Counter led strip lighting - Dimmer before or after transformer?

[yzman720]

Going to be putting some of the led strip lights in that have the sticky backing under some cabinets for counter lights.
The strips are able to be cut every few inches. They come in 5 meters and are 12v and have a working current of 3A. That's all the specs say. Pretty generic. I am buying a separate transformer from another company that takes the A/C to D/C and says 12V 3A 36W...again pretty generic. My question is can I put a dimmer switch before the transformer or does it have to be after? And Does the transformer's value look correct or am I missing some specs to be able to tell if it's compatible?

 

[gar]

170122-2137 EST

Do not be scared away by my comments.

I can provide some general information, but can not answer your specific questions.

To work with the many new types of products that electricians are now encountering it is more important to have a basic understanding of how individual components within those devices work.

Lots of devices may have the LED name attached, but that whole device does not necessarily exhibit the basic characteristics of of a single LED. All sorts of components may exist in the device such that looking at the input terminals of the device the actual characteristic of the LED are not seen

Clearly an LED light strip is composed of many individual LED chips. Are these series, or parallel, or series-parallel connections? I suspect your strip is series-parallel. The 3 A figure is a red flag to it bring simply a series circuit. A quick Internet search did not provide information on the internal wiring of typical strip lights.

You need some understanding of the basic characteristics of an LED diode. See figure 9 at http://www.mouser.com/pdfdocs/PhilipsLumileds_DS202.pdf . This is a plot of LED current vs the voltage across the LED chip and at a specific chip temperature, 25 C. Ignore temperature for the moment, assume it is constant. Suppose this LED is supplied a voltage that is proportional to your input AC supply. With 120 V input we apply 3.1 V to the LED. The LED will have 100 mA of current flow. Increase the input voltage to 130 V, an 8.3% increase in voltage. At the LED this is 3.1*130/120 = 3.36 V. From the Philips curve this results in a current of way off of the plot. Going the other way drop the input to 110 V, 92% of nominal. At the LED this is a voltage of 3.1*110/120 = 2.84 V. Now LED current is about 40 mA. Over a reasonable range LED light output is approximately proportional to current, see figure 7.

Thus, one wants to drive an individual or series string of LEDs with a moderately constant current source. If LED lights of some sort that include electronics or some means of current limiting are of concern, then a constant current source would not be required.

On to transformers and dimmers.

Magnetic core transformers can not tolerate an average non-zero DC input component. Excessive DC current can cause transformer failure. Ordinary run of the mill Triac dimmers can cause this problem.

This does not provide a direct answer, but hopefully you are able to understand some of this. Ask more questions. And possibly someone with direct experience can provide useful information. I want to know more about these strips. I need IR illumination with no visible light from such strips.

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[Electric-Light]

Going to be putting some of the led strip lights in that have the sticky backing under some cabinets for counter lights.
The strips are able to be cut every few inches. They come in 5 meters and are 12v and have a working current of 3A. That's all the specs say. Pretty generic. I am buying a separate transformer from another company that takes the A/C to D/C and says 12V 3A 36W...again pretty generic. My question is can I put a dimmer switch before the transformer or does it have to be after? And Does the transformer's value look correct or am I missing some specs to be able to tell if it's compatible?

http://catalog.gelighting.com/elect...12v-signage-drivers/p=geps12d-60u/d=0/?r=emea

Those from lighting component brands are built to commercial light fixture standards, such as encapsulated electronics and meant to work with controls that are accepted for lighting control such as 0-10v wall switch.

The ultra cheap stuff (you know.. adapter + dimmer for less than a combo meal at McD) from private sellers are Chinese flea market grade stuff that some guy imported himself from China himself and selling out of his garage and those are most likely the ones to not meet UL standards. Something big brands wouldn't sell for fear of having to recall.

They're general purpose 12v AC adapter and they should at least work when you plug it in, but safety, durability and quality are hit and miss. The dimming control is meant to be used with general purpose DC 12v but it might feel like the speed control on the adjustable chasing rate holiday decoration while a 0-10v from a lighting brand is durable and feels quality.

When you crack open a power adapter that came with your computer side by side with the same voltage "fits your laptop" adapter, the manufacturing standards are completely different and all but the components necessary absolutely necessary for it to work are omitted. Often the board is dirty as heck and looks like it was hand soldered in a sweat shop. This is what it is like with $4 shipped LED 12v supply.

 

[Electric-Light]

Thus, one wants to drive an individual or series string of LEDs with a moderately constant current source. If LED lights of some sort that include electronics or some means of current limiting are of concern, then a constant current source would not be required.
.

Even reputable brand like Lutron does not list its PWM based power source as FCC CLASS B. The PWM type most likely does hard switching and spew off a lot of interference beyond limits allowed for residential use. They have a constant current reduction model that does not hard switch LEDs but its expensive. If those internet LED dimmers rated for 5A 12v from the size of an Oreo cookie causes unlawful levels of EMI, it wouldn't surprise me. It most likely do not have any capacitors or inductors that do not contribute to the PWM's working. They're

Sylvania has 12v DC input dimming module for use with 12v input LED strings. The control is 0-10v analog. It is quite big, so I am assuming EMI measures are included.


----gar NOT related to rope LEDs but those 2' U or 4' Philips tube LEDs have 60 pcLEDs in a long string and operates from roughly DC 200v supply fed from series capacitor + bridge rectified fed from HF AC ballast used for fluorescent lamps. The absence of resistive ballast for every 3 LEDs certainly help.

LED light bulbs also have no isolation. The cheap one for example uses 8 components in the LED ballast including the fuse. It creates DC 170v p-p from bridge and runs 30 dies in series through 15 packages through a 3 terminal purpose made hard switching LED ballast IC

 

[gar]

170123-1208 EST

yzman720:

From what Electric-Light has said and trying to decode some of the information from the Internet I conclude the following:

1. The 12 V LED light strips are constructed with three LEDs in series with one or three current limiting resistors in the series. If we designed at 1.5 V per LED and 100 mA current, then the total current limiting (ballast) series resistance for the 3 LEDs would be (12-3*1.5)/0.1 = 75 ohms. Power input to this group of 3 LEDs with current limiting resistance would be 12*0.1 = 1.2 W.

2. The light strips are then made up from many of the series groups of 3 LEDs connected in parallel to a pair of 12 V bus lines. Current into the 12 V bus lines will be incrementally proportional the light strip length. Cutting of the light strip would be done at some multiple of 3 LEDs.

3. The high output light strip has 18 LEDs per foot, or a spacing of 0.67" or about 5/8" (0.625).

4. The high output strip has 6 groups of 3 LEDs per foot. Six times 1.2 W is 7.2 W per foot. In data I found for a WFLS-X3 on the Internet it specifies 4 W maximum per foot. This would imply a lower operating current level based on 4/6 = 0.67 W per 3 LEDs or a current of 0.67/12 = 0.056 mA. Current per foot of light strip at 4 W per foot is 4/12 = 0.34 A. A 3 A rating for a strip on this basis is 10 ft long.

5. Five meters is 39.34*5/12 = 16.4 ft. For your full length, 16 ft, light strip the power input = 12*3 = 36 W. This is 2.25 W/ft and thus I suspect not the high brightness strip. Meaning the LED spacing is about 1.3".

6. Your power supply current requirement will be dependent on the total number of feet of LED light strip you use. You can take short lengths, several feet each, and parallel these strips with each other and power from the same power supply.

7. If you connected all separate light strips in parallel, and use one dimmable power source, then all lights would dim together.

8. If you have separate low voltage dimmers on the output side of the power supply, then individual strips of LEDs can be separately dimmed from one power supply.

9. If the DC supply is internally adjustable for output voltage, meaning an input dimmer to a DC supply (transformer, rectifier, filter), or internal electronics for output voltage adjustment, then you need a different DC supply for each different LED area to be dimmed.

10. The way these LED light strips are being made is not power efficient because more than 1/2 the input power is being lost in the series current limiting resistors.

11. With resistors as current limiters light output intensity is not good with respect to source voltage variation, but much better than when no resistor is used.

If even more than 1/2 the power was wasted in the series dropping resistance, then light intensity variation could be reduced to that of the voltage variation.

12. Ask questions and provide feedback on what you are doing.

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[Electric-Light]

Gosh.. LEDs are still extremely complicated to use correctly. It seems like magnetic transformer is the only way you can dim 12v LEDs and stay compliant with radio frequency emissions requirements.

For the original poster: You would use a transformer like this one. I think this is like a regular low voltage MAGNETIC transformer with a rectifier output. You then use a AC power line side dimmer intended for MLV

https://www.pegasuslighting.com/12-...onstant-voltage-led-driver-nora-lighting.html

The pass through dimmer you put on the output side chops the DC into small bits and feeds it into field installed LED rope which can turn into interference emission antenna. If the frequency is high enough to avoid visible flicker, it may make noise you can hear or cause harmful radio interference.

Another method is solid state dimmable LED driver/transformer for 12v LED which is PWM method. It's best to not use this method unless it's a commercial property due to high levels of radio interference generated.

Gar, "
8. If you have separate low voltage dimmers on the output side of the power supply, then individual strips of LEDs can be separately dimmed from one power supply."
It would appear you need a linear regulator. The cheap ones online do not have a substantial heat sink and I believe they're RF spewing type. It doesn't look like enforced code regulates wiring done on Class 2 side even if it causes harmful interference. So, it's a partial regulatory vacuum. RFI emitting LEDs are still the wild west just like drone laws.

Lutron's official page says:


NOTE: This equipment has been tested and found
to comply with the limits for a Class A digital device,
pursuant to part 15 of the FCC Rules. These limits
are designed to provide reasonable protection
against harmful interference when the equipment is
operated in a commercial environment. This
equipment generates, uses, and can radiate radio
frequency energy and, if not installed and used in
accordance with the instruction manual, may cause
harmful interference to radio communications.
Operation of this equipment in a residential area is
likely to cause harmful interference in which case the
user will be required to correct the interference at his
own expense.
This device complies with part 15 of the FCC Rules.
Operation is subject to the following two conditions:
(1) This device may not cause harmful interference,
and (2) this device must accept any interference
received, including interference that may cause
undesired operation.

 

[gar]

170124-0858 EST

yzman720:

Are you going to use one long, 5 meter, strip, or will this be cut into smaller strips?

Is one dimmer to control the brightness of the entire 5 meter strip length?

If different sections of the strip are to be individually dimmed, then I would probably prefer to use separate dimmers at the 12 V level, but I don't know what is available, and what problems they might create, such as RFI (radio frequency interference), and their reliability.

One power supply that provides 12 V DC at 3 A will power one 5 meter 3 A light strip. This power supply will also power 5 light strips cut into 1 meter lengths from said 5 meter strip. Whether the supply will provide long term reliability or not will depend upon its quality of design and manufacture.

.

 

[iwire]

Guys, how about letting the OP respond before posting more walls of text?

If I was the OP I would have already run away.

 

[gar]

170124-0920 EST

Electric-Light:

I could not figure out what Nora Lighting is talking about. Looks like a translation from a far-east language. And even if it was clear what their words really meant, then there was still insufficient information to know what was inside the devices.

.

 

[kwired]

Going to be putting some of the led strip lights in that have the sticky backing under some cabinets for counter lights.
The strips are able to be cut every few inches. They come in 5 meters and are 12v and have a working current of 3A. That's all the specs say. Pretty generic. I am buying a separate transformer from another company that takes the A/C to D/C and says 12V 3A 36W...again pretty generic. My question is can I put a dimmer switch before the transformer or does it have to be after? And Does the transformer's value look correct or am I missing some specs to be able to tell if it's compatible?

Read all of 411.4 before proceeding.

Is your power supply and lighting units listed for use as part of same system?

Some of the comments so far may be reasons why this is required.