I am planning to light up 178 LEDs (3.4V-3.8V, 25MA) using a 18V/1A Adapter (DC radio shack transformer) and I need a little help.
Power Adapter says that is 18 volt but when I test it with volt meter it shows 22V. When I connect a string of 6 LEDs (plus small resistor) with it voltage is dropping to 21.4 (and shows 9.4ma current)
I don't have problem calculating my resistors (I am using
formatting link
but it seems my 18V (or 22V) Adapter is not really what is says (it drops).
My question is what should I put in that web site as my Source Voltage? (22V? 18V? or something else).
Circa 17 May 2007 06:37:52 -0700 recorded as looks like Homer sounds like:
Oops. You do have a problem. See my other post (not crossposted) in s.e.b:
Message-ID:
First, the "wizard" neglects to take into account your power source max current in its automatic design. With 44 legs at typical 25mA, you've exceeded your 1A limit by ten percent.
To give it credit, the "wizard" did give you this information at the end (did you read it?):
# together, all resistors dissipate 3593.75 mW # together, the diodes dissipate 16910 mW # total power dissipated by the array is 20503.75 mW # the array draws current of 1125 mA from the source.
So, by a couple means of calculation, you see that your 1A source just doesn't cut the mustard.
Thanks All for your inputs. Hearing all those bad things about not- regulated power supply, I decided to use a regulated (old Laptop Adapter) this time. It's 24V/1.7A and it should be sufficient for my project. I checked it already and volt meter shows exactly 24.1V. I will put 24V into wizard and go from there.
You need a different supply, or you need to run the LEDs below 25 mA. Try an experiment: put 5 LEDs in series with a 51 ohm resistor and put that in parallel with a 30 ohm resistance made from six 5 ohm, 5 watt resistors in series. Connect that circuit to your supply, measure the voltage, and observe the brightness. That experiment will approximate
35 strings of 5 LEDs + 51 ohm resistor for each string, and an additional string of 3 LEDs and a 390 ohm series resistor.
If the LEDs light up to your satisfaction, you're home free. Wire them up in strings of 5 in series, each string getting its own 51 ohm resistor in series. The last string of 3 is wired in series with a 390 ohm resistor. The 30 ohm resistance is not used - it is replaced by the other LED strings. Each string draws about 20 mA, for a total of .72 amps, so you should be good to go with that supply.
If you _must_ have ~25 mA, you need a different supply. I'd recommend you use a cat # 15737 PS ($7.95) and 15447-CB ($1.25) from MPJA, and qty 25 plus qty 5 of part # 512-LM317LZ from Mouser.
formatting link
and
formatting link
In lots of 25 they're 20 cents each, for $5.00, and the other 5 are 23 cents each for $1.15 Then do this for each string:
You'll do 29 strings of 6 LEDs in series, with 4 LEDs left over. The 4 LED string is wired exactly the same as the 6 LED strings, except that it is 4 LEDs, not 6.
The LM317 and 51 ohm resistor regulate the current to
Ed, I am lost. Are you commenting on not-regulated idea (my initial question) or 24V/1.7A regulated adapter? If it's the second one,
formatting link
is suggesting that I have more than enough current:
The wizard says: In solution 0:
each 8.2 ohm resistor dissipates 5.125 mW * the wizard thinks 1/4W resistors are fine for your application Help * the 560 ohm resistor dissipates 350 mW * the wizard thinks 1W resistors are needed for your application Help * together, all resistors dissipate 478.125 mW * together, the diodes dissipate 15130 mW * total power dissipated by the array is 15608.125 mW * the array draws current of 650 mA from the source.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.