I have four identical LED Christmas light strings, each originally powered by a 3 V / 525 mA wall adapter.

Measured with a bench supply:

At 3.45 V (original adapter voltage): ~680 mA

At 3.0 V: ~260 mA

I bought a single adjustable 3–12 V, 5 A DC supply to power all four strings in parallel, but even at minimum setting the output is 3.83 V.



The supply is sealed (no screws) and I’d prefer not to modify it internally.

At 3.83 V the current increases significantly, so I’m concerned about overstressing the LEDs.

I considered adding a series resistor or diode at the output to reduce the voltage, but I’m concerned about power dissipation and heating.

 

hi k,
I would use 1 or 2 series, 2Amp Diodes in series with +3.83V line.
E

 

Thanks.
Would it be safe to place the series diode(s) inside heat-shrink tubing,

 

Hi k,
Try a test without shrink tube, check the diode temperature, and then make a decision regarding the shrink.
E

 

I bought a single adjustable 3–12 V, 5 A DC supply to power all four strings in parallel, but even at minimum setting the output is 3.83 V.

I bought 4 of that type of supply from AliExpress. Two different variants from two different sellers.

In both cases, I found the "regulation" to be lacking because they used such small gauge wires in the output leads that the voltage at the plug end varied quite significantly based on the load.

You can read about my experiences here and here. The second post describes the "regulation" issue. I don't seem to have mentioned wire gauge in that thread. I think I calculated it to be about #26 AWG. That's small for a supply rated at 3A. At that current, it would drop around 0.75V (6-foot lead).

The built-in meter doesn't reflect the voltage drop issue because it measures the voltage at the supply end of the cable, not the load end.

 

I bought 4 of that type of supply from AliExpress. Two different variants from two different sellers.

In both cases, I found the "regulation" to be lacking because they used such small gauge wires in the output leads that the voltage at the plug end varied quite significantly based on the load.

You can read about my experiences here and here. The second post describes the "regulation" issue. I don't seem to have mentioned wire gauge in that thread. I think I calculated it to be about #26 AWG. That's small for a supply rated at 3A. At that current, it would drop around 0.75V (6-foot lead).

The built-in meter doesn't reflect the voltage drop issue because it measures the voltage at the supply end of the cable, not the load end.

Maybe I'm misunderstanding what you say but on my PS the output leads are definitely not 6 feet... They are more like 3 foot

 

I have four identical LED Christmas light strings, each originally powered by a 3 V / 525 mA wall adapter.

Measured with a bench supply:

At 3.45 V (original adapter voltage): ~680 mA

At 3.0 V: ~260 mA

I bought a single adjustable 3–12 V, 5 A DC supply to power all four strings in parallel, but even at minimum setting the output is 3.83 V.

View attachment 360442

The supply is sealed (no screws) and I’d prefer not to modify it internally.

At 3.83 V the current increases significantly, so I’m concerned about overstressing the LEDs.

I considered adding a series resistor or diode at the output to reduce the voltage, but I’m concerned about power dissipation and heating.

Why the concern? you say the original adapter voltages was 3.45v and so this new adapter is less than 0.5v higher. What is the current measurement with your bench supply set to 3.83v? is it like 770 mA?

 

can you share the led strings data sheet?

 

It sounds like you are expecting the total current in all four strings to add up to be a bit over 2 A, so I would recommend getting a diode rated at a minimum of 4 A.

At 2 A you would be looking at about 1.5 W of heat being dissipated in the diode, which is quite a bit. You may need to provide some kind of heat sinking.

The best solution would be to get a decent power supply that can actually deliver the current you need at the voltage you need.

 

Maybe I'm misunderstanding what you say but on my PS the output leads are definitely not 6 feet... They are more like 3 foot

Three or 6 feet, I still suspect that the gauge is small and there will be significant voltage drop.

If you'll always have all 4 on, you could arrange then as 2S2P and set the supply to 6V.

 

I suggest putting two of the 3 volt LED strings in series. Then you can adjust to put the right voltage on each string.