Constant Current vs Constant Voltage Dimmable LED Drivers

For my circuit, I want to use 110VAC mains power, via a TRIAC dimmer to an LED Driver power supply.

For this use case, I am confused on whether to use a constant voltage AC to DC LED driver power supply, or a constant current AC to DC LED driver power supply.

When would I use one vs the other? Which supporting components would I need to drive the LED array?

I thought that if I used a constant voltage supply, using the formula R = (Vs - Vf) / I, I would simply put a resistor in series (calculated by the above formula) to be sure "I" (current) never exceeded the LED array's maximum forward current.

For the constant current power supply, would I calculate the minimal value resistor that when I was at Vmin of the LED power supply, it would translate to a current just below whats needed to light up the LEDs, and when its at Vmax of the LED power supply, well, thats the maximum current I would have at my disposal to light up my array? (So in this case, I would need to find the power supply with the ideal Vmin and Vmax to take advantage of the LEDs' power capacity?)

Let me know if I'm on the right track here, or if I am missing anything... thank you.

• Why not just get a dimmable LED driver? That way you wont need an external circuit to do the dimming. Commented Feb 15, 2015 at 13:43
• I want to dim from the wall. Commented Feb 15, 2015 at 17:41

Sorry, Geremy, but you've misunderstood. The module you're looking at will simply not work well with the dimmer turned down. It will ALWAYS try to drive 1400 mA through your LEDs. If the input voltage is too low (because of your dimmer) it will get very unhappy and may do strange things.

The 17 to 29 volt range just means that it will successfully drive 1400 mA through an LED string when the voltage needed to produce that current varies between 17 and 29 volts. This voltage difference could be produced by differing numbers of LEDs in a string, or using the same number of LEDs, but of different colors.

• Thank you for your reply! Given this item as the reference (the voltages / current are different than what was mentioned before) meanwell.com/webapp/product/search.aspx?prod=pcd-60 as it claims to be a dimmer, how is it dimming (what is it doing at the output side to dim the LEDs) as I turn my TRIAC dimmer up and down, while still providing a constant current? Commented Feb 16, 2015 at 0:47
• Yes, that one is designed to work as you want. Just be aware that it won't work over the very lowest dimming range. On the data sheet, it specifies that it works with a "minimum conductance angle" of 30 degrees. Commented Feb 16, 2015 at 0:55

From further research, I've arrived at the conclusion that in the realm of LED Power Supplies, "Constant Current" is also known as "Constant Current Reduction".

If this is the case, I should understand that for a "Constant Current" LED power supply that is advertised as

• In: 110VAC
• Out:17V - 29V, 1400ma

this means, as I dim from the wall (mains power) to zero, the voltage will approach 17V, and the current will approach 0, and as I dim up from the wall, the voltage will approach no more than 29V, and the current will approach no more than 1400ma.