LED wiring Series or Parallel 110 or 12v (for Power Efficiency)

Simple basic LED series with resistors hooked up to rated (meant for laptop) power supplies. At this point is still looking really good.

This seems like a good way to start. Of course get the UL done on my side of things, whatever legal is needed is a given for me. Really don't wanna buy houses for people cause I sold some lights.

Given the manufacturing tolerances of LEDs, resistors, etc., it's possible that if you design for 'perfect' components you might end up a bit too too high on some LEDs. It won't be much ... but it's worth designing 5-10% under spec, just in case.

If you look at the datasheet for your LEDs you'll see the light output vs. input current is a curve, not a straight line. Designing for 5-10% less current will hardly affect the output at all.

Without looking at the data sheet... dropping 75 volts is probably out of the question - the absolute maximum standoff is ~36 volts if my memory serves me.

For heat - you can always add a series resistor and dissipate most of the heat in the resistor and just enough to regulate current in the LEDs in the 317.

I've been going crazy with these warm white LED strip light thingees. I bought 5 meters of the stuff (~$30) and mounted one meter over the kitchen sink - since I already had a 12V power supply nearby.

On high, it is a lot of light so I added a dropping resistor and SPDT switch so I have a night light as well as enough to do dishes. So far it is great. Built in resistors for each three leds in series and the leds and resistors are spread out so there are no hot spots.

My wife doesn't like it because she associates the lights with Christmas. She'll adapt...

Having 4 meters leftover... I mounted some in test tubes small ones are 12 leds each and larger 24 per tube then added a mosfet transistor, and photo transistor, to switch them on at dusk. I have five of them to light a path.

Now what to do with the remaining 2-3 meters?

You could also use an LM3914 - it can do current limiting on up to 10 strings of LEDs with just a couple of external resistors.

from what I can tell these tapes can be custom made also. If I can get the LUX, beam angles and nm correct this way I'm sure it will be worth testing against my old school LED design.

I'm at 132V total now. I don't see how I can use LM317s without one of them having a huge drop. I must be missing something.

The beam angle is very wide - fully half the light on my kitchen sink goes out the window... A polished aluminum channel or angle would have been better to maximize the indoor lighting - and lots easier to mount.

The stuff is peel off sticky on one side, but it is hard to get it pulled taut when working close to the ceiling and trying to span a meter in length. The adhesive on mine is by 3M and pretty aggressive stuff - that test will come in summer...

it seems I also saw some regulators with like 10 Vout pins. But I got no clue on that stuff yet. I wonder if it will turn out to be as simple as the rest of this is.

Thanks for the 3914 reference I'll look at that since I've got less than 10 strings per fixture going at the moment.

What about voltage drop with these I've only got 132V total as soon as I split it I'll be dropping a lot from 110V. surely with 10 series capacity these will be able to have more drop tollerence or maybe I'll figure out just how much power I will lose by keeping a LM3914 cool and find out it's nothing.

------------------------------- 57V + 29.7V + 19.8V + 25.5V = 132V

US mains are 120VRMS nominal, +/- 10%, which means that the input to your string could be anywhere between 108 and 132 volts.

It seems like since your string drops 132V with 30mA through it all would be well since the maximum mains voltage would be 132 and all that would happen if the mains voltage dropped would be that the string would dim a little.

In truth though, there is some tolerance to the LEDs' Vf, and that needs to be taken into consideration.

Do you have links to the LEDs' data sheets?

BTW, as others have posted, working with mains voltages is dangerous even if you know what you're doing, and if you don't you could easily kill yourself or someone else or burn someone's house or place of business down.

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With that in mind, you might want to proceed like this: (View with a
fixed-pitch font)

First, set up three strings like this:

    3.8Fv      3.3Fv      2.2Fv      1.7Fv
---[5LEDs>]---[3LEDs>]---[3LEDs>]---[5LEDs>]---

Each string will then drop about 44V, so if you connect them up in
parallel, like this:

                          +--[STRING1>]--+
                          |              | 
                          +--[STRING2>]--+
                  +----+  |              |
MAINS>-----P||S---|~  +|--+--[STRING3>]--+
           R||E   |    |                 |
MAINS>-----I||C---|~  -|-----------------+
                  +----+

you'll have 44V across and 30 mA through each string for a total of
90mA.

You'll drop about 1.5V across the bridge, so the transformer will have
to put out about 46V with a 90mA load, which is 4.14VA

Hammond makes a transformer, a 164F48, which has a 115V primary and
two 24V secondaries rated for 125mA that can be connected in series to
get 48V and Mouser has 3 of them in stock for USD 8.68 QTY 1.

Hammond rates their output voltages with 115V in, so instead of 48V,
with 120V in it'll be about 50V out.

With the 10% variation in US mains voltage, we'll have:

MAINS  Vin   Vout
-------------------
 LOW   108    45
 NOM   120    50
 HIGH  132    55

On top of that, small transformers like this typically regulate at
about 30% from no load to full load, so for this one that would be 48V
out with a 125 mA load and 62.4V with no load, which is about
115mV/mA.

Then, since we'll only be presenting a 90mA load, that'll be a
decrease of 30mA from full load, so we can expect the output voltage
to rise to about 49V for low mains, 54V for nominal, and 59V for high
mains. 

So now, since we only want 44V across the strings and we want to
protect against the high mains, we'll have to drop the extra 15V with
resistors, like this:

                           
                          +--[R]--[STRING1>]--+
                          |                   | 
                          +--[R]--[STRING2>]--+
                  +----+  |                   |
MAINS>-----P||S---|~  +|--+--[R]--[STRING3>]--+
           R||E   |    |                      |
MAINS>-----I||C---|~  -|----------------------+
                  +----+

Since we want to drop 15V across each resistor and the current through
each one will be 30mA, Ohm's law says:

          E      15V
     R = --- = ------- ~ 500 ohms
          I     0.03A

510 ohms is a standard 5% value and recalculating the current into the
string gives:
          
          E     15V
     I = --- = ------ ~ 29mA
          R     510R

Then, the power it'll dissipate will be:

     P = I²R = 0.029A² * 510 ~ 429 milliwatts, 

so a good choice would be a 510 ohm +/- 5%, 1/2 watt carbon film
resistor.

Now there's only one thing left to do, and that's to protect the LEDs
from spikes. We can do that by placing a 68V 1500W transzorb

http://www.onsemi.com/pub_link/Collateral/1.5SMC6.8AT3-D.PDF

across the output of the bridge, like this:

                           
                          +--[R]--[STRING1>]--+
                          |                   | 
                          +--[R]--[STRING2>]--+
                  +----+  |                   |
MAINS>-----+  +---|~  +|--+--[R]--[STRING3>]--+
           P||S   |    |  |K                  |
           R||E   |    |[TVS]                 |
           I||C   |    |  |1.5SMC68AT3        |
MAINS>-----+  +---|~  -|--+-------------------+
                  +----+
 
Just for grins, at low line the output of the bridge is going to be
49V, so with a string dropping 44V there'll be 5V across the resistor,
so it'll only allow:

         E      5V
    I = --- = ------ = 9.8mA
         R     510R

into the string.

At nominal mains, there'll be 54V coming out of the bridge, so that
translates to about 19.6mA into each string.

Anyway, that's basically how to do it passively at a safe voltage and
how to protect the LEDs.

It can be done actively so that the illumination from the LEDs remains
essentially constant over the +/- 10% range of mains voltage
variation, but it gets more wasteful of power and more expensive.

While it is theoretically possible to run a string of Leds into hundreds of volts, there may be practical reasons not to.

I've never researched it, so I don't know. I do notice that traffic lights using many leds. 6 series strings on the ones I looked up, and that's only ~30 volts a string. Perhaps there's something (some good reason) I'm not aware of that makes it desirable to limit the voltage. (like - hypothesizing here - junction capacitance being unequal might dump too much voltage across one led on the instant power is applied). Or maybe something as innocent as worried about water condensation across the circuit boards.

Anyhow. For the small sacrifice in efficiency (and bragging rights) low voltage has a lot of good reasons to use it. Like not having to meet the same stringent electrical codes, safety, etc..

I'm running ~26 gauge flat two wire "telephone" wire out to individual led modules - even the 1 meter strip (which uses ~12 watts) I just tucked the wire behind a piece of molding around the window to hide it

- and put a "solid state fuse" in series to protect it.

The idea of using an LM3914 is ingenious, and an "off label" use.

You'd have to watch the total "package" power dissipation to stay safe. Each output may be able to safely supply a certain maximum current - but the whole chip may overheat if ALL outputs are dropping voltage and current (dissipating watts of heat) at the same time.

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Thanks JF

I do have links but also a little fear to give away some itelectual property for example the specific nm of my LEDs would give a lit away.

Thank You very much for the plans. I'll study them well. :)

hard to believe that 48 little LEDs made for such a good thread!

Cheers to all the great input!!

Thanks again!!

An LM317 has a maximum rating of ~37 volts difference between input and output. So, if you had say 168 volts on the input, and the output was connected to the series string of LEDs, the output pin would be at 132 volts. Therefore the 36V difference between input and output would be just within the rating. If you configured the LM317 to provide a constant current of

18 mA, and put a 1000 ohm resistor in series between the 168 volt source and the input pin, the resistor would drop 18 volts, so the input pin of the LM317 would be at 150 volts, and the difference voltage would be 18 volts instead of 36.

You configure the LM317 as a constant current source by connecting a resistor from its output pin to its adjust pin, and connecting the load to the adjust pin. The formula to determine the resistance or current is 1.25/R = I

For example, to get ~ 18 mA:

----- 68 ohms + ----|LM317|---/\/\/---+ ----- R | | | +--------------+--LOAD--+ | - -------------------------------+

1.25/68 = ~.018

In a like manner, if R was 1 ohm, I would be 1.25, if R was 10 ohms, I would be .125, if R was 100 ohms, I would be .0125 and so forth.

You should also check the power dissipated in the LM317. Power is the LM317 input - output voltage difference times the current.

Ed

--
Nice. :-)

sorry Ed, I must be too thick in the head like Dad used to say...

If I start with 110V from the wall and my fixture is 132V total then I will have to split my LEDs into a minimum of two strings. As soon as I split up the 48 LEDs up the smaller individual strings total V would be greater than the drop limit of 36V for a LM317.

If I split them evenly.

132 / 2 = 66V per string 110 - 66 = 54V drop

In my mind I keep coming back to the comments about the reliability of power past a laptop charger is very good. and the comments about not running the LEDs at their full ratings.

This gives me a clean reliable 19V

Also everyone is saying to under drive me LEDs

I could build strings that are around 20.5V without resistors.

Even if my LEDs won't run at MAX brightness (which is not good anyway apparently) at least all power would be being used to make light and none being wasted in resistors or 317s or anything else. (Unless I am a retard which is very possible. This is pretty new stuff for me) 19V is also pretty low so even if it shorts it would not be a fire starter. Running at 110V is a bit scary.

Yep. You seem to be hinting at selling this to the public. I don't know if that requires certification where you live, but it's likely (and for good reasons!).

If any part of whatever it is your selling has mains A/C wires inside it then it had better be rock-solidly built.

Laptop power supplies have already been through the certification process and they mean that no part of your gizmo has potentially lethal wires inside it.

If you rectify & filter 110 volts you get about 155 volts DC.

If you rectify & filter 120 volts you get about 170 volts DC.

My line voltage is usually around 127 volts, which gives

127*1.414=179.578-.6, or 178.978 after the diode's forward voltage drop. If you use a full wave bridge, you lose another .6 volts.

--
You can't have a sense of humor, if you have no sense.