LED bits and pieces…

1206 SMD LEDs

I picked up some 1206 SMD LEDs the other week, they’re bound to come in handy. One of my pet hates is when I get some or other board with SMD LEDs on and the maker has read the LED datasheet. Yup, it’s all in spec but I don’t like getting blinded every time I power up the board. To that end, I wanted to get a good idea of what resistor values would be appropriate with these LEDs rather than just “correct”.

Here’s all the data I have on them:

Forward voltages @ If = 20mA

Forward voltages @ If = 20mA

Standard resistor values would be 150Ω for Red and 100Ω for pretty much everything else. I tried those to confirm that they were eye-burningly bright. And they were.

That's better...

That’s better…

I’m using 4.7kΩ with red, white and blue, 10kΩ with green and 2.2kΩ with yellow. That seems to give light that’s bright enough to see, even on a sunny day, without blinding me. Having these little boards setup also lets me play with these values so I can customise them for any application.

1W LED “Beads”

I’ve had some of these for a little while now and so I decided to blat a few (it turned out to be 8) onto a bit of prototyping board, just for the hell of it. I’d noticed before that both LED and resistor (2W 6.8Ω) got pretty hot so I wanted to see how a bunch of them got on together. Here’s the board:

LEDs and Resistors

LEDs and Resistors

It’s a bit bright! Although it only draws about 7.5W rather than the 8W I was expecting. Have you got your shades on?



The proto board gets fairly toasty but I have run it for about 8 hours without any ill effects. That’s not long enough though and I think I’m going to have to do something to dissipate the heat when I use these in anything more serious. I’m not happy with the mahoosive resistors – which also get pretty hot – so I can’t see this combination being much use for anything.

AMC7135 LED Driver

Had a 100 of these turn up in the post this morning – tiny SOT-89 packages that can sink a constant 350mA.

AMC7135 Pinouts

AMC7135 Pinouts

Coincidentally 😉 that’s exactly the current for the 1W LED “beads”. Had to have a play with one of these…

AMC7135 bodged on to an adapter board

AMC7135 bodged on to an adapter board

The datasheet for these is confusing! Well, not really but I got confused by “Out” and the 1W LED I put onto the board got a bit dead. Never mind.

Here comes the sun...

Here comes the sun…

According to my bench PSU it is pulling around the 350mA as advertised. The AMC7135 is pretty toasty, as is the LED but no smoke so far. It’s a much neater solution than those enormous resistors but will still need to sort out some kind of heatsink. I’ve got a fan on my desk and that’s gently wafting air around – if I turn it off then everything feels hotter and the current goes down as the AMC7135 starts to limit things to stop itself burning out.

I need to test this but I believe that the Vdd signal to the AMC7135 can be PWM from a micro-controller. It’s just acting as a switch and the voltage through the LED is separate. If that’s the case then it should be possible to run the LEDs at 12V (say) whilst providing PWM control from an ATtiny45 (or similar) to act as a dimmer. Will try this soon, what can possibly go wrong?

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