3 wire strings and MFC removal

[David Hayes]

I have 4 sets up 500 string white LEDs (2 brands) with a standard 3 wire MCF that is supplied by 24VAC plug pack transformer. The MFC is switching the negative rail and the DC output of the MFC is around 27V.

I cut off the MFC and tested the strings on a power supply (which has constant voltage and current setting available) and found that when the strings are dimmed to about 20VDC, the last 12 or so LEDs in the string are still on yet the rest are out. Both manufactures exhibit the same behavior.

Looking at the strings the resistor values are different in the end of the string compared to the other constant current resistors in the string. If I then want to connect these to a DC controller with an appropriate DC PSU, when dimmed the last LED's will remain on. Also, I fear they will burn out when the rest of the string is at full brightness (approx. 28VDC).

When reconnected to the original MFC and put on a fade pattern, I don’t see this happening. Not sure why this is the case when connected to a direct DC source.

Has anyone had this issue or can explain why this happens? I have read the 101 manual and subsequent info on MFC's and light strings.

Thanks
Dave

 

[David_AVD]

It's possibly that the last section has less LEDs than the other sections, so its "forward voltage" is about 3V less.

Almost all LED strings that came with a 24V AC transformer will work fine on a DC supply in the region of 27V - 31V.

I usually adjust the DC supply on a modified set to match the intensity of another set running (steady) on the original MFC as a reference.

 

[David Hayes]

It's possibly that the last section has less LEDs than the other sections, so its "forward voltage" is about 3V less.

Almost all LED strings that came with a 24V AC transformer will work fine on a DC supply in the region of 27V - 31V.

I usually adjust the DC supply on a modified set to match the intensity of another set running (steady) on the original MFC as a reference.

Thanks for that David. I'll check the LED count and trace the wires, so I can see what the sub count per section is. What gets me though, is why it doesn't exhibit the same behaviour on the MFC when they are set to the fade function?

 

[David_AVD]

The MFC is applying phase (or PWM) control which chops up the full voltage time wise, where as you're simply dropping the DC voltage to below the level required to light the LEDs.