How do dimmable LED light strings, with only a plug (no controller) work?

I know just enough about controlling LEDs to be dangerous. My understanding is that they dim (and/or, change color) via PWM (Pulse Width Modulation). As opposed to old-school incandescent bulbs which would be dimmed by varying the… voltage(?).

Recently I came across several outdoor LED light strings (the ‘bistro’ style, with ‘Edison’ style, screw-in bulbs) which claim to be dimmable. But they only have a plug on the end, and no (LED) controller (that I can see). Apparently you can buy a ‘standard’ plug in dimmer, which apparently does, indeed, work.

I guess I was just trying to understand how that is supposed to work?

The full story is: I had bought some of these strings at the recent ‘Spring Black Friday’ sale at Hazard Frought. It was an impulse purchase, because of the sale (they were only $17.99 each!). A few days later, I decided to double check the price on Amazon, that was when I noticed all the other strings claiming to be dimmable.

Subsequent research revealed a single comment on the Harbor Freight listing claiming those strings are actually dimmable as well, even though this is not an advertized feature.

I am currently awaiting delivery of some Zigbee DIN rail dimmers to test this theory out. I didn’t want to destroy the light strips / bulbs though, so I thought I would seek some additional advice in the meantime.

Here’a good primer on how dimming works: Dimming LEDs - The difference between leading and trailing edge

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Thanks for that link, I think I understand now.

It looks to me like this is a form of PWM as well?

It seems to me that a growing number of these LED elements/bulbs are made to withstand this kind of switching/dimming nowadays. Am I remembering correctly that just a few years ago, this was much less common (in other words, only incandescent could be dimmed, not LEDs, at least not without a dedicated controller)?


More specific to my application, I have ordered some of these Gledopto GL-SD-003P DIN rail AC dimmers (mfr, blakadder, Z2M). They appear to be rated up to 100W @ 110VAC, but I did not see any minimum amperage requirement listed. Nor could I find detailed information whether they cut the leading or trailing edge. I don’t think I will bother contacting the seller nor manufacturer.

Instead I think I will just try them out when they arrive and report my findings. I do feel a lot more confident about it now, thanks @tom_l! :slight_smile:


FWIW, I have wanted to put some kind of lights up in my tree in the back yard for a long time now. These Harbor Freight lights actually appear to be quite decent quality, with a 2’ spacing between lights (many others are 3’) and put out quite a bit of light (almost too much, which is why I started looking into dimming). And they were a very good price (on sale). Since it now appears my scheme might actually work, I will allow myself to begin to get a bit excited about all of this now. :smiley:

Thanks again, Tom! :slight_smile: