i have a
http://www.amazon.co.uk/gp/product/B0076UK0D4?psc=1&redirect=true&ref_=oh_aui_detailpage_o04_s00
Output: (8-12)X3W 680mA±5% 36w led constant current driver
I want to use PWM to dim say 12x 3w leds using the arduino.
Is there any easy way or decent boards to use?
You could use an npn-transistor on the negative output, like this one that will give you a lot of headroom: STP16NF06
You may want to consider a heatsink on the transistor for longer life and cooler operation. If you use an Arduino UNO you could hook up your PWM output to the Gate without a resistor (as seen on a Adafruit guide), but maybe try to limit the gate current with a resistor that would drive ca 30mA (which may still be way too much, but study the datasheet if you need to find exact current to saturate the gate pin).
That is a constant current LED power supply.
It is not designed for dimming in any form. It is specifically designed not to be dimmable. You will need to find one that provides a dimming control input.
{And - believe me - this is not the first time someone has tried and caused themselves trouble - and come here about it.}
Interesting Paul. What happens when a constant current source is pulsed to say 50% duty? Will it not provide designed output at 50% of the time and then appear as dimmed?
I did a simple sketch and hooked up a 3x1w led run by a 280mA driver, and it seems indeed to dim well with this design and the NPN transistor. But I would of course not use this approach, if you say that the driver or other parts of the circuit will be overheated etc. What is the downside?
It is a switchmode converter - hopefully contains an isolation transformer (it is big enough).
It functions by generating a form of "PWM", so its output is not exactly constant. If it had a capacitor on its output, then it would feed a constant current into the capacitor, so that while the load was turned on, the capacitor would (eventually) stabilise at a voltage according to the load. When the load is turned off however, the capacitor absorbs the constant current and charges to a higher voltage. As the load turns on again, the capacitor delivers a much higher current to the load as it discharges from that higher voltage. In essence, the capacitor ensures that the load receives the average current delivered by the constant current supply, but in bursts of a far higher current when it is switched on.
If the constant current supply does not have a capacitor on its output, then it will have an inductor in series with the output (which is how a switchmode converter operates). When you switch off the load, the inductor in attempting to keep the current constant, will deliver a very high voltage "spike" which will eventually destroy your switching component (or some part of the constant current converter).
Either way, you are in trouble.
And do note that when I said
Paul__B:
hopefully contains an isolation transformer
I was not joking.
The smaller versions of these do not contain an isolation transformer which means that the output is directly connected to the mains.
I hope I do not need to explain the implications of that in your circuit.