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LED amperage question


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I have a quick question and Ill delete this topic after I get an answer.  How many mA do I get out of a LED?  The same ammount as it draws?  Less than that?  Or supply minus the draw? 
The max supply I may have is 13.5V @ 1A.  I am using a LED as an indicator(with a proper resistor) of a complete circuit but will have a pushbutton to bypass the LED.  I want as little of mA as possible to result (when not bypassed).  (I am powering ni-chrome so if I get to much...BOOM if it's hooked up ;D)

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How many mA do I get out of a LED?

If you shine a very bright light or same kind of LED on an LED it will generate only a couple of uA when used as a solar cell, because its chip is so small.

I am using a LED as an indicator(with a proper resistor) of a complete circuit but will have a pushbutton to bypass the LED.
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Sorry I should have wordered the questions differently.  I was meaning when 13.5V @ 1A with the right resistor is applied to a LED what is the resulting mA comming out of the - lead?

About the pushbutton:  What I meant was a SPST NO pushbutton wired in parallel with the LED and resistor.  Would this waste? The reason for the bypass is because I need the max power for my ni-chrome but also want to be able to test the connections without lighting the ni-chrome.  The ni-chrome only needs to be red hot for about 1 second so will the waste use enough to cause a no light?

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They way the system is there are several positives to multiple nichromes and one negative and the LED and push button is on the negatove side.  Therefore, pushing the buttons (the multiple ones) does a continuity test but if you hold down the one button and press one of the others causes a fire (then its safer that way also).
Thanks for your help.  Im not giving a full descripton at this point of the project because I am working on posibly marketing this.  It's not to hard to figure out but my design is much easier to use then home-made ones and safer because of the good parts (not just a touch and fire)

I am using RF-45 cables because I can get them very cheap in the 100' length.  Otherwise the cables can be very expensive.  I have another question:  I want to make another box with a much simpler design for fireworks that can be launched at a closer range.  I want to use parallel cable because of the 25 wires and the wide (or once was) use then can be cheap.  I have a hard time finding a 25' or so parallel cable on ebay (the cheapest by far).  Al the cable are parallel to printer.  The parallel side is 25 female and the printer side is 36 (i think) male.  How does that work???  If I cut off the printer connection will there be 25 or 36 wires in there?

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  • 3 weeks later...

An LED will only draw the current it needs to light up. Usually this is something around 5-10 mA, so if connected to a power supply able to deliver 1A, it will still leave most of that 1A for powering other loads.

As for printer cables, the usual 25-pin D to 36-pin Centronics has 17 signal lines, plus ground, so the number of cores in the cable can be anything from 18 to 25, and there may or may not be a screen, depending on the quality of the particular cable under consideration. On the D25, pins 18-25 are all ground, while 1-17 are various data and control signals. The cables usually come with molded-on connectors, so if one cuts it open, it will be a matter of using a multimeter, ohm-meter, or continuity meter to figure out which pins in the D25 goes with which core.


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Not exactly.... a LED will draw whatever current you allow it to by means of the current limit resistor! As you allow more current flow, it will get brighter and as it gets brighter, the LED (diode) forward voltage increases as dictated by the diode equation. Therefore,  the "exact" amount is a little harder to calculate with 2 variables that are changing, and are non-linear!! In the "old days" most LED's needed around 20mA to be "bright". Nowadays LED's that only need 2mA to be bright can be found if you dig a little.

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