10W 9-12V LED

Another arrival from China 😆

This thing is bright, very bright! I had an initial play using the bench psu to limit the current to around 1A – it seemed to drop about 10V at that so all within spec, even if it does get a bit toasty.

I’ve been playing with the AMC7135 350mA constant current drivers (for instance, to configure a 1W bead to light up the understairs cupboard at work). One thing you can do with those is connect them in parallel to get more current. In this case, 3 x 350mA = 1.05A which ought to be near enough.

3 x AMC7135 + "the beast"

3 x AMC7135 + “the beast”

In the above photo (I know, it’s not the best photo ever) you can see the 3 x AMC7135s on a SOIC adapter board and the LED mounted on a heatsink (which is then sat on an old CPU heatsink just to get it up and into the draft of the little 12V fan at the top of the picture).

That all works fine but I’d like to use one of these on my computer desk since the lighting in the man lab is fairly crap. I’ve got an old ATX PSU on that desk so the power side is ok, I wouldn’t want it on full blast all of the time though.

It’s not entirely clear if you can use PWM to switch the AMC7135 fast enough to control LED brightness. I gave it a go and my conclusion is that you can’t. That needed another solution – switching a FET in-line with the LED ground. I’m using an ATtiny45 for this – eventually it will have “+” and “-” buttons to control the brightness in (say) 10 steps, for now I’m just varying the PWM from a duty cycle of 0% up to 100% and then back down again. That should let me see if it works or not.

Firstly, I tried to use a chunky IRFU9024 power MOSFET. That worked but it got very hot, very quickly so it was time for some more learning. The problem was that the 5V of the Attiny isn’t driving the MOSFET fully on so it’s just acting as a large (and expensive) resistor. It needs to be fully on or fully off to not dissipate power (therefore heat) so that component wasn’t going to work well.

Luckily, I still have some logic-level power MOSFETs kicking around (Infineon IPS135N) so I could just substitute this smaller MOSFET for the larger one. These are fully on by the time you reach 2.2V so absolutely no problem there and this one doesn’t even get warm.

Tiny45 (left), logic level MOSFET (right) and "demo" LED

Tiny45 (left), logic level MOSFET (right) and “demo” LED

That all seems to work just fine. I need to think about a chunky heatsink and/or fan to keep the LED and the AMC7135s cool. I’ll also require some kind of flexible arm to mount everything on so it’s not a done deal yet. At least all the “proof of principle” stuff is done so I know it works before I start trying to put it all together.

Here’s a video…


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