Can a Standard Power Pack's Variable DC Be Power & Dimmer for LED Strips?

[tehachapifan]

So, I plan to have many LED's on my layout for streetlights, etc., including some 12v DC LED light strips here and there. Instead of buying a separate driver/dimmer, could an unused DC power pack be used on the variable DC side to drive and dim the 12v DC light strips? I realize that the independent LED's will still require resistors.

Thanks!

[peteski]

I don't see why not. LEDs have not special requirement for power. As long as the LEDs themselves have current limiting resistors and the power pack can supply sufficient current, it should work.

The only think I would worry about is the power pack itself. These are usually not very robust (especially the low-en d  ones) - I would not put a load on it for more than 50% of its rated current.  Also, power packs are usually vague on how much current they can supply so it might be difficult to determine.  Some power packs show combined wattage rating (for both DC and AC output).

[tehachapifan]

Thanks, Peteski! I just looked again and the power pack I had in mind goes up to 14v on the variable DC side, not 12v like I thought. I'm guessing maybe it isn't the best option after all for the LED strips, which already have resistors and call for 12v DC. Wouldn't want to accidentally overpower these.

[peteski]

Thanks, Peteski! I just looked again and the power pack I had in mind goes up to 14v on the variable DC side, not 12v like I thought. I'm guessing maybe it isn't the best option after all for the LED strips, which already have resistors and call for 12v DC. Wouldn't want to accidentally overpower these.

The couple of extra volts might not be all that critical. It all depends on the resistors used with the LEDs.  in many circuits the with LEDs (because they are so bright) aren't operated at their maximum rated current (the resistor has a larger value than required).  Extra couple of volts on the supply side would make the LED glow brighter but probably still not over their maximum rating.

For example if you have a white LED rated for maximum current of 20mA (which has a forward voltage or Vf of 3V) and a 1k resistor in series with it (a commonly used resistor value) then the current through the LED @ 12V is (12-3)/1000 = 0.009 (or 9mA).  So it is running at about half of its maximum rated current.

Now power that LED circuit with 14V and see how much current is flowing: (14-3)/1000 = 0.011 (or 11mA). Still well below maximum safe current.

But if you can live without the dimming features, there are lots of "wall-wart" type of 12V supplies available rather inexpensively from many electronic surplus dealers. Such as All Electronics or Electronic Goldmine to mention few.  You can pick up a nice regulated 12V supply capable of supplying few amps for less than $20.

[tehachapifan]

Thanks, Peteski! I probably will just buy a 12v power supply as you suggest.

[Scottl]

What I have learned is that LED strips that are really long (like 5m) need a lot of power, far more than a normal powerpack will provide.  Something like 10A at 12V for many.  Depends on what you are using.

[N2deep]

Hello,
I'm new to this forum but this is the exact question I was looking to answer. I have several unused dc controllers. Originally planned on using some of them to control incandescent bulbs in scenery but would like to go with these led strips for everything. I understand the led requires at least 9V to come on so does that mean that dimming or adjustments are not possible?

[peteski]

Hello,
I'm new to this forum but this is the exact question I was looking to answer. I have several unused dc controllers. Originally planned on using some of them to control incandescent bulbs in scenery but would like to go with these led strips for everything. I understand the led requires at least 9V to come on so does that mean that dimming or adjustments are not possible?

Most of those strips have 3 white LEDs and a resistor in-series connected to the power rails.  That pattern repeats throughout the strip.  Since a white LED needs close to 3V to give out some appreciable light then you are right, around 9V is needed for them to light up.  But they are still dimmable.  They will probably start fairly glowing around 7 or 8 volts and up to their full as-designed brightness at 12V.

But it is the current rating of your power pack that is the important factor. Most power packs supply 1 amp or less.  If you are only powering few dozen LEDs then that is ok, but if you have several feet of LED strip you will overload the power pack.  Even a 1 amp power pack is not designed to continuously supply 1 amp.  It would most likely overheat after few hours of such current draw.  It might even start a fire.  I wouldn't recommend a constant current load of more than about 50-60% of the power pack's rated current.  That is why one is better off buying an inexpensive power supply which is robust enough to provide a higher continuous current. But even with those, I wouldn't recommend a continuous current draw of more than 70-80% of their rated current.

[N2deep]

Thanks peteski. So I'm reading where each one of these 3 led's draws 20ma and a whole 5 meter role takes 2 amps. Sound right? Would that mean that even a 1 amp controller could handle about 100 led's safely? Don't get me wrong, I'm not trying to get out of spending $20 for a better power supply, im just trying learn and understand.

[peteski]

If the specs state that every group of 3 LEDs uses 20 mA @ 12V, and 5 meter strip consumes 2 A @ 12V, then there are 100 sets of 3 LEDs in 5 meters (or 300 LEDs).

According to that, 50 groups of 3 LEDs (150 LEDs total) would consume 1 A @ 12V  Most power packs aren't very forthcoming as far as the current rating goes.  If the pack you have does supply up to 1 amp then I would not want to put a constant load on it of more than 700ma.  35 groups of 3 LEDs (105 LEDs total) would consume 700mA @12V.

I keep mentioning @12V because if the voltage is higher, the LEDs will consume more current and if it is lower, less current.

[N2deep]

Outstanding. You are correct on the specs. I will get out all the controllers and check their amps tomorrow.

[daniel_leavitt2000]

I looked at this, and played around with a few strips with a Tech II 2800. If you are powering more tab one strip,I would recommend a dedicated power supply and a 60-100 watt dimmer switch.

[N2deep]

I looked at this, and played around with a few strips with a Tech II 2800. If you are powering more tab one strip,I would recommend a dedicated power supply and a 60-100 watt dimmer switch.

I checked my extra controllers. I have two Tech II 1400 and they are only rated at 1.2 amps. I quickly found other regulated power supplies at 12v 5A for $12 but nothing higher yet. I'm very curious about the 60-100 watt dimmer switch you mention. Is this a dimmer designed for LED's?

[daniel_leavitt2000]

I was looking through power supplies myself. There are a few here:
https://www.superbrightleds.com/moreinfo/enclosed-power-supplies/12vdc-enclosed-power-supply/70/

But I was able to find a much cheaper one with similar specs on eBay:
http://www.ebay.com/itm/AC-110V-220V-TO-DC-12V-Regulated-Transformer-Power-Supply-For-LED-Strip-Light-/261470580279?pt=US_Lighting_Parts_and_Accessories&var=&hash=item3ce0dc6e37

My guess is that Superbrightleds.com over charges for a lot of their components, but its a good place to start.

Here is the 60W dimmer:
https://www.superbrightleds.com/moreinfo/single-color-dimmer-switches/slvdx-60w-led-switch-and-dimmer-for-standard-wall-switch-box/1181/

And here is the 100W variant:
https://www.superbrightleds.com/moreinfo/single-color-dimmer-switches/lvdx-100w-led-dimmer-for-standard-wall-switch-box/1244/

I have not been able to find an equivalent on eBay.