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large led circuit
SavannahLion:
I have 73 blue leds to be lit. I don't have the specs on hand but I believe they're 20mA at ~3v (I think it's more like 3.3 or 3.5v, I need to double check). They're all going to be on/off at the same time.
What I can't work out is,
For so many LED I can see the benefit of upping the voltage. 12v, 24v that kind of thing. The dominate wiring I have on hand is CAT5 which works well at 5v but am I correct in looking at the reference tables that 24awg isn't going to be enough to handle higher voltages in that configuration? The sheets I found specify about 3.5 amps for chassis wiring which seems like it should work but specifies 1/2 amp for "power transmission"?
The second question is should I try to hang one high power fet or transistor to control all the chains together or have a separate transistor on each chain? Except for soldering work, I can't really tell what the electrical difference between choosing 1 fet over many. The cost seems roughly the same between one very high power fet vs many smaller fets.
Howard_Casto:
Ok, so you are going to have to explain your particular application because if you need 70+ leds to be lit all at once with no individual on/off control the easiest and cheapest thing to do is buy a blue led Christmas light string. You can get those in the usb/battery powered variety as well which would cover your low voltage requirements.
MonMotha:
The voltage handling capacity of the wire is related to its insulation, not the conductor size. You can safely put 24V (or even up to 48V) on typical telephone wire. The telephone company does it all the time.
The current handling is related to how much voltage drop you can tolerate as well as how hot you're willing to let the wire get. Voltage drop is influenced by the length of the wire while heating is to a much lesser extent (the longer it is, the more power it will dissipate, but the larger the space it has to dissipate it over). As a total WAG, I would say AWG24 is probably fine to about 2A over short distances for chassis wiring type applications.
So, if you need 70 LEDs, each 3.3V @ 20mA, you might find the following configurations work well:
For a 12V supply, 3LEDs in series by 23 strings = ~10V on the LEDs (R = 2/0.02 = 100ohms). Supply needs to handle 23*20mA = 460mA. Not too bad.
For a 24V supply, 7LEDs in series by 10 strings = ~23V on the LEDs (R = 1/0.02 = 50ohms). Supply needs to handle 10*20mA = 200mA. Easy peasy.
You could go to 48V, but there's not a ton of reason to.
For these solutions, the worst current seen (that coming off the supply in total) is well within what you can run some distance over typical AWG24 telephone cable.
If you only have 5V at your disposal, you have to run them completely in parallel. That's 70*20mA = 1.4A on the supply, which is modestly hefty for a typical wall wart, but not a huge deal (a typical powered USB hub has a 5V@2A supply). Your resistors each dissipate 1.7*.02=34mW for a total of 2.38W wasted, whereas the 48V solution only wastes 200mW across all 10 resistors (20mW each).
In general, even the 5V solution is manageable. You'd probably want to either double- or triple-up the wiring from the supply to the LED group or run each LED's wiring straight back to the supply.
EDIT: Erm, I did the math up there for 70 LEDs, not 73. Same idea, though. If you have an "odd string out", you just re-size the resistor on it. If it's substantially off, like 3 left vs. 7 on a "typical" string, you can run a few strings of 6 to balance the power back out so you don't end up with one big, hot resistor on the short string.
Nephasth:
--- Quote from: Howard_Casto on June 08, 2013, 12:31:20 am ---if you need 70+ leds to be lit all at once with no individual on/off control the easiest and cheapest thing to do is buy a blue led Christmas light string.
--- End quote ---
Or those 12V LED strips that are cuttable every 3 LEDs so you can arrange them however you want. Here's a pic of 5 meters (300 LEDs total) of RGB LEDs lit up only in blue with just a spare PC power supply I had laying around:
Will you be controlling them with software? Or will they be switched only?
SavannahLion:
@Howard_Castro
I really can't. Not for this one. Sorry. :dunno
The Christmas lights idea is perfectly valid and one I looked at but it proved too impractical and cumbersome in this particular case.
@Nephasth
I'm looking long and hard at those strips. I like them but the "groups of three" actually works against me.
They'll be controlled via a microcontroller. I was working it out last night and decided to split them into fourth groups. It changes the make up a little bit requires an additional circuit but I get some gains out of it.
@MonMotha
Ah... that clarifies a lot of stuff. A ton actually.
Thanks for explaining what those ratings really mean.
I was calculating my mA all wrong and was coming out with some huge numbers that didn't really make a whole lot of sense even while all my other numbers seemed to fit. Thanks a bunch.
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