Hi all,
I see some similar threads from last year, but I'll try this question again.
I use triac boards to drive incandescent lights and they dim/rise very well.
But, if I use LED lights (Christmas bulb strings not fixture bulbs), they don't behave.
The LED striongs don't dim completely off, and don't ramp up to bright.
The triac boards for AC dimming are similar to:
The board has suggested values for high, low, and gate impulse which wok fine for AC incandescent lights #define GATE_IMPULSE 5 #define FREQ 84 #define FULL_ON 26 #define FULL_OFF 245
I can just flip the lights on/off, but can't fade. Thoughts?
...Most LED "light bulbs" from the home improvement store (which have an internal switching power supply) are not "dimmable". Some are rated for 90-270V (or something like that) and they try to maintain constant brightness no matter what!
There are 12VDC LED strips/strings that can be PWM dimmed (like a regular little LED) with a MOSFET driver, or variations of WS2812 NeoPixels which are individually serially-addressable and each LED has it's own driver.
Correct, but the OP cannot see the little box that converts the AC to current regulated DC for the LED lights, so he thinks it is all AC, just like the old incandescent lights.
Hi all,
Thanks for the replies....
The lamps being driven are Christmas strings of LED bulbs, not "fixtures" with driver circuits.
That said, I think I found a strange solution for the Krida triac board. If I have a small incandescent load (nightlight bulb) in PARALLEL with the string of LED bulbs, then the triac behaves normally and dims (PWM) both the incandescent and LED bulbs at the same slope.
Hi Paul,
In the world of LED christmas ligh strings, I do not see any sort of in-line device that is converting the 120VAC into DC to drive the LED bulbs. The string of 100 LEDs is powered directly off of AC...
Most LED are driven with a constant current (or controlled current) source but some LED strings are made that way. 120V / 100 LEDs in series is 1.2V each (about 1.7V peak). Or if they are wired back-to-back there would be two groups of 50 in series with 2.4V each. And back-top back wiring also prevents reverse over-voltage.
...I knew that would make them go off, because TRIACs usually have some small leakage current that might be enough to light the LEDs. With a higher-current (lower resistance) load, the voltage will drop and most of the current will go through the incandescent lamp.
Good to see you have found a solution, the Lamp will be providing the proper resistive load.
The LED bulbs may have some extra components in them, like a dropping capacitor.
The capacitor will introduce a phase shift in the current waveform with respect to the voltage waveform and may have been causing zero crossing switching problems if your dimmer has that facility.
