@RichardO, thanks for the thinking.

I have Arduino Mini as PWM source. This should be (close to) 5V.

Sounds good.

The leds are to be used for a project in a church in December. Its typically quite chilly at that time of a year. So I am lucky in that sense.

Call me paranoid but I would still plan on the project working at higher temperatures. There are few things worse than having a design fail because the conditions were worse than expected. _Always_ assume the worst possible scenario.

I am using a heat sink calculator to test my heat sink design. I will check max junction temp as you indicated.

Good. Make sure to double check the maximum temperature rating for the LED's you are using. Remember that it is the junction temperature that matters. Be warned that your LED's are going to need a massive amount of heat sinking. I am wagering that you will need a fan. If I were you I would plan on that in the mechanical design.

I did not account for a heat sink for the FET.
I saw that power supplies have some adjustable output range. 4.5-5.5V for the red and blue. 6-9V for the green.
Thus I expect to have ~< 2V @ 700mA drop over the FET. Do you think a heat sink is still needed then?
Fans are not easy to be added.

That sounds marginal to me. A TO-220 transistor package is only going to dissipate about 2 watts for power without a heat sink -- in the best of conditions. Check the spec's for the part you plan on using to find the exact limitations.

*** Wait a minute. Worst case, there is going to be more like 5.5 volts across the FET with the green LED circuit powered from 9 volts and 2.5 volts with 6-volt power. That's more than the TO220 isw likely to handle without heat sinking.

Brightness calibration between the different leds I planned to do via an led specific PWM offset programmed into the Arduinos.

That might be good enough. If not, then changing the current sense it not going to be terribly painful.

You need to order a selection of power resistors to have them on hand when you need them.

I already bought a batch of 100 leds for ~ $30 (chinese site, brand: Genesis). Trying these first and if not succesful will follow your advice and switch to e.g. Cree leds with proper data sheets.

Can you post the spec's for your LED's here? I am dubious that you got 3 watt LED's. A true 3 watt LED is usually mounted on a star board with a metal core for heat spreading when mounted to a heat sink.

 

https://www.aliexpress.com/item/9W-...-RGB-Six-Legs-700mA-3-2-3-4V/32818187268.html

 

Tilt part of my mini moving head proto.
You can see the heat sink. When driving two leds at full current (~700mA) then I still can touch it for several seconds, although it feels pretty hot. With the 3rd led on I can only touch it for less than second. Still need to check with decent thermometer.

 

https://www.aliexpress.com/item/9W-...-RGB-Six-Legs-700mA-3-2-3-4V/32818187268.html

It looks like an interesting part. From the best I can tell from the pictures it has a pad on the bottom to reflow solder to the heat spreader. If this is not done right (and even if it is) it will be very hard to get the whole 9 watts of dissipation.

I would contact the vendor to get the complete data. This is very important if you want your LED's to work right and to last.


Here is a site that has a bit more information. Still no where near enough.


https://www.aliexpress.com/item/3W-...&terminal_id=63861125bf7d4c748014d9b340a515e9



Here is a site that shows the LED on a star heat spreader. Hopefully this is what you are getting.

https://www.aliexpress.com/item/3W-...&terminal_id=63861125bf7d4c748014d9b340a515e9


Product Description

Specifications:

1.Model Number: DRP-3WRGB

2. Product name: Led Light Bead

3. Shapes: Round Six Legs

4. Wire's Material: Gold Wire

5. Power: 3W

6. Forward Voltage: R: 2.0-2.4V G: 3.2-3.4V B: 3.2-3.4V

7. Forward Current: R: 350mA G: 350mA B:350mA

8. Luminous Flux: R: 30-40LM G: 80-90LM B: 30-40LM

9. color-rendering index: >70

10. Beam Angle: 140 degrees

11. Working Temperature:-20 centigrade to 60 centigrade

12. Wavelength: R:620-625nm

G: 520-525nm

B: 460-465nm

13. Lamp Body Material: Copper Bracket

14. Life Span: 50,000 Hours

 

View attachment 146884

Tilt part of my mini moving head proto.
You can see the heat sink. When driving two leds at full current (~700mA) then I still can touch it for several seconds, although it feels pretty hot. With the 3rd led on I can only touch it for less than second. Still need to check with decent thermometer.

If you look at the post I just did you will see that they say the current rating is 350 ma per LED, not 700 ma. What kind of heat sink do you have the LED's on? I don't see it in your picture. Actually I am not seeing your picture of the LED's at all.

(That is odd. I only see one picture but it shows two attachments.)

 

It looks like an interesting part. From the best I can tell from the pictures it has a pad on the bottom to reflow solder to the heat spreader. If this is not done right (and even if it is) it will be very hard to get the whole 9 watts of dissipation.

Indeed it has a round pad (~5mm diameter) at the bottom, which I connected via arctic silver 5 to the heat sink. I do not use a starboard (yet).

Do you suggest that the led connections RGB+/- also have to contribute to heat transfer to allow full current, and thus I -have- to use an starboard with soldering pads integrated, instead of a direct connection to the heat sink?

I would contact the vendor to get the complete data. This is very important if you want your LED's to work right and to last.

Done that. But no more response yet than what mentioned on the aliexpress site.

Here is a site that has a bit more information. Still no where near enough.

That's the same supplier, with same level of info than for my product (see my link). But this is about 3W variant instead of 9W.

If you look at the post I just did you will see that they say the current rating is 350 ma per LED, not 700 ma. What kind of heat sink do you have the LED's on? I don't see it in your picture. Actually I am not seeing your picture of the LED's at all.

(That is odd. I only see one picture but it shows two attachments.)

I have the 700mA 9W version.

Can't you see the picture at all?? I t should show a breadboard on the right and stepper motor, heat sink and black led lens on the left.

 

Yes the 6 pins do contribute to the transfer of heat to the star spreader.

The best way to attach the LED to the PCB spreader is to use the reflow technique and actually solder all of the pins…and the small round “dot” underneath the LED.

But of course only the best LED companies use this technique, and most of the cheaper ones use a thermal compound on the “dot” and other methods to solder the pins.

The choice is yours as to how the LED is mounted, but I would derate accordingly.

 

You can see the heat sink. When driving two leds at full current (~700mA) then I still can touch it for several seconds, although it feels pretty hot. With the 3rd led on I can only touch it for less than second. Still need to check with decent thermometer.

Waaaaay too hot.

Bob Pease "5 second rule": If you can hold your finger on a hot device for 5 seconds, the heat sink is about right, and the case temperature is about 85 deg C.

Note that the device in your case is the LED and not the heat sink.

 

I have the 700mA 9W version.

Can't you see the picture at all?? I t should show a breadboard on the right and stepper motor, heat sink and black led lens on the left.

Typical Chinese spec's. I doubt that there is any difference between the 3 watt and 9 watt LED's. I would wager they are all 3 watt parts.

OK I see it now. A better picture showing the mounting of the LED to the heat sink is needed.

If your LED is getting that hot either it is not transferring enough heat to the heat sink or the heat sink is not big enough.

Imagine how hot a 5 watt incandescent night light bulb runs. You don't want your LED anywhere near that hot. Only real good heat sinking will prevent this.

 

I do not use a starboard (yet).

A star board is not a regualr PCB It has a metal core to spread the heat better than a standard board. I don't think you can easily make one yourself. That is why you buy the LED already mounted on the star board.

 

@Patrick Meijers
I have attached a _real_ data sheet for a power LED. Hopefully it will help you understand more about the requirements of using one.

I built a bicycle headlight using this 40 watt LED.

Some details on the heat sinks I used in bicycle headlight:
The heat sinks I used are 1.8" square by 0.75" tall finned heat sinks each 6.1 degC/Watt.
I needed two of them to get the thermal dissipation I needed to prevent overheating in still air and get full brightness in moving air.
I am running at 13 watts maximum. This is about the limit of both my batteries and heat sinks.

 

@Patrick Meijers
The heat sinks I used are 1.8" square by 0.75" tall finned heat sinks each 6.1 degC/Watt.

Any reference to that?

I also read that corroded aluminium has ~10 times better radiation than polished/shiny aluminium. That is also something that I can still do to gain little bit.
(Note that I can make the sinks myself on my CNC from a spare bar of aluminium - as I did in the proto picture - to reduce cost.)

 

Any reference to that?

I also read that corroded aluminium has ~10 times better radiation than polished/shiny aluminium. That is also something that I can still do to gain little bit.
(Note that I can make the sinks myself on my CNC from a spare bar of aluminium - as I did in the proto picture - to reduce cost.)

Note that I don't think you can make your own heat sink that accepts the LED unless it is on a pre-made star board. Manufacturers make heat sinks specifically for star boards.

But if you want to give it a try... Read on.


The heat sinks I used were Thermalloy 6002B-2. These are inexpensive stamped and bent sheet aluminum. What made them special is that I already had some.

There are lots of better options available from electronic distributors. Digi-Key and Mouser have search engines that should help. Buying stamped or extruded heat sinks is a lot easier than machining them.

Typically, aluminum heat sinks are anodized black to increase the emission of IR wavelengths.

If you are a real masochist you can use lye and black RIT dye to do this yourself. I just realized that I know at least two people that have done this. I never have never done it myself so I don't know the exact process. I am sure that someone on this forum does. If not, Google is your friend.

Here is an example of an extruded heat sink -- a Wakefield 657-15ABPE. It is only about a dollar from Digi-Key:

Description: Heat Sink TO-218, TO-220, TO-247 Aluminum 6.0W @ 38°C Board Level, Vertical. 1" by 1.65" by 1.5" tall.

 

What is the voltage across R1 with a green LED and a 5 V power supply?
If it is greater than zero, then the circuit is working exactly as is "should", it is simply that that doesn't correspond with what you want.

Sorry if this has already be covered, but I searched on R1 and found nothing.