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| spartadrive_in |
 Posted: Wed Nov 14, 2007 9:41 pm Post subject: LED Sign |
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Joined: 14 Jan 2007
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Location: San Diego, CA
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I want to make a sign out of LEDs. It will just be one message so I'm not interested in scrolling programmable signs.... at the moment 
Example:
Eat at Joe's!
Can I just wire them in series and add one resistor before power supply? Also how much power so I need? Ratio? Equation please??
thank you. |
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| Harold14370 |
| Posted: Thu Nov 15, 2007 1:18 am Post subject: |
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You can wire them all in series but remember that there is a forward bias voltage so you will need enough voltage in your power supply for the forward bias voltage multiplied by as many as you have in series.
When you buy your LEDs they will have a specification sheet with the forward voltage and current. Let's say yours are 1.7 volts and 100 milliamps. If you have 100 of these in series you will need at least a 170 volt power supply.
Suppose your power supply is 200 volts and you want to run 100 diodes in series with 1.7 volt forward voltage at 100 milliamps. The voltage drop across your resistor needs to be 200-170=30 volts. You will need a series resistor of 30 volts/100 milliamps = 300 ohms. The power consumed (neglecting losses in the power supply) will be 200 volts multiplied by 100 milliamps = 20 watts. |
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| spartadrive_in |
| Posted: Thu Nov 15, 2007 9:59 am Post subject: |
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ok soudns good. but the sign that i wanted to make ideally i want to run with a 9 voly batt. maybe even two. it's a novelty sign. for your room.
can you help? |
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| Harold14370 |
| Posted: Thu Nov 15, 2007 10:58 am Post subject: |
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| spartadrive_in wrote: |
ok soudns good. but the sign that i wanted to make ideally i want to run with a 9 voly batt. maybe even two. it's a novelty sign. for your room.
can you help? |
Sure, just put strings of 2 or 3, maybe 4, in series and then wire those strings in parallel. You might want to consider powering it with a d-c supply (like a battery charger) so you don't have to keep replacing batteries. |
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| spartadrive_in |
| Posted: Thu Nov 15, 2007 3:04 pm Post subject: |
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after looking around on the web that's exactly what i came up with. i even found a calculator that will tell me what resistor values to use.
my other concern was how long would that 9 v last? equation? |
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| Harold14370 |
| Posted: Thu Nov 15, 2007 3:43 pm Post subject: |
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| spartadrive_in wrote: |
after looking around on the web that's exactly what i came up with. i even found a calculator that will tell me what resistor values to use.
my other concern was how long would that 9 v last? equation? |
A typical 9 volt alkaline battery has about 500 milliamp-hours of capacity. So, based on how many LEDs you are using you will have to calculate the total current of the parallel strings and divide by that number. For example if your total current is 500 milliamp it will last 1 hour. |
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| spartadrive_in |
| Posted: Sat Nov 17, 2007 12:25 pm Post subject: |
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Could I use a power supply from like a printer to power all this? It already converts the AC wall power to 30V DC. (some have 9V)
Am I right when I assume a power supply is a power supply, for any application?
Thanks. |
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| Harold14370 |
| Posted: Sat Nov 17, 2007 2:52 pm Post subject: |
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I wouldn't say they are the same for every application. The first thing you will have to check is if the power supply is rated for enough current output for you application.
The other problem you might run into is power supply ripple. A real cheap rectifier might just be transformer with a diode that blocks the negative half of the a-c current. A little better would be a full wave rectifier. Better yet would be a rectifier with some kind of filter or regulator on the output.
Ripple is the a-c component of the voltage. This matters because you will have to limit the peak current in your circuit to the maximum rating of your LEDs. Then they will not be as bright if there is a lot of ripple as if they has a constant current near their rated current. You can get some idea of the power supply ripple by setting your multimeter on a-c volts and checking the power supply output. |
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| spartadrive_in |
| Posted: Sat Nov 17, 2007 3:13 pm Post subject: |
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Thanks Harold this is lots of good info for my build.
Another one:
I was thinking of putting in a pot for brightness control. Which one do I use? I tried with a guitar volume pot and i noticed it only made on/off. A linear 10k pot was better but didn't have that gradual build up.
So what value?
Thanks!!! |
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| Harold14370 |
| Posted: Sat Nov 17, 2007 5:12 pm Post subject: |
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| I would need to know your circuit design. You want the pot to add just enough resistance to shut off the LEDs, and not too much more. |
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| spartadrive_in |
| Posted: Sat Nov 17, 2007 8:06 pm Post subject: |
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Well I don't have it designed yet. Would be a LINEAR though?
Also, I was thinking of wiring each letter in series then connecting all the letters in that "word" in parallel. Then I would assume I connect all the "words" in parallel with each other to be complete. That would mean different resistor values per letter, correct?
ex. "Eat at Joes" |
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| Harold14370 |
| Posted: Sun Nov 18, 2007 10:42 am Post subject: |
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I think your dimmer will work better if all the strings are the same. I don't see any real reason to make one string per letter of the sign. You can run wires wherever you want in the back of the sign.
Here is an example of how you might size your pot. Lets say you use the 30 volt power supply, and you have 10 strings of 12 2-volt LEDs each with a rating of 20 milliamps. The total power supply current is 10*20=200 milliamps so your power supply needs to be rated for 200 ma. The voltage across each string of 12 LEDs is 24 volts. Your resistor needs to be (30-24)/.20, or 300 ohms. There are 10 of these strings in parallel giving you an effective parallel resistance of 30 ohms. So if you put a 30 ohm resistor in series with the power supply you will cut the current, and the intensity of light, in half. By the same token a 60 ohm pot will reduce the intensity to 1/3, a 90 ohm pot will reduce the intensity to 1/4, etc. |
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| spartadrive_in |
| Posted: Sun Nov 18, 2007 6:03 pm Post subject: |
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Ok sounds good. I found this calculator and everything is checking out with what your math said:
http://ledcalc.com/#calc
So would the pot need to be a linear pot?
I have this Class 2 power supply that came from a printer. It's output is 30V at 500mA. Since you said I need it to be rated at least 200mA is it ok to have more. So can I use this one that's rated at 500mA? |
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| Harold14370 |
| Posted: Sun Nov 18, 2007 10:34 pm Post subject: |
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| spartadrive_in wrote: |
Ok sounds good. I found this calculator and everything is checking out with what your math said:
http://ledcalc.com/#calc
So would the pot need to be a linear pot?
I have this Class 2 power supply that came from a printer. It's output is 30V at 500mA. Since you said I need it to be rated at least 200mA is it ok to have more. So can I use this one that's rated at 500mA? |
I think a linear pot would work ok. It's all right if your power supply has a higher rating, but what I calculated was just an example. Is that really how many LEDs you need? |
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| spartadrive_in |
| Posted: Sun Nov 18, 2007 11:28 pm Post subject: |
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| No, I don't really know. But the example is helping me with a starting point. I will reduce/add LEDs as I see fit. |
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