What do I need to run 200 LEDs on 110V

[NoTime]

{Wait, stop the presses! I've had a brainstorm. A single component solution @ 19.95 bucks is available in the lighting section of your local home despot.}

What does BOE mean?

I don't like the data sheet. I'd guess 70 ma peak would thermally degrade the diodes. 60 Hz is so slow it's nearly DC for the thermal mass of the die embedded in plastic, wouldn't you say? The data sheet is just not good enough to tell us. I'd use a bridge, though. For the cost of three more diodes, the average current is doubled for the same peak current. I was thinking 44 series diodes and a bridge.

But then again, maybe you're right aiming at 70ma peak. Without better data or prior expertise, experimenting is in order.

Data sheet looks fine to me.
It has all the information you need to work this out, including any thermal effects.

BOE = Back of Envelope. Quick and dirty but close.
Using a bridge bumps the freq to 120Hz, but doesn't add that many LEDs to the string.
I thinks it's around 28 or so.
Don't forget you're effectively lowering the supply voltage + adding resistance.
Peak current drops quite a bit.

There shouldn't be an issue with 70ma peak thermal since the actual duty cycle is low.
I also took worse case so the actual peak will be lower.
No idea if if actually would flicker with the single diode.
Experience says some people might see it and some wouldn't.

Homework assignment: Show the math for working the bridge case out
The answer is definitely not a count of 44.

 

[kevin071572]

Have you looked into just buying a strand of white-LED xmas lights?

OmCheeto-Great Idea, but this is the off-season and Ebay and Amazon don't have good enough deals on LEDs from christmas lights. Most of the strings I seen are approximately $1/LED.

Phrak, NoTime-You both were complaining about the data sheet, is there a better brand of LEDs besides the Nichia's? I'm not rigid about the brand, I just want decent LEDs.

NoTime-what did you mean about the home depot single component solution for $19.95?

 

[OmCheeto]

OmCheeto-Great Idea, but this is the off-season and Ebay and Amazon don't have good enough deals on LEDs from christmas lights. Most of the strings I seen are approximately $1/LED.

I found the box the lamps came in. The manufacturer, http://www.neo-neon.com/default.asp", apparently no longer makes the Christmas light strings. They appear to have expanded into just about everything else though. They even have a large selection of LED Disco Balls.

Moddersmart has white LED's in various sizes for $0.25 a piece. Loose of course.
The different sizes have luminous intensities varying by a factor of 10.
This is another parameter you'll want to consider. A 20,000 millicandela(mcd) lamp reflecting off of a tile countertop may be uncomfortable to look at.

 

[Redbelly98]

Phrak, NoTime-You both were complaining about the data sheet, is there a better brand of LEDs besides the Nichia's? I'm not rigid about the brand, I just want decent LEDs.

They were discussing running the LED's from 120 V AC, and concerns about peak current associated with AC operation.

As long as you still plan to run from a DC supply, those LED's will work just fine Have you gotten a price from a supplier?

 

[NoTime]

NoTime-what did you mean about the home depot single component solution for $19.95?

That was Phrak, but if what I saw when I was in there the other day is what Phrak is referring to, I think they are incandescent light ropes. Not sure. I didn't look that close.

PS:I don't have any issue with the LED datasheet.

 

[Phrak]

Data sheet looks fine to me.
It has all the information you need to work this out, including any thermal effects.
...There shouldn't be an issue with 70ma peak thermal since the actual duty cycle is low.

This is why the data sheet isn't fine. Say we drive 70 mA peak at 1/10 Hertz. There will be a long period where the current is over 30 mA. So how low a frequency is too low? The data sheet doesn't say. It provides no thermal response curves to answer the question: how much average current can be tollerated at a given duty cycle and given frequency without overheating the die at the end of each cycle?

Theramal response curves are readily available for transistors. Is there something I don't know about diodes?

 

[Redbelly98]

I thought Kevin had decided to go with a 12 V DC supply, in which case none of that matters. This stuff about peak currents and duty cycles is just a side-discussion between Phrak and NoTime, not relevant to what Kevin actually plans to build.

Please correct me if that is wrong.

 

[Phrak]

That's true Redbelly. Your right, I think, that omCheeto has the right idea. Experiment with diode count, and buy a premade product. Reverse engineer it if you like.

LEDs are too nonlinear for simple analysis (roughly exponential in current vs. voltage), another reason that explains our wide disagreement. Spice would be nice.

 

[Phrak]

Phrak, NoTime-You both were complaining about the data sheet, is there a better brand of LEDs besides the Nichia's? I'm not rigid about the brand, I just want decent LEDs.

That was me complaining, but just about the data sheet, not the LEDs. When you go to use something in a nonstandard manner, like driving LEDs with 60Hz AC, you don't always get the information you want.

NoTime-what did you mean about the home depot single component solution for $19.95?

That was actaully me. It was just a joke (buy a fluorescent under-the-counter lamp.)

 

[NoTime]

This is why the data sheet isn't fine. Say we drive 70 mA peak at 1/10 Hertz. There will be a long period where the current is over 30 mA. So how low a frequency is too low? The data sheet doesn't say. It provides no thermal response curves to answer the question: how much average current can be tollerated at a given duty cycle and given frequency without overheating the die at the end of each cycle?

Theramal response curves are readily available for transistors. Is there something I don't know about diodes?

Try looking at Page 8.
There is enough information there to work this out.

If you can't tell me where you got a count of 44 from, then I don't see any point to continuing this conversation.

 

[NoTime]

I thought Kevin had decided to go with a 12 V DC supply, in which case none of that matters. This stuff about peak currents and duty cycles is just a side-discussion between Phrak and NoTime, not relevant to what Kevin actually plans to build.

Please correct me if that is wrong.

You are correct, it is not particularly relevant to Kevin's current plans.

 

[Redbelly98]

Mind you, I have nothing against side-discussions and I think they're partly why Physics Forums is such a fantastic place. But I did want to clarify that for Kevin's sake.

At any rate, the main choices now seem to be either:
1) build a 12V DC LED string, or
2) find something pre-made if available, for less money and takes less time than making one's own.

 

[kevin071572]

Try looking at Page 8.
There is enough information there to work this out.

Page 8? Looks like a flower petal with numbers around it. :surprised

I thought Kevin had decided to go with a 12 V DC supply, in which case none of that matters.

Redbelly- Thanks for keeping that clear, I was starting to wonder if that was all just to run on DC or not.


Like I said earlier, I knew this was the place to figure out this little wiring question. I just didn't know it would spark such a "discussion"

I now know that I can string 200 LEDs together and I know that I can run them on a 12 VDC power supply, which will make building it soo much easier.

 

[Redbelly98]

Page 8? Looks like a flower petal with numbers around it. :surprised

It's the page that says "8" on it, which is p. 9 in the .pdf file numbering.

 

[Phrak]

Try looking at Page 8.
There is enough information there to work this out.

If you can't tell me where you got a count of 44 from, then I don't see any point to continuing this conversation.

I'm sorry to have frustrated you NoTime. I think it has become rather pointless, but from page 8, I assume I cant rely on the Duty Ratio plot, and use the max rated current of 30 mA instead.

$$\frac{(1.414)120V - 1.4V}{44} = 3.82V$$

At 3.8V per diode the forward current is about 30 mA.

 

[NoTime]

Mind you, I have nothing against side-discussions and I think they're partly why Physics Forums is such a fantastic place. But I did want to clarify that for Kevin's sake.

Side discussions are fine by me.
Originally, I just wanted to clear up some ambiguities with Phrak's schematic for the benefit of other people that might be reading this.
LED's are a fairly popular topic.

Good point about the clarification.
It's a good thing you mentioned this

 

[NoTime]

Page 8? Looks like a flower petal with numbers around it. :surprised

The flower petal is a color map.
Here is one painted in with the colors.
http://www.google.com/imgres?imgurl=http://www.ligitek-led.com/rimages/24/chromaticity-diagram.jpg&imgrefurl=http://www.ligitek-led.com/chromaticity-diagram.htm&h=655&w=559&sz=422&tbnid=FWHpX9a-z-sJ::&tbnh=138&tbnw=118&prev=/images?q=Chromaticity+Diagram&hl=en&sa=X&oi=image_result&resnum=3&ct=image&cd=1

It's the next page, I was refering to.

Redbelly- Thanks for keeping that clear, I was starting to wonder if that was all just to run on DC or not.

Only the part about dynamic resistance is important to you.
You can use 12v, 3.6v and 30ma in the calculator link.
Your project will work, but the final measured current will be around 10ma rather than the expected 30ma.

Have fun

 

[NoTime]

I'm sorry to have frustrated you NoTime. I think it has become rather pointless, but from page 8, I assume I cant rely on the Duty Ratio plot, and use the max rated current of 30 mA instead.

$$\frac{(1.414)120V - 1.4V}{44} = 3.82V$$

At 3.8V per diode the forward current is about 30 mA.

That explains a lot!
Nice touch with the bridge diode drop, but I will note that the nominal line voltage, in the US, is 118v rather than 120v. It also varies some.

I don't think this side discussion is pointless.
Since you showed your work it isn't frustrating either

Quiz questions:
Compute the RMS current for the 100ma 10% duty cycle square wave given in the data sheet.

Compute the duty cycle for your solution (120hz).
Assume a cutoff voltage of (where no current flows) of 3.819v.

Compute the duty cycle of the portion of a 60hz sine wave that exceeds the 120v RMS value.
Assume only the positive half of the full cycle conducts (no bridge or switch diode).

 

[NoTime]

You can use 12v, 3.6v and 30ma in the calculator link.
Your project will work, but the final measured current will be around 10ma rather than the expected 30ma.

I made a mistake here.
Use 20ma in the calculator link.

This will work out quite closely to 20ma (about 17ma) using the 68 ohm resistor the calculator will show.
The reason it works is because the 3.6v is Vf at 20ma.
The calculator shows standard resistor values instead of exact values, or why the final value is not exactly 20ma.


My mistake was to use the 3.6v (Vf @ 20ma) instead of V_threshold which is about 2.9v.
If you use the 47 ohm resistor then the final current will be around 25ma, somewhat distinct from 10ma.

 

[Phrak]

Talk to me as if I have 23 odd years in electrical circuit development or design in power conversion, instrumentation, controls, fixturing, and fixing other peoples design errors, neither pushing papers nor peripheral support, interspersed with firmware coding and software engineering in test, engineering tools, and simulators, with exerience in primary and root cause failure analysis.

 

[NoTime]

Talk to me as if I have 23 odd years in electrical circuit development or design in power conversion, instrumentation, controls, fixturing, and fixing other peoples design errors, neither pushing papers nor peripheral support, interspersed with firmware coding and software engineering in test, engineering tools, and simulators, with exerience in primary and root cause failure analysis.

Fair enough.
Part of what I said to you made me realize what I had forgotten.
So I give you credit for fixing my mistake.

Your number of 44 also solves for a 20ma RMS current, except you got the count right.

One of the good things about PF is that if you do get things wrong there are people to set you on the right path,

Thanks.

 

[Phrak]

Fair enough.
Part of what I said to you made me realize what I had forgotten.
So I give you credit for fixing my mistake.

Your number of 44 also solves for a 20ma RMS current, except you got the count right.

One of the good things about PF is that if you do get things wrong there are people to set you on the right path,

Thanks.

You're a better man than me, NoTime.

I've been too under the weather to respond lately. I may still be too dopey.

We don't really see this exexcise as the same problem. In my view, the objective is to obtain the most luminosity with the least component count and cost, without thermally degrading the components. Thermal degregation is everything. This means keeping the peak current within bounds while delivering the highest possible average current. These things are diodes, and these sort of curcits the dynamic impedence is not a useful number given the wide swing in bias voltage. The current through one of these LEDS varies roughly as the exponent of the bias voltage, I'm sure you know. Roughly,

$$I = exp(kE)-1$$

$$P = c(E exp(kE) - 1)$$

So as the voltage increases from zero the current creeps slowly up with a ugly shape peak in as the AC peaks. The idea in any design is to keep the peak instantanious power down. But it's a little more than that. Thermal degragation is usually about wire bond integrity and more importantly mirgration of dopants at the junction. In this case you might add degregation of the fluorescent dyes. Beats me. The curves you see in the data sheet only hint at the thermal limits. You might ask yourself why these boundries in their Allowable Forward Current area plots are drawn where they are. There are six different boundries going on; I don't know where they all come from.

You know about thermal circuits right? In any case this is all for general consuption anyway. The heat driven into the die is the instantanious resistive power lost integrated over time. The heat conducted out depends on the bulk thermal resistance as the heat is conducted away via leads and plastic case. So in the simple situation of single pulse power, for instance, the instantanious temperature of the die ramps up like voltage across a capacitor in parallel with a resistor, being charged with a current source. To make a long story short, after approximating the repetative current duty cycle if you know the normalized thermal frequency response curve you obtain the heating obtained as if it were heated by a constant DC bias.

As there is no frequency response curve provided by the manufacturer, we can only guess based upon where the boundries show up in the Allowable Forward Current plots. The thermal response curves are based upon duty cycle and frequency. As Nichea Corp was not gracious enough to let us know the test frequency used in obtaining their Duty Ratio vs. Allowable Forward Current plot, we can just guess or defer to experimentation. [Edit: If it's 10KHz or better which is pretty typical for strobed character/segment multiplexing, then it's useless. If it's 1000 Hz, which should be more typical for scrolling displays that use descete diode assemblies it could be somewhat usefull.] The latter (experimentation) is what I recommend. The 44 diode count solution I had is pretty dismal: assuredly within max allowable current spec, but probably about as bright as running on 10mA DC bias.