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Best Wiring Method?

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a b B Homebuilt system
April 11, 2011 10:31:09 PM

As part of a custom build I need to wire up 5 Lamptron LED buttons like these. Each momentary button requires 12v to be lit fully.

I figured I could grab a single molex cable to provide the 12V.

1) I'll be able to power all 5 through a single molex cable right? Forget about connectors, I'm talking about the power draw.

2) Can anyone suggest a clever way to connect this up? I have this cable that connects the various terminals. The non terminal ends are pin headers.

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April 11, 2011 10:42:14 PM

LEDs only draw a few watts. A molex connection will supply plenty of power....


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April 11, 2011 10:53:11 PM

LEDs draw typically 30-100 milliamps - so ~0.3 to ~1 watts at 12V

You should be able to power a few dozen LEDs off a single molex before the power supply even begins to feel a difference.

Make sure your LEDs have resistors connected up or they'll break in seconds (and damage the PSU)! LEDs within toggle switches often have resistors built in, I'm not sure about PC case mod kits.

You can work out the resistor values via Ohm's law if needed:

Vpower = Vled + Vresistor

.'. Vresistor = Vpower - Vled

.'. Rresistor = (Vpower - Vled) / (Iled)

Vpower is the supply voltage (12V you've said)
Vled is the LED's voltage drop (1-2V usually, 1.8 is a reasonably safe guess if you don't know)
Iled is the LED's ideal current (50 mA [0.05 A] is usually a safe guess if you don't know)
Rresistor is the approximate resistance you'll need (in serial) with the LED. If you get say 3kOhm, its usually safe to take a 2.8k or 3.3k if you can't find a 3k

So if you're supplying 12V, the LED has a 1.6V drop (red LED typical value) and wants a 25mA current - Rresistor = (12 - 1.6)/0.025 = 416, so a 400 or a 470 resistor would generally be Ok.

-Mark

Note: The above is just a guideline, if you don't have exact values then the chance of the LED going pop does increase a bit!
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April 11, 2011 10:54:28 PM

Oh also, remember to connect the LEDs in parallel!
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April 12, 2011 3:01:29 AM

Luxeon and Cree LED's used in some of the best flashlights can handle voltages in the 6V ballpark, some maybe up to 9V. I haven't kept up with the latest products recently but I don't know if any can go as high as 12V. The more common LED's used in christmas lights, fan lighting, etc. will use less voltage, maybe on the order of 1V each. Typically LED's do have resistors wired in parallel with them, or enough LED's wired in parallel to limite the voltage that each one sees. If you have 5 LED's then you can probably wire them in parallel, but check on the specs for the LED's and you might also need to supply some resistors.
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April 12, 2011 12:59:47 PM

cadder said:
Luxeon and Cree LED's used in some of the best flashlights can handle voltages in the 6V ballpark, some maybe up to 9V. I haven't kept up with the latest products recently but I don't know if any can go as high as 12V. The more common LED's used in christmas lights, fan lighting, etc. will use less voltage, maybe on the order of 1V each. Typically LED's do have resistors wired in parallel with them, or enough LED's wired in parallel to limite the voltage that each one sees. If you have 5 LED's then you can probably wire them in parallel, but check on the specs for the LED's and you might also need to supply some resistors.


You'd want the resistor wired in series :)  Also, you'd want the LEDs wired in series if you're not using a resistor - parallel would increase the current draw. You want the voltage across each LED to be just sufficient to get the ideal forward current flowing through it - using Ohm's law and Kirchkoff's laws.

For a 6V LED, you could use the +5V and +12V rails to supply the LED - giving a 7V potential across it. If you do this, it is really important to use resistors if the LED doesn't have them built in, otherwise you can jeb up the power supply and any 5V components in the PC.

If the LED has a resistor built in, it will only work within a narrow range of voltages. If it doesn't have a resistor built in, then you could run it off almost any voltage provided you used the correct series resistor.

Red LEDs typically have a 1.6V drop across them, so 7 of them in series (7*1.6V=11.2V) would run nicely off a 12V supply, or three in series across a 5V supply (3*1.6V=4.8V).
You can find the voltage drops for other colours via a quick google search.


You could run a small LED off a mobile phone battery, or off rectified mains power depending on the resistor used - but for mains, this would waste quite a lot of energy through Ohmic heat in the resistor.



-Mark
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April 12, 2011 7:43:37 PM

YES, in SERIES. (Sorry if I get the words mixed up.) The idea is to limit the voltage to each LED, and to do that you have to wire something in SERIES with something else.
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Best solution

April 12, 2011 10:44:01 PM

cadder said:
YES, in SERIES. (Sorry if I get the words mixed up.) The idea is to limit the voltage to each LED, and to do that you have to wire something in SERIES with something else.


Series, parallel - easy to mix up in a quick forum post :) 

To the OP, this is a setup like:

12V----Resistor----LED1---[LED2---LED3...LEDn]---0V






If you want lots of LEDs (of the same type) to run off one power supply, you could put them in parallel, then put that parallel group in series with one resistor, rather than having a separate resistor for each LED - again, use Ohm's law for this (if each LED wants 50mA, then you'll want the current through the resistor to be 50mA * number of LEDs)

For this arrangement, I'm talking about a setup like this:

12V-----Resistor------(Parallel array of LEDs)-----0V
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a b B Homebuilt system
April 18, 2011 10:06:47 AM

Humm this seems a bit of a pain in the arse just to make some lights appear on some buttons. I'm seriously just considering regular buttons!

Does some clever person know of a prebuilt resistor "board" that will achieve the setup that Battlesnake69 has described i.e:

12V-----Resistor------(Parallel array of LEDs)-----0V
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a b B Homebuilt system
June 7, 2011 3:48:40 PM

I ended up stealing the multi fan resistor board off my Lian Li case to power the LED buttons.

The buttons I picked up from Scan already have resistors built in so I had no need to fit any.

Thanks for the help everyone.
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a b B Homebuilt system
June 7, 2011 3:48:53 PM

Best answer selected by Rusting In Peace.
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