Modifing LED strings for SSR operation

J

jpb

New member
I have cut this out of the Big W post

wjohn said:
Another update

Coles branded 126 LED strings. these are two channel strings, RGB & Yellow. Power is 24 VAC.

I take off the Multifunction controller and test the cable. The Common wire will be POS and the two strings will be NEG.

I use a 30 VDC Bench power supply and start the string off at 18~20V DC, and slowly bring the voltage up until all the LEDs are illuminated.

Once the Strings have passed the visual test, I measure the Current on both channels to get an idea of the total power budget. Next, I cut the string between two LEDs and take a current measurement to see how hard the LEDs are being driven. Nominally, I am looking for ~10mA to no more than 15mA per LED.

As the Strings are RGBY, and two channels, I like to swap the LEDs around to make the string two colours (R and G) (B and Y) or solid colour.

While it is good to get Solids from RGBY, you need to be careful. RED LEDs have a lower forward voltage:

RED ~1.7V
GREEN ~2.2V
YELLOW ~2.1 V
BLUE ~ 3.2 V

Why bother? well the trick they use is to group LEDs by mixing the colours RED and YELLOW, Blue and Yellow etc etc. When you start swapping them around, you are changing the effective Forward Voltage drop for that group of LEDs. e.g. if the strings is groups of nine LEDs, RED will be only ~16V, while blue will be 28V.

What happens the the REDs? they will conduct more current, and potentially over driving them to failure. you will need to add a current limiting resistor using the standard formula R = V - V(fwd) / I R = (24 -16) / 0.20 A = 400 ohms So a value great than 400 ohms is safe, I go 470 ohm.

A resistor needs to be added for each group (14 total) for 126 LEDs in groups of 9.

Hope this helps
Click to expand...

Could a similar out come be achieved by isolating one section of the LED string and putting a resistor substitution wheel and a meter to measure the current in series with the string and then power it by rectified 24VAC?
Start with a high value on the wheel and then slowly decrease the restistance until the LEDs are on and the current is around the 10mA. This value can then be added to each section of the string.

I am hoping to stick with my existing 24VAC transfomers and then use a 50c rectifier package like this one on the light string where required.
http://www.altronics.com.au/index.asp?area=item&id=Z0073

It is just a thought and January is the time for thinking and planning.

Jon
 
I guess that should work. Measuring the current on a rectified 24Vac may be harder especially with a cheap multimeter.

This is assuming you use an SSR unit with TRIACs (AC switching) and not MOSFETs (DC switching).
 
Here is a good standard set of voltages for today's LED's. Not including the .5 or 1 watt LED's


http://www.theledlight.com/color_chart.html


You will see a nominal operating current for each LED color. If you read the actual specifications there is also a maximum current. This can be twice the nominal current and will be the possible point that will destroy the LED's junction and the LED will not light anymore. The LED's junction can open and the string of LED's will not light or short and the string will still light but not that LED.
 
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