Hello to all…

I have been perusing the forum now for some time, and only just recently registered.

I have a bit of a quandary.

I’m looking to add a supplemental lighting system to an existing structure, and am hoping you all can be of some help.

I want to keep this on the cheap, as much as possible, and most importantly a modular type system where components can easily be replaced and/or swapped out as need be.

My thought is to use a common ATX power supply to run approximately sixty to seventy 5050 SMD’s.

Output of supply’s 12v leg is 11.95v (rated for 14amp), 3v leg is 2.97v (rated for 18amp). (actual volts measured with a FLUKE 87.

I’ve worked with LED’s quite a bit in the past, but nothing of this magnitude, mostly with accent and homemade replacement lighting in cars, boats, motorcycles, etc.

My wiring will be a “loop” around the interior of the building, approximately 300ft. total. Using the “loop” I plan on connecting both ends back to the power source. I hoped to use 20-2 solid wire for the “loop” itself, and approximately 16in of 22-2 solid wire for the pigtails for the LED’s themselves.

To keep things as simple and “off the shelf” as possible, I have two options I’m eye-balling.

1 – Run the SMD’s directly off of the 3.3v. leg of the power supply. At a rated draw of a bare 5050 being 20ma @ 3.2-3.4v I’d have plenty room to spare, drawing (by my calculations) 1.2amp total draw.

2 – use 5050 LED’s, prewired with resistors for automotive use, and running them off of the 12v leg of the supply. I have a couple of these, and have tested them with my FLUKE 87, finding their draw at about 45ma. Which would put 60 of them at 2.7amp total draw.

I’ve used several internet based parallel/series LED wiring wizards, and see the options of running series/parallel setup’s. I don’t want to use this, as if one bulb would fry, I’d have a whole string of them out and have to test which ones a bad, etc. I’m not a Christmas light fan….and really hate doing that.

Again, I’m having my amp draw numbers of the LED’s confirmed, and will be able to post back with better numbers.

I lean toward using the 12v. leg of the power supply, as to me (guidance needed here as well), it would make more sense to push a higher voltage through the line right from the start, than the lower 3.3v.

I’ll be using this simply as a kind of elaborate night light to accent the interior only (obviously) at night, and had planned on working out a simple photocell as an eye to control power to the power supply. HOWEVER, I also have several old power supplies for some Malibu lights laying around that already have photo cells incorporated into them. I know these put out (rated) 12v AC (11.8? actual), and know that using them would necessitate building a full wave bridge rectifier to convert the AC to DC.

The specifications for these supplies have long since rubbed off, but I do know that at one time I ran approximately twenty to thirty incandescent bulbs off of one of them following it’s specs so I’m sure it could handle the current without any problem.

I’ve built rectifier assemblies for a motorcycle in the past, but that was about nine years ago. I also would have no clue how much amp draw such a rectifier would handle, how to calculate that, or what components to use anymore.

So there’s a lot of questions in there…

Am I on the right track? What would my voltage drop be? With the “loop” I plan on using should I even be that worried about a voltage drop? (I believe computer power supplies are regulated, and in THEORY would internally compensate for any drop) Is my wire gauge sufficient enough? 12v or 3.3v leg? Computer Power supply vs. Malibu light supply w/ rectifier? Bridge rectifier designs?

Thanks to everyone in advance, any and all guidance is very much appreciated.

 

A simple power supply does not compensate for current dropped by wiring resistance.
Look online for a wire guage resistance chart and figure out how much the current will drop.

you CANNOT and should not connect a bunch of LEDs in parallel directly to a power supply without series current-limiting resistors unless you measure all the LEDs then select only ones that have EXACTLY the same voltage because they are all different. Then adjust the voltage for the brightness but not exceeding the maximum allowed current in the LEDs.

12V is too high for one LED with a series current-limiting resistor because the resistor gets hot. 70 hot resistors make a lot of heat.

I made a nightlight with 36 Luxeon SuperFlux LEDs. The LEDs have 4 pins to take away heat to the copper on the pcb. The maximum allowed continuous current is 70mA so I used only 53mA. i connected groups of 3 in series then in series with a current-limiting resistor. Boy oh boy the LEDs got very hot even though I drilled many cooling holes in the enclosure.

 

A simple power supply does not compensate for current dropped by wiring resistance.

- Possibly a 12vDC supply as available from "superbrightleds.com"?

Look online for a wire guage resistance chart and figure out how much the current will drop.

- I've looked in to that, but frankly have no clue what i'm doing there.

you CANNOT and should not connect a bunch of LEDs in parallel directly to a power supply without series current-limiting resistors unless you measure all the LEDs then select only ones that have EXACTLY the same voltage because they are all different. Then adjust the voltage for the brightness but not exceeding the maximum allowed current in the LEDs.

- essentially you're saying no to running the 5050's dirctly from the PS? (another reason I wasn't liking the 3.3v leg direct wiring)

12V is too high for one LED with a series current-limiting resistor because the resistor gets hot. 70 hot resistors make a lot of heat.

- so goes back to running multiples of three/four LED's off one reistor, all parallel w/ source? (christmas light sorta thing) The resistor driven LED's will be well over 4ft apart (if it makes a difference), and hung from rafters.

 

A simple power supply does not compensate for current dropped by wiring resistance.
Look online for a wire guage resistance chart and figure out how much the current will drop.

It depends on the current, the capacitors, the wire length, and the voltage.

You won't get effects from 2mtr + 1 Amps.

If you have 5 metres + 50m distributed wires + small caps on a SMPS (only 1000uf) + dealing with 30 to 50 volts at 5 Amps, you would see some effects from the wires.

For instance if you install an additional switcher at the end of the chain, and turn it up, you'd notice flickering, and difficulties to gain high currents.

you CANNOT and should not connect a bunch of LEDs in parallel directly to a power supply without series current-limiting resistors unless you measure all the LEDs then select only ones that have EXACTLY the same voltage because they are all different. Then adjust the voltage for the brightness but not exceeding the maximum allowed current in the LEDs.

Depends on the LEDs. Power LEDs can sustain direct connection + parallel operation. But, the more in series, the better.
Piranha superflux LEDs work perfectly connected in parallel.

I made chains from ordinary 3mm + 5mm LEDs, most of them still work (at 20 volts), some actually failed!

For paralleling LEDs you need to use one batch, one manufacturer, or it will fail.
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12V is too high for one LED with a series current-limiting resistor because the resistor gets hot. 70 hot resistors make a lot of heat.

I made a nightlight with 36 Luxeon SuperFlux LEDs. The LEDs have 4 pins to take away heat to the copper on the pcb. The maximum allowed continuous current is 70mA so I used only 53mA. i connected groups of 3 in series then in series with a current-limiting resistor. Boy oh boy the LEDs got very hot even though I drilled many cooling holes in the enclosure.

[/quote]

I made a light with amber superflux LEDs, the 70mA type, actually NOS from the US.

The PCB turns hot, but not that much (it is freely suspended in air).

You can for instance use a larger VGA cooler, and glue it to the PCB or use the screw mounting originally intended.

You don't have to run the fan at full 12V!

The large coolers cost about 10 dollar (rectangular), while smaller one's can be had for about $2.50

Making and fan blade size varies a lot. There are also very cheap ones made from thin aluminium, and not much structured. They are only good for a little cooling.

I rather use power LEDs and attach them directly, than superflux LEDs.

Soon I will made some panels from straw hat LEDs!

I have not yet decided how to drive them, very likely I will use a LM2576 circuit. It depends on the inductor- it simply won't be able to crank out so much current at some stage. I really have difficulties to exceed 1 Amps for these- maybe due to DMM, and small capacitors?

I guess my DMM can not capture the current right, because one of the regulators causes the main power bus to drown down so much, all of the large white LEDs go dim a little. When I reduce it just a little, the voltage is normal. Maybe these are effects caused by the long cables I use.

It is very interesting anyway. And I will make more and much different types of LED lamps soon!

 

A simple power supply does not compensate for current dropped by wiring resistance.

- Possibly a 12vDC supply as available from "superbrightleds.com"?

Look online for a wire guage resistance chart and figure out how much the current will drop.

- I've looked in to that, but frankly have no clue what i'm doing there.

you CANNOT and should not connect a bunch of LEDs in parallel directly to a power supply without series current-limiting resistors unless you measure all the LEDs then select only ones that have EXACTLY the same voltage because they are all different. Then adjust the voltage for the brightness but not exceeding the maximum allowed current in the LEDs.

- essentially you're saying no to running the 5050's dirctly from the PS? (another reason I wasn't liking the 3.3v leg direct wiring)

12V is too high for one LED with a series current-limiting resistor because the resistor gets hot. 70 hot resistors make a lot of heat.

- so goes back to running multiples of three/four LED's off one reistor, all parallel w/ source? (christmas light sorta thing) The resistor driven LED's will be well over 4ft apart (if it makes a difference), and hung from rafters.

ATX power supplies are not so good for currents in excess of 3 or 4 Amps.
They are made for symmetrical loading among a few other things. They don't have much of a protection against short circuit. I saw 3 or 4 of them going bang over the years- one even catched fire.

12V is not a high voltage for LED chains. The more in series, the better. 30 or 40V are typical.

You can for instance get a 12V -> 35V converter, connect some 15 LEDs in series or whatever it takes, and then crank it up very carefully. You must measure the current when the LEDs are hot.

 

A constant current source (typically switching) is the accepted method for powering LEDs.

A fixed voltage may or may not work, or burn them out, based on environmental factors, such as a very hot environment where heat cannot be dissipated effectively.

The most common current source is a resistor in series with a voltage source, though the downside is power wasted as heat.

Doing a search for LED Drivers will result in several constant current drivers to match your LEDs.

 

A 300ft. loop can certainly produce interesting effects.

I have seen effects from much less at suitable currents.

I do not know much about these SMD LEDs but smaller LEDs normally don't take direct connection so well. Piranha LEDs do as mentioned.

It is not difficult to use a MC34063 as booster from the ATX supply. It has current limiting feature.

As for LED drivers, 300mA, 700mA and 1A are common. Most of these will produce a certain range of voltages, intended for certain types of LEDs. They are not made for SMD LEDs.

If you parallel chains for 300mA, you could use a 1W driver, but maybe will end up with too many wires.

If you use many in series, you really get a window of a few volts, which is certainly manageable with a voltage regulator (SMPS chip).

For 1A or 2A, use an ATX supply if you like.

Running a single LED directly from a voltage source is almost not a good idea, especially a non-power LED. I use 10W LEDs with 12v supplies, but they all vary a little. 1/2 a volt means a level of heat production which is not sustainable. So I had to add a diode for the PC ATX supply, then when I moved it out, using a 12V electronic transformer, I had to add another dropping diode. Otherwise, it turned very hot quickly.

The thing is power LEDs can tolerate overcurrents, at least for testing, while small LEDs can not, they do not have cooling. If small LEDs heat up so much the color goes off, it is often too late. It is also more difficult to monitor heat developement.