Need to dim some fairy/christmas LED lights - use a resistor?

[SolderSucker]

I've recently bought a string of 120 LED lights that plug into the mains (voltage details further down this post).

However, they're a bit too bright for my tastes so I'd like to dim them a bit. I guess the best way is to put a suitable resistor in series but as the mains plug/controller is a sealed unit I guess my best option is to cut into one of the cables and insert a resistor there.

Voltage details on the plug/adapter/controller (which has a button to cycle through various settings (flashing, slow bright to dim, etc)):

Primary: 220-240 VAC, 0.12A

Secondary: Urated: 31V, Uout: 34V

P rated: 3.6W

The main light string unplugs from the controller adapter (no doubt to use the light string with different adapters in different countries) and the two pins for the light string where it connects to the adapter are marked as positive and negative.

I know little about mains adapters, etc so is the 'Secondary' above for the secondary winding? I guess so as I can't imagine it's the voltage for the LED bulbs.

Whatever I do I want it to be safe of course.

[ajb]

These days almost every consumer wall wart style power supply is a switch mode affair, so the 'secondary' ratings don't correspond directly to the characteristics of the transformer. The specified secondary voltage will be the actual output voltage.  31-34V would correspond to about ten LEDs in series, so that's probably how the string is set up. If at any point some of the LEDs are off while others are on, then half of those series strings of LEDs will be arranged antiparallel to the other half, and the output of the adapter will be a full bridge. The adapter will switch the output polarity to select which set of strings are on, or rapidly swap polarities to illuminate both at once (well, alternately, but persistence of vision completes the illusion of simultaneous illumination).

The two different output voltage ratings are a little funny but may be an artifact of there being some sort of current limiting internal to the adapter. You could measure the open circuit voltage and the loaded voltage to get a better sense of what's going on.  Just be aware that if it does the antiparallel thing then you'll probably need to measure with an oscilloscope rather than a meter.

The built in current limiting may be (or probably is?) just a fixed resistor, but it's possible it's more sophisticated, or current limiting is built into the converter itself.

An additional resistor inserted in the string is probably the easiest way to go, although you'll need to find the right place to do that so that it ends up in series with all of the LEDs. A spot near the plug where there are only two wires in the string would be a good place to start.  It's possible that if the adapter has some sort of active current limiting that it will act up with additional resistance, but I doubt that's the case.

You'll of course need to ensure the resistor is rated appropriately for the power you'll be dissipating, but if the whole adapter is only rated for 3.6W that shouldn't be too hard, maybe 1/2W at most?

[SolderSucker]

Thanks a lot for the very helpful information and advice.

You say maybe use a 1/2 watt resistor, in order to dim the bulbs would I be looking to use a very high value resistor in the mega ohms range, or much lower?

Also, how about safety while the lights are in use with a resistor in the string, any concerns there?

Thanks again