Hi guys,
Below is a picture of how I understand the Big W Led's should be wired up using a big 24v AC transformer. Am I right? Do I have to use the 180 Ohm resistors? If not, could someone please correct me.
Cheers, Terry
Terry
The resistor value will need to be calculated for each of the colours.
The actual voltage output of the transfprmer will need to be taken into account as they are usually higher than the nominal rating, ESPECIALLY under light loads.
For the Big W Leds refer to
http://doityourselfchristmas.com/forums/showpost.php?p=94315&postcount=35 for my measurements on all colours except the warm white.
For AC the peak voltage needs to be taken into account.
24v = 33.9v Peak
26v = 36.7v Peak, probably the most likely value 300w Toroids though i haven't measure one for a while
28v = 39.6v Peak, typical of the voltage out for smaller transformers with a 24v rating.
Example
RED set based on measurements in other post.
29.6v forward voltage for 20mA
10v need to be dropped across a resistor from the peak of 39.6v (28v RMS)
= 500 ohm resistor
or
4v need to drop across a resistor from peak of 33.9 (24v RMS)
= 200 ohm resistor
if a 200 ohm resistor is used based on expected 24v RMS voltage and 28v is actually supplied then the peak current would be 50mA, this value should be inside the peak rating for most LEDs.
All the other colours would use lower value resistors.
When used with AC most 24v nominal LED strings i have found to be fine without resistors just watch the actual transformer output voltage.
The pain of taking into account all the voltage calcs for AC was one of the driving forces for switching to DC.
DC is just so much easier for low voltage lighting especially LED's.
Cheers
Aussiephil