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]

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.

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.

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.

[Le Chuck]

When do we get to start seeing pics of what you've been working on?  Is this all the same project or what?  I'm totally curious.

[Nephasth]

@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.

You could alway make your own circuits with the SMD LEDs so you're not restricted to the groups of 3: ebay link.

[SavannahLion]

When do we get to start seeing pics of what you've been working on?  Is this all the same project or what?  I'm totally curious.

It's all the same project... er... most of it I think. I'm not exactly sure if I asked anything about any other ideas unrelated to my current project.

I'm currently working on the electricals and part of the cabinet in an effort to avoid the "it's almost done, now let's see how well it plays." In other words, the electricals and the internals of the cab are being constructed first then the shell is getting "wrapped" around it. The cabinet is going to be unplayable until it is actually completed.

That's the idea anyways, we'll see how well it works. 

[SavannahLion]

This is just a taste of what I'm working on.

I'm kind of too lazy to figure out how to convert my cell phone video to something compatible with the forum or upload it to YouTube, so you'll have to be satisfied with the stills. 

I am working on a write up, mostly while I'm at work to avoid erm... during my breaks.

Short explanation. The photos show red and blue LEDs. The red is what I tested with while trying to figure out my blues. Sharp eyes will spot that the blues are actually what's connected. Note the difference in the green LED on the red board in each photo. LED2 is acting as a light sensor.  I worked out a pretty decent auto-ranging solution (I hesitate to call it an algorithm) that seems to work most of the time. When it doesn't, it does correct itself in short order. I'm working on fine tuning the software.

The attached circuit diagram is the most basic and depends on the standardized features found with the board I'm using. It's Arduino compatible so I imagine any Arduino (though I don't use the platform) board has the same features. The components inside the dotted line are crucial to the circuit but are part of the red board itself. I just can never remember how those pins are connected. I did leave out that AREF is connected to +5v through an inductor. Another detail I always seem to forget.

For the curious, the question I asked relates specifically to LED1 though it isn't specifically shown (I still haven't selected a MOSFET to use). I was short on pins which prompted the question, but I decided to design a different circuit which netted me a 17% gain in available I/O pins. None of the information is wasted though, the new design draws heavily from this particular thread and I now have much finer control over the LEDs and have self diagnostics.