LED Strip Lights

[Cavester]

Hi,

I am hoping there is an answer for what seems like a simple questions, yet my sparky was not sure.

I wish to put various strip LED's under the pelmets of windows etc. however I need the drivers to be concealed yet accessible fro any future maintenance etc.

The question is, is there a maximum run of cable permissible between the driver and the LED strip if I wanted to host the driver in a remote cupboard?

Thanks.

 

[ban-all-sheds]

There could well be, either for reasons of voltage drop, or EMR from the cable.

The people who can tell you if/what the limits are will be the makers/sellers of the supplies

 

[winston1]

There could well be, either for reasons of voltage drop, or EMR from the cable.

The people who can tell you if/what the limits are will be the makers/sellers of the supplies

LED strips run on DC. A decent driver will output DC only so there should be no EMR problems.

The max cable run depends on the size of the cable. The longer the run the thicker the cable needs to be.

 

[bernardgreen]

A decent driver will output DC only so there should be no EMR problems

SHOULD be no priblems but there often are problems.

Many switch mode 12 v DC power supplies produce 12v DC (eff) where eff = effective. The output is a high frequency fluctuating voltage the averages out at 12 volts but may have peaks of more than 30 volts. A lead carrying the high frequency fluctuating voltage can act as an aerial to radiate interference

Many switch mode 12 v DC power supplies that produce 12 v DC (eff) fail to mention the (eff).

For incadescent lamps this type of fluctuating voltage output is as effective as steady 12 v DC.

The spikes on an (eff) supply can degrade the LED elements in the LED strips

 

[winston1]

SHOULD be no priblems but there often are problems.

Many switch mode 12 v DC power supplies produce 12v DC (eff) where eff = effective. The output is a high frequency fluctuating voltage the averages out at 12 volts but may have peaks of more than 30 volts. A lead carrying the high frequency fluctuating voltage can act as an aerial to radiate interference

Many switch mode 12 v DC power supplies that produce 12 v DC (eff) fail to mention the (eff).

For incadescent lamps this type of fluctuating voltage output is as effective as steady 12 v DC.

The spikes on an (eff) supply can degrade the LED elements in the LED strips

You may well be right for those supplies sold as LED drivers. For the LED strips I fitted in my wardrobe I recycled on old set top box 12v power supply which had smooth DC output.

 

[SimonH2]

Even where the DC output is relatively smooth, there is an amount of ripple and potentially conducted emissions from within the circuitry. I get to read a friend's RadCom (journal from the RSGB), and they have a regular column on EMI - from many sources. Just last week I was reading of a problem a member had which was tracked down to the PSU for a novelty illuminated sign a neighbour had in his "man shack" - needless to say, cheap unit which (IIRC, I could be confusing it with another similar article) when dismantled had no filtering components fitted.
Even ignoring that possibility, LED strips are typically voltage driven and volt drop in the cables will have a significant effect on light output. Many quality LEDs are current driven, and within the capability of the driver will be unaffected by cable length - but I doubt many LED strips fall into this category.
So try and fit the PSU close to the LEDs. With pelmets you should be able to hide them either underneath or on top, other locations you may need to be a bit more creative. Also, some PSUs are smaller than others, so it may be worth looking at alternatives to find ones that are easier to hide.

 

[bernardgreen]

needless to say, cheap unit which (IIRC, I could be confusing it with another similar article) when dismantled had no filtering components fitted.

Far from uncommon that the PCB has provision for the filtering components and these components are fitted in the devices used for any product testing. Then to reduce costs they are not fitted in production runs.

Many quality LEDs are current driven

All LED elements are current driven, the ultra cheap LEDs used in festive decorations which can be connected directly to a 3 volt battery have resistive semiconductor material included in the structure of the LED element thus creating a current limiting resistor as part of the LED element.

For what it is worth I find running 12 volt strips on 9 volts still provides adequate light and should prolong the life of the strips. ( 9 volts DC and not a 12 volts PSU dimmed to 9 volts ).

 

[SimonH2]

OK, the LED chip itself is current driven, but the assembly the user deals with can be either voltage driven with internal current control, or current driven (basically an LED with no extra components - typically needing 350mA).
While playing with some 12V white LED strips, I found the turn on voltage to be about 7 volts. Above that, the LED voltage isn't going to change all that much, with the rest dropped in the current limiting resistor (for resistor controlled ones) - so less than 5V across the resistor that sets the current, so (say) a 0.5V drop in the connections is going to give 10% drop in current and hence brightness.

Some of the better strips have a constant current chip in each group of LEDs - so are much less sensitive to voltage variations.

 

[bernardgreen]

Some of the better strips have a constant current chip in each group of LEDs - so are much less sensitive to voltage variations.

But these cannot be dimmed by a simple reduction in supply voltage, depends on the application as to which type is "best"

 

[aptsys]

You may well be right for those supplies sold as LED drivers. For the LED strips I fitted in my wardrobe I recycled on old set top box 12v power supply which had smooth DC output.

Take your 'smooth' DC output from a switcher and have a look at it with an EMI receiver and you'll see it's anything but smooth. Here's a recent measurement of a high quality PSU from a very well regarded manfuacturer into a 50W load

 

[SimonH2]

Everything is relative ! It's pretty well a given that any PSU will have some noise on it's output - but some manufacturers at least filter it as far as is practical; others (like the Chinese Export ones with filter components left out) simply DGAS.