Whether or not you can dim a lamp (or string of lamps) depends on a few factors.
If a string of lamps has a multifunction controller, they are generally not suitable for external dimming. This is because the electronics in that controller is designed to have constant power. Dimming can lead to erratic operation and damage to the multifunction controller. Even switching power (fully on and off) to a multifunction controller at a relatively slow rate (seconds to minutes) via another AC controller can be problematic, as its electronics can interfere with the correct operation of the upstream controller outputs.
Incandescent lamps simply pass a current through a filament wire to make them glow white hot. Flood lights, fairy lights, halogen lights, etc are all incandescent types. Some are 240V (like older household globes) while others are low voltage (6V - 36V usually).
In general, incandescent lamps will work equally as well on AC or DC, so ones that originally were supplied with a 24V AC (for example) plug pack can be used with an equivalent DC supply and DC controller.
Despite what some may say, you can dim halogen lamps (240V or low voltage) just fine. If you run a halogen lamp at low levels for a long long time, the output can start to deteriorate (the glass starts to blacken), but all you need to do is run them at 100% for a while to "rejuvenate" them.
A lot of LED strings are supplied with an AC plug pack that drops the 240V AC mains down to 24V AC or other similar voltage. Most times, these LED strings can be used (without the plug pack) with a low voltage DC controller. Some experimentation may be required to find what DC power supply voltage works best with them. Some strings may also need to have resistors added to each segment to ensure correct and even current draw of the LEDs.
Some mains voltage (240V AC) LED strings can be dimmed with an AC controller, but erratic operation can result. This is due to the fact that the LED strings don't draw enough current for the correct operation of the controller's TRIACs. The TRIAC is the component that switches the power on and off to an output channel. One way around this is to provide an additional (resistive) load on each channel. This is sometimes called a "ghost load", as it's often hidden away and not part of the visible display. They are often incorrectly called "snubbers".
Most CFLs (Compact Fluorescent Lamps) can not be dimmed as they contain special drive electronics. Some are marketed as "dimmable", but they must be used with special 240V AC dimmers and the dimming range is often quite poor.
As explained earlier, a lot of off-the-shelf light sets are supplied with a 240V to low voltage iron cored (older heavy type) transformer. The success of switching the 240V supply to these sets will depend somewhat on the exact situation. If you switch them at very low speeds (say no more than every minute), your AC controller may work quite well with them.
Dimming larger toroidal transformers is possible, but usually an AC controller that is designed to handle inductive loads is required.
Never attempt to dim or switch a switchmode plug pack with an AC controller. These plug packs can draw huge inrush currents that may damage the controller. The switchmode plug pack will often be destroyed (in spectacular fashion!) by attempted dimming.
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