Wiring up LED's? Amateur Question 2

[plangetee]

I have a question with regards to LED's. I have a power supply that supplies 3, 41/2, 6, 71/2, 9 and 12VDC (nominal). Materials I'm using:
Aforementioned power supply
Circuit board
~80 Red LED's, "high" intensity (

http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_333526_-1

)
Connecting wires, soldering gun, etc etc

Assuming a voltage drop of 1.9V across each diode
How would I light 80 LED's if the combined voltage drop in SERIES is 80*1.9V?
All LED's will always be on when the power source is on. (The LED's will not be on-off intermittently or controlled by an IC)
Advice?
Thanks

http://politicalplangetis.blogspot.com/

 

[plangetee]

Some additional background information:
Most LEDs have their characteristics specified at a current of 20 mA. If you want really good reliability and you are not certain you don't have worse-than-average heat conductivity in your mounting, heat buildup in wherever you mount them, voltage/current variations, etc. then design for 15 milliamps. I am designing for 15 mA.
The 1.9V drop is safe to assume for high-brightness, high-efficiency and low-current red. This is what I am using.

http://politicalplangetis.blogspot.com/

 

[IRstuff]

You are supposedly a EE, so why wouldn't you make use of series-parallel configurations to get what you want?

TTFN
faq731-376

 

[plangetee]

Thanks for the snarky comment, lol. I thought more about it and I think I have an answer, I'll throw all 80 LED's in parallel, and add an appropriate sized resistor in series with this parallel string to eat the additional voltage, and I should be good to go.

http://politicalplangetis.blogspot.com/

 

[MikeHalloran]

Well, that's one way to go.

Be sure to size the resistor for power dissipation in addition to impedance.


Mike Halloran
Pembroke Pines, FL, USA

 

[plangetee]

Mike,
What truth do you see to this statement?
"Do not put LEDs in parallel with each other. Although this usually works, this is often not reliable. LEDs become more conductive as they warm up, which may lead to unstable current distribution through paralleled LEDs. LEDs in parallel need their own individual dropping resistors. Series strings can be paralleled if each string has its own dropping resistor."

This makes me wonder, should I have a resistor in series with EACH of the LED's in my parallel chain? Or is this statement not true.

http://politicalplangetis.blogspot.com/

 

[djs]

I would suggest wiring up the LED's is strings of five or six in series, each string with it's only dropping resistor.

 

[IRstuff]

You should have one resistor for every parallel leg. LEDs are not unique in their dislike of being directly paralleled.

TTFN
faq731-376

 

[plangetee]

Guys, thanks for your feedback
I also found a great little tool recommended to me by the store I'm purchasing my LED's from (Jameco). VERY useful for my application.

http://led.linear1.org/led.wiz

http://politicalplangetis.blogspot.com/

 

[itsmoked]

What are you trying to grow? If you're going to use a truck load of those gum drops just go buy a grow light on ePay. That's more cost effective and you avoid all the packaging headaches.

If you really want to build your own then use HBLEDs not those gumdrops. Go look at LEDGroupBuy or some other aquarist DIY lighting site and see how to do it the 'modern way'. That way you use 6, 12, or 24 LEDs to get you 2, 4, or 8 times more light.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mikekilroy]

did someone remove my comment/idea to just stick them all in series with a small dropping resistor and place them across the 120vac line?

 

[plangetee]

Mike - I didn't see that comment?
itsmoked - I'm not using the LED's to grow anything.

http://politicalplangetis.blogspot.com/

 

[mikekilroy]

hmmmm, well I wrote a bit on it, I will again

why not stick them in series and put across the line? If I was doing it, I would start with a 10k resistor in series with them to limit current, lowering it in steps until the series ammeter showed 15ma or the brightness you want.

 

[VEBill]

Perhaps there was a concern with the OP (considering the obvious lack of experience) being advised to start playing with direct connectio to the AC Line.

 

[mikekilroy]

could be. If one buys a $ 15 equiv 30-100 hi power LED stick lite from ebay for use on 120v wall power it is the same way.

 

[IRstuff]

Being that it's past Labor Day, Costco already has LED Christmas lights on sale, so one could replace the lights with different ones...

TTFN
faq731-376

 

[itsmoked]

mikekilroy; That would toast them on the negative cycle of the powerline. You need to shunt them with reverse diodes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mikekilroy]

I've not toasted any yet and have done this quite a bit. I often use a single LED in series with dropping resistor off the line to revive old 6/12/24v gel cells since the 60 hz hits seems about the best way to knock crap loose in the batteries and revive them in a week or so (assuming not dried out).

 

[VEBill]

I don't believe that it's wise to recommend to a rank amateur that he should change his original approach of employing a low voltage DC power supply, to an approach of directly playing with AC line voltages.

Of course the approach can be made to work (á la Xmas tree LED strings), that's not the point. The issue is directing an obvious neophyte to play with AC line voltages, with not a word of caution. It seems arguably a bit careless.


We cannot address the question of the optimum series-parallel arrangement for the LEDs without knowing more details of this available power supply.

A power supply that provides "3, 41/2, 6, 71/2, 9 and 12VDC" sounds suspiciously cheap and perhaps of limited capabilities. We need to know the maximum current output at each voltage setting. It's likely that the optimum voltage setting is the maximum 12v, but this assumption should be confirmed.

 

[OperaHouse]

I've never seen a LED that can't take 200V reverse voltage regardless of the fact that the printing presses are stuck on 5V.