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I'm looking at using 12VDC or 24VDC LED light strips for over and under cabinet lighting. These would be the LED strips you can buy in 100 foot lengths and cut to the desired length. They have solder tabs for connecting power.

My questions are:

1) Can I use a standard 120VAC to 12VDC or 24VDC wall transformer connected to the LED lights, and control the brightness of the LED lights using a 120VAC lighting dimmer wired to the receptacle?

2) If so, would I have a wider range of control with a 24VDC transformer as compared to a 12VDC transformer?

I would make sure that the transformers are rated to carry the load.

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  • \$\begingroup\$ When I first read this I immediately thought "Argh! No!". But after some further contemplation, you probably could get this to work if the power supply is simple enough. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 9 '17 at 0:55
  • \$\begingroup\$ Ignacio, thank you for the quick response. Can you describe what you mean by a "simple enough" power supply? Don \$\endgroup\$ – Don Mar 9 '17 at 1:04
  • \$\begingroup\$ More often these come in 5m reels with hundreds of varieties of power, color, density, and many actually rated for cars 12~14.5V so if you want to dim you need a dimmable supply unless you get clever with 2 or 3 levels. ledsupply.com/blog/ultimate-guide-on-buying-led-strip-lights FYI \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 9 '17 at 1:06
  • \$\begingroup\$ A lighting dimmer works by clipping the AC sine wave either before or after its zero crossing. This translates reasonably well to DC PWM as long as the power supply doesn't try to smooth the wave. So theoretically it should work as long as the supply only has a transformer and rectifier, no filter. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 9 '17 at 1:07
  • \$\begingroup\$ I agree and 120Vac triac dimmers are usually not compatible with std PS, as most these days are regulated. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 9 '17 at 1:08
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To directly answer both of your questions:

1) Probably not. Check out this rundown on wall warts. Some have linear regulators and some have switching regulators. In both cases, the transformer and rectifier will output a voltage higher than the desired output, and either a linear regulator or a switching regulator will drop the voltage to the desired one. This is useful because it helps maintain voltage across a range of loads (at least, sort of and in theory).

2) Your range of brightnesses won't vary based on strip voltage because of how the power strip is designed. I go into a lot more detail here about LED strips, but the quick version is this: The LEDs are arranged in bundles. Each bundle has the LEDs in series, allowing them to be powered by 12V (3 LEDs) or 24V (6 LEDs). You put a lot of bundles in parallel and that makes an LED strip. So, you won't get more control over brightness just because the operating voltage is different.

So, you may be asking, "How on earth do I do this then???" Well, it turns out the answer is fairly straightforward. You get a 12V or 24V LED dimmer. These go between your power supply (which provides 12 or 24V) and the LED strip. I've linked one here, to provide an example. I have no experience with that particular unit, and StackExchange isn't for product recommendations anyway, but it should help you find the right unit for your needs.

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Nope just rectify it and use PWM it is far less complex. The dimmer might cause harmonics on the transfomer and a range of other nasty things. A 555 timer wired for pwm with a full bridge rectifier would be much better. The DC system simply has less that can go wrong with it so use it. Messing with the wave form of AC on inductive devices is a bad idea.

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  • \$\begingroup\$ It would help if you provided details that directly answer the OP's question. Saying, "[...] just rectify it and use PWM [...]" would be either obvious to the OP, and they wouldn't have asked, or incomplete, leaving the OP (still, and pun intended) in the dark. \$\endgroup\$ – Hari Ganti Mar 9 '17 at 1:53
  • \$\begingroup\$ I think I've fixed that. \$\endgroup\$ – Fil Nick Mar 9 '17 at 2:00

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