# Thread: run multiple LEDs off one resistor?

1. ## run multiple LEDs off one resistor?

I found this article on the web and wanted to make sure I was reading it right, I will be buying 225 LED's that run 2.3v@20mA
I want to connect as many as I can together and use one resistor per grouping. I will be using a 12volt switchable wall transformer rated at 500mA.
From my calculations I can connect 5 LEDs with one 100ohm resistor? But it seems the power supply should be able to handle a lot more than just 5 leds, what am I missing? Heres the example I found. I will need to connect 200 LED's and try to run them off as few transformers as possible, is there something else i could use to get my current up without having to buy a 4amp (200 x 20mA) transformer?

A red, a yellow and a green LED in series need a supply voltage of at least 3 × 2V + 2V = 8V, so a 9V battery would be ideal.
VL = 2V + 2V + 2V = 6V (the three LED voltages added up).
If the supply voltage VS is 9V and the current I must be 15mA = 0.015A,
Resistor R = (VS - VL) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200ohm,
so choose R = 220ohm (the nearest standard value which is greater).

2. The same current flows thru all devices in series.· Each LED is rated at 20 mA. 20 mA across a 200 ohm resister is 4 V.· If your power supply is 12 V, this leaves you 8 V for a stack of LEDs. Each LED is rated at 2.3 V, so you can run 3 or 4 LEDs in series with your 200 ohm resistor.

If you decide to run 5 LEDs in series, you'll need 11.5 V across the stack of LEDs. That leaves 0.5 V across the resistor.·0.5 V and 20 mA gives 25 ohms for your resistor.

I am surprised that four 12 V 500 mA switchable wall transformers cost less than a 4 A transformer & a few components, unless you already own the transformers.

3. i am buying them for another project and thought they would be good to have for all the little projects I am doing anyways. I found them for \$3.49 a piece, no minimum, and only \$5 shipping, but the company is not very reputable and I am not learning that even after a week they have not shipped....hmmm www.x10.com

4. I hope you used a throw away E-Mail address when you ordered from X10. If not, be prepared for tons of Spam!

X10 is generally pretty good with shipment. I always receive my orders within 2 or 3 days.

X10 AC Adapters are not regulated and at \$3.49 it is guaranteed that they are unregulated. I just tested a couple of my X10 12V adapters and they put out 16.5 volts, so keep that in mind when using them with your LED's.

I recently purchased 20 12V AC adapters from All Electronics. These are rated at 1 Amp and are regulated. I use them with my LED light fixtures and they work great.

http://www.allelectronics.com/cgi-bi...213&type=store

They also have some 500 ma adapters but they are the same price so it would be best to go with the 1 amp adapters.

My Light Fixtures use a total of 132 LED's and the 1 amp adapter powers·them just fine with room to spare.

Kenny

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Kenny Gardner
GAP Development Company
http://www.gapdev.com/
·

5. X10 does send a lot of spam. When I ordered from them last year they automatically signed me up for daily email specials - though you can adjust this to weekly or monthly thru their web site. However since you've placed an order from them their is no way to NOT receive email specials for at least a few months.

6. Kenny,
how do you run 132 leds off a 1amp power supply? if the leds are 20mA, which from my browsing most are, it would take a 3amp power supply to run them. (132 x .02A=2.64A) can anyone give me a brief outline of voltage and current trends in a series and parallel circuit? What would be the easiest, less involved, method of wiring 200 leds and to run them off one power supply? Can someone supply me with a parts list and possibly the formulas to justify the specific parts (resistors, power supply...) This would really be helpful, i dont want someone to do all the work for me, so thats why I am asking for formulas so I can learn this stuff. Thanks again, this is a great forum. Mark

7. Hi Mark,

See if this helps.· It's pretty simple.

http://www.plasma-ireland.com/lp/applications.html

Chris Isaacson

8. Hi Mark,
how do you run 132 leds off a 1amp power supply?
if the leds are 20mA, which from my browsing most are,
it would take a 3amp power supply to run them. (132 x .02A=2.64A)

It would take 3 amps if you ran them all individually (with their own resistors). If you ran them all in series, it would only take 20 mA, but would require a 462+ volt power supply! (3.5v * 132).

My Light Fixtures are made up of groups of 3 LED's. Each group has its own current limiting resistor to limit the current to 20mA per group. There are 44 groups of 3 LED's for a total of 880mA. There are actually 4 strings of 33 LED's, or 11 groups of 3 LED's. Each string uses 220mA. A "string" is a 1/2inch wide strip of balsa wood with the LED's glued and wired to it.

At 12 Volts, the best you can do with White or Blue LED's are groups of 3. With Red LED's, you can wire 5 of them in series. In series, their voltages add, so at 3.5volts the most you can hook in series is 3.
Can someone supply me with a parts list and possibly
the formulas to justify the specific parts (resistors, power supply...)

For 200 LED's, you will need a 1.5A or 2A power supply.

1.5A Wall Wart
3A Table Top
For the LED's, the least expensive place to buy them is eBay.
The guy from Hong Kong is the brightest and the cheapest. But he rips you on the shipping and it takes 2 weeks. You also may get screwed. I placed 3 orders and on one of them I ordered 300 LED's but only received 100. The other 2 orders (500 LED's) were fine. But, if you need a lot and you want them cheap, he's the only game in town.

mckenkenken is a bit more expensive but he's reliable and ships fast. If you only need 200 LED's, then you might want to go with mckenkenken.

To calculate reistor values, see here:
and here:
I got my Light Fixture idea from Project X's Fish Tank Light Bars. I expanded upon John's Light Bar concept to go from 9 LED's to 132 LED's! Instead of using PCB's, I just glued and hardwired the LED's and resistors to strips of Balsa Wood.

Attached is a schematic and a couple of pictures. I just quickly took the pictures so they are a bit blurry. I'll be taking better pictures for posting on my Web Site in a couple of days.

Kenny

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--

Kenny Gardner
GAP Development Company
http://www.gapdev.com/