!

 

[mattl]

I used 5 10 ohm resistors in parallel to create the impossible to find 2 ohm resistor.
 
I picked up these LEDs, 50 for ~$18.


 

 

 

[mattl]

Quick update.  I can't find  any capacitors rated more the 50v DC locally so I opted to use the center tap winding of the transformer.  After the rectifier, capacitor and LM317 I'm getting 40.7v.  So I just connected 12 LEDs.  They are running at 560ma and 3.3v, and are quite a bit cooler running than my other setup with the constant current supply I bought.

 

Now I can add a small second transformer to supply the remaining 6 LEDS or figure out how to wire them into the other 3 strips. and run 12 instead of 10 on the other lights.