View Full Version : Dimming LED Light Strips - What's the Best Way?

Bob Deroeck
11-19-2017, 12:35 PM

I'm looking at installing LED undercabinet light strips for a kitchen remodel. For the high-task area of the countertop I'm planning on using LED high density light tape (950 lumens/meter), which may be too bright. So, I'm thinking of installing a dimmer. The wiring is in the walls and it can easily accommodate either of the following options.

Option 1. Light switch with 120 volt on/off switch and dimmer, followed by a 120 volt dimmable power supply stepping down the voltage to 12 volt which then goes to the LED tape.
Option 2. Light switch with 120 volt on/off switch, followed by a standard (non-dimmable) power supply stepping down the voltage to 12 volt, followed by a 12 volt dimmer which then goes to the LED tape.

I'd appreciate recommendations on which of these two options is "best" from a performance standpoint only. I consider the cost differences between the two options to not be significant. I also consider the difference in effort to install either of these options to not be significant. There is plenty of "room" to "hide" from view the components for either option, so the cosmetics of the installation is not a concern. My concerns are "noise" or "hum" from the transformers, RFI from the transformers or dimmers, the dimming range of the light, and the quality of the light (color, flickering, other?). I'd also be interested in any other "performance" considerations that I may be overlooking.


Bob DeRoeck

Peter Kelly
11-19-2017, 1:35 PM
1 under-cabinet strip of 120 LEDs / 950 lumens per meter shouldn't need dimming unless you're really light sensitive or have a lot of white surfaces. Electrical layout should be option 1.

I'd recommend grooving out the underside of the cabinets and installing the lights in a channel with a diffuser lens. Countertop materials with a reflective finish like quartz will show the LED dots otherwise.



Frank Pratt
11-19-2017, 7:19 PM
Something you need to consider with the higher density LED tapes is heat dissipation. Sticking the tape to wood alone will greatly reduce the life span. You may find that in as little as a year the LEDs get much dimmer. It's heat build up that does that. Most vendors that sell the tape also carry aluminum extrusion that can be surface or flush mount & with or without a diffuser. The aluminum helps keep the LED temperatures down.

Jim Becker
11-19-2017, 9:39 PM
My under cabinet LED tape strips from Lee Valley have their small dimmer (low voltage side) for exactly the purpose you state...to slightly adjust the light output so it does the job without overpowering things. If you choose to dim on the 120v side, you need to be sure that the dimmer you use is LED compatible and that the transformer you use is also dimmable.

Bill George
11-19-2017, 10:52 PM
To dim just use a lower volrage DC power supply. They or at least mine anyway worked fine on 9 vdc. When something electronic dies witn a wall PS I always save the power supply and recycle.

For my kitchen project the first OEM 12 volt PS generated so much EMF/RFI it made my AM radio useless with the static.

I always make sure whatever I use is UL approved, a lot of the Chinese electronics may not be.

Peter nice drawing, you are talented.

Pat Barry
11-20-2017, 9:04 AM
I bought the tape backed strip from HD and it came with a dedicated dimmer and it works great. I bought a long strip, cut it to length 2X, wired the two ministrips end to end with some light gauge wire (28ga) installed the strips into a cabinet to light up two different sections.

Frank Pratt
11-20-2017, 12:25 PM
I've also used lots of the Lee Valley LED tape & extrusions. Quality seems good & I've had no call backs at all.

Curt Harms
11-20-2017, 8:23 PM
I did our LED under cabinet lights before there were good switching/dimming options. I wanted dimming but couldn't figure out a way without a fairly large PWM device. I found a web site (http://ledcalc.com/#) that discussed using a resistor. I was able to get the dimmer bits I needed at the local Radio Shack (R.I.P) 100 ohm wirewound resistors and a mini 3 position toggle switch. I wired it so in one switch position the power ran from the 12 v. power supply through the switch to the lights. In the other switch position the power runs through the resistor. Not real energy efficient because the resistor gets a little bit warm but the watts are so low that I feel it is insignificant. I have the toggle switch mounted under a cabinet so it's invisible unless you bend down and look up. So far so good.

Jason Roehl
11-21-2017, 7:09 AM
On top of what the others have said, nothing should hum--I don't think anyone uses an actual transformer anymore for AC-DC conversion, it's all done in circuitry.

Alan Rutherford
11-26-2017, 3:45 PM
LED's will dim by reducing the voltage but the right way to do it is with Pulse Width Modulation (PWM) which electronically pulses the full voltage at very fast intervals. Shorter pulses and longer gaps between the pulses give dimmer light. You can do it with one of these (https://www.amazon.com/Leegoal-LEDwholesalers-Lights-Dimming-Controller/dp/B003L4KKF2/ref=sr_1_2) for $6-8. I've done it this way in 3 kitchens. I don't think you'll need the dimmer, though. I only use ours as a dimmer in the middle of the night. It also serves as the on/off switch and the 12v supply is on all the time except when I unplug it if we're going to be gone long enough to matter.

Get some of those white plastic molding strips from HD or similar, maybe 1/2" wide , 1/4" thick and stick the LED's to the plastic. Then use foam tape to stick the plastic to the cabinets. Or screws if you must. Get the LED's with wire attached on the plastic and working before you put them up. It's a lot easier that way.

Our LED's under the cabinets and 3 parallel strips over the sink:

[Edit: I didn't really answer your question. From a performance standpoint, I'd feed the 12v power supply from the wall switch and not use a dimmer. Mount the LED's on plastic strips. I solder everything except at the screw terminals on the power supply.]