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

Parallel is frowned upon because individual LEDs vary and there's no way to control how much current goes down each path of the circuit. ie. Some LEDs might get more than their maximum rating.

12V seems a bit small and 110V seems too big for your LED mix.

12V will work but you'll have a lot of little strings of LEDs.

eg. You can only put three of the 3.8V LEDs in a series. For 18 LEDs you'll have 6 separate series.

Yes, but remember that all LEDs in a series will have the same current passing through them so mixing 20mA and 30mA won't work.

Technically, no, but see next question.

Option 1: Choose your resistor carefully.

Resistors have a problem though, getting exactly 20 or 30 mA can be fiddly and if you get it a couple of mA wrong the difference in brightness between LEDs can be noticeable.

If this isn't a problem for you then the resistor method will work.

Option 2: Use a LED driver. There's lots of circuits, pre-built boards, etc. out there which provide a fixed current from any reasonable voltage.

You get one that matches your LED current (eg. 30mA) then simply connect a series of LEDs to it.

I'm not sure what you mean by this. When you build your LED circuit you're going to limit the number of amps that can pass through it. Each circuit will have a current requirement.

So long as the total requirement for all your circuits is within the power supply's capabilities then you can just connect them up, nothing extra needed.

There was a long thread here back in '04 regarding regulating current to LEDs using a current mirror and similar techniques. Give that a read for some options:

or just do a search in

for "led current mirror group:sci.electronics.basics"

--
Rich Webb     Norfolk, VA

For 120VAC the current can be controlled by a so called "wattless dropper", this is a series capacitor who's Xc is large compared to RL.

Back in the days of hybrid TVs, Thorn consumer electronics tried this to replace the wasteful dropper resistors in the 300mA series heater chain.

IIRC the capacitor value was 4.3uF for 300mA on 230V mains at 50Hz, you'll also need to include some series resistance to absorb turn on surge and spikes.

You can use 2 series chains of LEDs in inverse parallel or a single chain after a bridge rectifier.

I'd be inclined to go for a simple and dumb 12V arrangement. You are dissipating under 3.7 watts in the LEDs so even if you waste 25% more power in the series resistors they will remain fairly cool and waste less than a watt. You avoid a lot of safety issues by NOT having high voltage running to the fixtures.

Unless the 12V supply is very well smoothed and regulated, you want considerable 'headroom' between the total LED Vf in each string and the supply, otherwise the current control will be poor e.g. a 5% increase in supply voltage causing the LED current to double. The 3.8V LEDs have too high a Vf to place three in series from a 12V supply for this reason.

The resistor chosen for each series string handles the current limiting and should be chosen to run the LEDs at no more than 80% of their maximum rating if you want reasonable reliability.

I would arrange the 30mA LEDs in series strings as follows: 9 sets of 2 x 3.8V + 1 x 1.7v giving 9.3V total Vf 2 sets of 4 x 2.2V giving 8.8V total Vf 1 set of 5 x 1.7V giving 8.8V total Vf 1 set of 4 x 1.7V + 1 x 2.2V giving 9V total Vf

The 20mA LEDs cannot be mixed in the same string as the 30mA ones and 3 x 3.6V doesn't leave much headroom to stabilise the current so I'd run 4 strings of two and an odd one out which will need its resistor calculated separately.

Each string requires its own series resistor chosen for the correct current based on the difference between the total Vf and the supply voltage. All the strings WITH their individual series resistors are connected in parallel.

You need a 'transformer' (actually a DC output power supply) intended for LED applications. An AC output one for halogen lighting is unsuitable. You can use multiple fixtures in parallel up to the rating of your power supply, (though it would be smart not to exceed 80% of its rating.

--
Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
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CORRECTION 1 set of 5 x 1.7V giving 8.5V total Vf

Sorry about the stupid maths error. It only affects the serise resistor for this string and doesn't make any difference to the over-all design.

--
Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
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to=20

chain.

You need to make sure the capacitor is rated for mains connection, and = even=20 with the series resistor the turn-on surge and spikes from transients = can=20 easily exceed the LED's maximum surge rating (usually 3 to 5 x nominal) = and=20 cause deterioration and failure. Much better to use an inductor or = ballast.

You may even be able to adapt a ballast from a small fluorescent lamp to =

work on an LED string.

But probably best is an electronic switching circuit such as=20

which is available = from=20 Mouser and others for a dollar or so.

chain=20

The two chains in inverse parallel have a huge problem if one of the = series=20 fails open. Then the other string will see the full reverse voltage and = one=20 or more devices will also fail.

You can make a constant current regulator from an LM317 or even a common =

7805 regulator:

There are many drivers available pre-made for 120 VAC line use, and it = may=20 be better (and safer) to use them. Jameco has some for less than $20, = but=20 designed for 350 mA 1 watt LEDs:=20

79_-1

Also please see the recent thread on LED lamp innards. There are some = pearls=20 of wisdom and information among the swine of bickering.

Paul=20

Thanks to every response. I don't feel nearly as insane as I did a few short days ago.

I guess to be sensible, I'll start out simple (LEDs with correct/safe resistors) and then also experiment with LED drivers, droppers and regulators for possible energy savings over my 1st design. It's always nice to have "improvements" in those areas to market if I get that chance. :)

My fixtures energy draw is pretty low now at less than 3.5w. Seems great when I compare it to what it is replacing. However I can easily see running 20-50 of these at once so it will add up.

Oh and I found some more LEDs with 30mA to replace the ones I have that are 20mA so I would not have to mess with it and leave me cleaner wiring in my series chains which are long because my LEDs are 3-4" apart in most cases. Also I trying an experiment where I mixed the

20mA with some LEDs that are 40mA in a series. I averaged the mA based on the number of each type LED. I did my resistor calculation based on this average. So far it lights up! we'll see if it blows up! :)

Oh I was thinking of using laptop power supplies for this projects future. They give 19V which makes for longer LED series strings and they are dirt cheap ($5-10) to buy because of their popularity. What I don't know is how reliable they are for power output??? Can I trust them??? If I make the jump to drivers that guarantee my voltage this may be the way to go! (LED power supplies are like $70.)

Thanks again!

No, you want to set the current lower than the *minimum* LED rating. They're in series, so each gets the same current. However, you want to add the voltage of each LED to calculate the ballast resistor.

It's unlikely that you'll need to drive them to their rated maximum. The eye won't be able to detect the difference between 20mA and 30mA (or 10, for that matter), anyway.

They're quite reliable. Go fer it.

--- If what you're looking for is minimum power waste, then you should implement series strings running at high voltages.

For example, if you ran the 30 mA strings you described earlier, you'd have 18 LEDs dropping 3.8V each for a total drop of 68.4V, 18 LEDs dropping 1.7V each for a total of 30.6V and 9 LEDs dropping 2.2V each for a total of 19.8V.

Putting all of those 30mA LEDs in series would result in a total drop of 68.4V + 30.6V + 19.8V ~ 119V, which is very close to nominal mains voltage in the US, suggesting that you could full-wave rectify the mains and run the LEDs without a current limiting resistor.

You'd need some kind of spike protection in order to protect the string from transients, but we can address that later, if you're interested.

---

--- If you run the 20mA LEDs at higher than 20mA, then their life will be shortened.

---

--- My suggestion is that you tailor your strings so that you can run them directly off the mains using a full-wave bridge to drive the LEDs and a small resistor and a Zener TVS to quell the spikes.

Would you like a schematic?

-- JF

Hey JF!!!

I may be a noobie but I was so sure that this is where I was heading. (Resistors just seemed to be against everything I was trying to do. It even seemed like adding another LED would be better than a resistor, because that way all the power is being used to make light no matter what. ???? wasn't sure if this was true.)

I found some more 30mA LEDs so they all match now, but was already calculating things to run at reg mains. USA 110V because of the new

30mAs I have over my 110V @ 132V and wasn't sure what to do with the extra 20V because a resistor was a billion ohms or something.

Yes I would LOVE a schematic!

Would you like a piece of my new company? :) Hell Yeah!! I love folks who help get things done!

My new specs are...

3.8Fv 3.3Fv 2.2Fv 1.7Fv 30mA 30mA 30mA 30mA 15LEDs 9LEDs 9LEDs 15LEDs

But for a newbie, it would be much safer to use a wall-wart or laptop = power=20 supply. They are pretty much fail-safe if overloaded or shorted, and the =

voltage won't kill you (unless you try really hard).

The power supply will take care of that problem, too.

Again, using a power supply is safer. And if there is a fire, the UL = rating=20 on the PSU may be worth an awful lot. Fire inspectors don't take kindly = to=20 homebrew gadgets across the house wiring.

There is a schematic, for the bulk of what is required, in the Supertex = data=20 sheets, and they also probably have application notes. Their circuits = work=20 from about 12 VDC to 400 VDC, but I think it's best to stay under what = is=20 considered "safe", which is about 25-30V. Or even 50V, which is what = POTS=20 phone lines carry, but that is current limited to 20mA or so, which is=20 marginally safe.

If you want to make such a device commercially, you should contract a=20 licensed professional engineer to make sure the design will meet UL and=20 other standards. Pay somebody a modest amount now so you won't have to = pay a=20 huge lawsuit settlement later.

Don't become a statistic...

Paul=20

It's not a good idea to do this. The thing is that the resistance of a LED isn't constant.

The resistance of a LED drops *rapidly* as the voltage increases past the threshold voltage, and that's precisely the voltage you're aiming at when you design your circuit.

In a series of LEDs, the LED with least resistance will get the most volts. Increasing the volts will lower the resistance even further... and... do you see where this is going?

You can probably get away with it if you're only trying to run them at (eg.) 10mA but if you're anywhere near their rated current then you're taking a big risk.

You won't be using anywhere near their rated maximum current so they should be quite reliable.

he eye

that

Yep. Aim for a few mA less then the rating (eg 25 instead of 30), you won't notice the difference and it should simplify the circuitry (or at least let you use cheaper/easier to find components).

I disagree. I've been using capacitive reactance for lighting small LEDs for years now with no ill effects. Use a full wave bridge, small series resistor, and zener across the led string to limit the peak voltage.

I concur that AC rated caps would be the better choice - but I've been using 450 volt polyester film caps with no problems (on 120VAC).

I think PE Johnson had a circuit posted showing a pretty good (bullet-proof) circuit for running off 120 VAC and protecting the leds from transients..

Don't do it. Fluorescent lamps require a high voltage strike to initially ionize the gas - you may still need the same protection that running off the mains directly would require.

That does look intriguing. 8-450 VDC input.

That's true, but the likelihood is very remote - maybe up there with shark bite in a Kansas cornfield...

You design the best you can and make compromises. Absolute bullet-proof starts getting expensive, and usually requires some other trade-offs like size, complexity, efficiency.

That would be my choice for a few high power, high cost leds. Downside might be efficiency, and if one were using batteries to power it....

=A0The eye

r that

OK not just this specific post but there seems to be a lot of warning about overloads. I want to take this seriously specially because I can quite be sure of what the end user would "plug" these things into. However I also am looking for maximum efficiency. I set up a spreadsheet that allows me to combine diff LEDs @ diif quatities and does the math based on ohms law and all that stuff and tell me what resistor to use. I know to round up to the next available to be safe. Are you telling me that I need to change the current/mA settings in my spreadsheet to lower than the manufacutrer suggestion of typical mA/ current??? I don't want to loose anything if I don't have to.

I also don't want to spend the next 2 years replacing products I've sold because the gain a history of being over-sensitive to their power source.

Thanks for the amazing discussion. This is better (down to earth) than any basic LED circuit design thread I've seen for sure.

Mad Props to all who have chimed in!

just want to throw in that I'm using 5mm old school LEDs because of their low power and nm selection. I am looking at tape and other LEDs but the total flux seems to suck so bad. I'm sure I just to learn more about LED shopping or get the cash to make a custom order.

Filter caps salvaged from the mains input circuit of old TVs/monitors/whatever are rated to take full on mains on a full time basis - not to mention subtracting the summed Vf of a decent size string of LEDs.

The "electronic transformer" for low voltage halogen lights is superficially similar to a CFL circuit - the main difference is having a large toroid transformer instead of a ferrite cored ballas in series with the tube.

However; introducing current control into this type of half-bridge converter is not trivial.

and I've been able to back down to 15 from the 18 sets so it's now...

15 - 30mA IF 3.8VF 15 - 30mA IF 1.7VF 9 - 30mA IF 2.2VF 9 - 30mA IF 3.3

total 48 LEDs per fixture