Planning DC Load Planning for LED lights

[kidcole]

Anybody got a reasonably accurate load figure for LED bulbs (per bulb) in amps at 12V ? Or check my numbers here:

I was using an estimate of .025 WATTS per bulb last year for LEDs. This year I may have 3200 LED bulbs on my DC controller (all 16 channels). So that should be 3200/.025 or 80 WATTS <total>. Running at 12v, then I should draw approx 6.7 AMPS for the entire controller .. This just seems so small ...

The DC power supply I have is 72 AMPs .. So it seems I have a screamin ton of extra power on my hands .. But it had me questioning my numbers. Maybe I should dig up a smaller supply !

[John]

Denny,

Your math looked OK to me. Why buy another power supply, the huge one you have would just set there idleing, with no strain at all

[Al in Raleigh]

Dennny, which LEDs are you referencing? If you are talking the Mighty Minis my number are different.

[mschell]

Denny,
I won't disagree with the numbers, but you're making certain assumptions that may or may not be true.

The thing about LED's is that you need to know how much current you expect to pull thru each LED or LED string. In the incandescent area, the filaments of the lamps have a resistance, and run at a nominal voltage, say 120V AC. That's why they can say - this string pulls .34 A per 100 lights.

However, with LED's you tend to start with a voltage, say 12V, and use an external resistor to limit the current that flows thru the LED, since it's more like a direct short when the LED lights up. If you don't limit the current thru the LED, it will burn up. You also need that resistor to drop the voltage that the LED "sees" to again, not burn it out.

All this to say that the resistors that you use will use up power, even more so than the LED. So you need to know what resistors are being used to limit current/drop voltage in your strings or fixtures, or measure the actual current/power draw with an meter. This will give you a more accurate view of your power consumption, rather than assuming that each LED or fixture is a certain power draw and multiplying it by the number of LEDs.

One more thing - there is a voltage drop associated with long wire runs from your controllers to the LEDs - this is the resistance of the wire over long(er) distances. This also will consume some power and your LEDs may be slightly dimmer. So you can't assume that supplying 12V to the controller will equal 12V out at the LEDs. This many change the chosen resistor values you use in your Mighty Mini floods and elsewhere.

Hopefully I haven't confused you too much, but there's more to this than meets the eye...

[John]

Oh, if you are also going to be building the MightyMinis, you need a 24 volt power suppy, not 12 volts

When I checked Denny's math, I "ASSUMED" he was basing his wattage using a 20ma current per LED (.024 watts), forgot to consider the power loss dropped across the resistors. My pizza arrived while I was typing my reply, and was hungry and didn't stop to ask more application questions

But Al and Mark are both correct depending on the current per LED and dropping resistors, which will vary also depending on color of the LED.

Regardless, with a 72 amp power supply, you should still be "safe" from overload. Cavet: The 2nd generation LOR DC board can handle 4 amps per channel which is 64 amps, but the board itself is only rated for 20 amps per 8 channel bank.

[Al in Raleigh]

Mark is correct. The explanations are for our group's edification. The LEDs need a certain current regardless. If you put too much voltage across them they go poof. If you put too little they ignore you. It really is that simple.

With LEDs it is all about the current. I know Denny is working on another project than the Mighty Mini.

[kidcole]

Good info guys. Yes, this is not for the mighty mini's. I am also buying the components to build the mighty minis, but I may wait until very close to December before I decide to use them or not. If I can get everything else done then the mighty minis will also come to life. Or they might have to wait until Christmas 2011 ..

For the connections to the LED strings, I guess I was thinking I could just remove the rectifier on each LED string, then wire that string (intact) to the 12v DC controller. I guess I will have to dig in deeper on this resistance thing. I can see a need for a really good LED primer document to help more people move to DC on the LED strings, particularly how to calculate the resistor needed...

My DC controller will be right beside the gadget with the LEDs, but there will also be a twin gadget about 20 feet away. So I will have some longer wires out to that one. I will show the gadget once I get it working .. :-)

[Al in Raleigh]

As Denny knows, LEDs look better with DC. You are using AC strings and cutting them to size. Which strings Denny?

[mschell]

As Denny knows, LEDs look better with DC. You are using AC strings and cutting them to size. Which strings Denny?

The normal "commercial" LED strings put many LED's in series to handle the 120V AC. If you have 30 LEDs and what the engineers call the "forward voltage" of each LED is 3V, then 30x3=90 Volts. You then take a resistor and put it in to drop the other 30 V and along with the rectifier, you're good to go.

The problem is that with the LEDs in series, you can't apply 12V and expect the LEDs to light. With 12V, you can probably only get 3-4 LEDs per channel, if the sets were originally 120V. AC or DC doesn't matter, the total voltage is the determining factor.

The Mighty Mini's require 24V, because they have 6 LEDs in series, and with a 3V forward voltage, 6x3=18V as a minimum.

Hopefully we haven't bored you guys with all the details, but give you enough information to be dangerous

[John]

For the connections to the LED strings, I guess I was thinking I could just remove the rectifier on each LED string, then wire that string (intact) to the 12v DC controller. I guess I will have to dig in deeper on this resistance thing. I can see a need for a really good LED primer document to help more people move to DC on the LED strings, particularly how to calculate the resistor needed...

Denny for experimenting to find the correct resistor value, cut say 5 or 6 LEDs that are wired in series from a commercial sting. Then take your multimeter (on milliamp setting) and a potetiometer (5K should work) and connect this serial wired mess to your 12 volt source. ( and remember to get the polarity correct)

Adjust the potentiometer until you get a current of ~20 mA, then disassemble and measure the resistance across the potentiometer at the setting that gave you the correct current. Use resistors of the nearest standard value.

If you're lucky, you'll get a current of 15-20 mA with the potentiometer turned down to 0 ohms and you won't need resistors.

If you can't get at least 10 mA at 0 ohms, repeat with 4 LEDs.

This measurement and calculation may need to be repeated with different colors or strings from different mfg. as has been mentioned, different LEDs work on different voltages ~2 volts or ~3 volts and may need a different current rating

This will at least give you an idea as to how much cutting and rewiring will be required to use your 3200 LED commercial strings by cutting out 4, 5, or 6 series wired LEDs at a time, adding resistors, weatherproofing and then rewiring each group in parallel

This is something I thought about doing a few years ago, but quickly ditched the idea when I realized having hundreds and hundreds of homemade connections out in the yard, and buying bags and bags of resistors just wasn't worth the time and effort for me.

If you decide to proceed with your gadget, I wish you luck and look forward to seeing the final result.

I hope if I made any errors in my explanation, (its been a few years since I played with this,) then Al or Mark will jump in with corrections.

[kidcole]

I just realized I might end up having to wire hundreds of LEDs in Parallel, with hundreds of little resistors, to make my idea work for 12V. Yuk !

Al,

how did your friend Geoff wire his LED lights to the LOR DC board ? He had hundreds of them on about a dozen (or so) channels. It sounds like commercial strings won't work for me .. but I certainly don't want to be trying to make my own waterproof LED sockets and soldering thousands of LEDs.

[John]

Denny, I'm not Al, but Geoff is from OZ.

All of their LED strings are designed to work on 24 volts, since they are outside and their mains voltage is 240 Volts, which cannot be used outside there for holiday lighting. (Safety reasons)

Most decorators in OZ have standard LOR controllers set for 240 volts, and the LED strings (and their incadescent strings also) have their own transformer/ and or circuits to drop the voltage down to 24 volts and they just use their standard controllers.

I supppose they could cut off the stepdown and rectifier pack off and use the LOR DC board fed with 24 volts.

Many of the DIY guys from down under do solder hundreds of LEDs to make their own strands.

I'm not sure which method Geoff used, maybe he shared that with Al, and Al will reveal the secret to all of us.

[Al in Raleigh]

John is correct. Geoff uses commercial 24Vac LED strings with 40 LEDs per string. He did not mention what his DC supply is but he uses LOR DC controllers which says it would be less than 60Vdc.
Al

[mschell]

Denny, I was hoping that you weren't planning to split up the 120V strings. I can see where it can get confusing, since in NZ/AU and even in the UK I believe, they do use 24V strings.

Hopefully, we haven't discouraged you from your project, and that you can find an alternate approach to making it work. There are 12V LED rope lights easily available in the US, but they can only be cut at certain intervals.

As for that 72A 12V power supply - if you or the guys in the lab can find an extra one, I probably can put one to use...

[Al in Raleigh]

I've had limited luck with rope lights. I think that I must mishandle them or something. What we need is a programmable, constant current source controller. You could use cheapo LEDs cut to length with excellent results. Hmm. Maybe, I will look into it. Naaa, I got too much on my plate now.

Denny, now you know why I decided to use AC incandescent minis with right angle clips. It makes life a little easier.
Al