Forward voltage for footswitch LED?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Normster, Jan 2, 2006.


  1. Normster

    Normster Member

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    I'm feeling like such an idiot. I'm ordering parts for a d-clone footswitch and don't know which LED to use. In the Mouser catalog, one of the specs is forward voltage. (2V vs 12V) The footswitch will control a 12V relay. Which one do I order?
     
  2. VintageJon

    VintageJon Member

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    Well, not knowing the design I would have to guess the 12V as it has a 12V relay...

    That being said a 2V or 12V Forward Voltage for an LED is news to me... They are all around 1.8V forward in my experience.

    Can you supply Mouser PN's? I may be able to to advise better if I can download data-sheets...

    Happy New Year and May The Tone Be With You,
    Jon
     
  3. Normster

    Normster Member

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    Thanks, Jon. I finally got it figured out with some help from Don Symes on ampage. The voltage is apparently dropped to 2V via a 1K current limiting resistor. I saw the resistor on my schematic but it didn't register that it was for the LED. :jo
     
  4. Slick51

    Slick51 Colonel Curmudgeon Silver Supporting Member

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    VintageJon/Normster

    Some of the newer ultrabright LEDs like the super bright blue have forward voltages (V sub F, or Vf) near 4.0 VDC!

    I use the Everlight Super Bright 5mm/T-1 LEDs, like the water clear lens/blue light LED on the third line here:

    http://www.everlight.com/prd_list.asp?lang=en&g=21&p=51

    or at Mouser, http://www.mouser.com/catalog/624/43.pdf . Vf = 3.8VDC.

    You have plenty of power, so you're fine. The forward current (If) is 20 milliamps (mA) for most LEDs, so we have everything needed to calculate the minimum load resistor to drop enough voltage to keep the LED from frying.

    Ohms Law, V=IR (V in volts, I in amps, R in ohms, solved for resistance; R=V/I), is all we need. The form here would be

    Minimum resistance = the difference between the supply and the LEDs required voltage (the amount we want to drop) divided by the LED current, or

    Rmin = (Vs-Vf)/If

    Use 12 volts as the supply voltage, 3 volts as the LED Vf (to make the math easier to show here), and 20mA as the forward current (but it must be in amps, not milliamps, for the units to work out, so multiply the 20mA X 1000, giving If=.02 amps (A)).

    Plugging it in and turning the crank, we get

    Rmin = (12-3)/.02 = 9/.02 = 900/2 = 450 ohms.

    Anything larger than 450 will increase the dissipation, lower the voltage, and therefore the LED brightness, but you have plenty of brightness to spare, so the 1000 ohm resistor is a great choice, and gives you a margin of safety. Also, for example, say most LEDs are around Vf = 2VDC... The resistor for that case would be 300 ohms, so the 1K load is fine there too.

    Buy several, and see what you like best. Some can be seen better on a bright stage/outside. IMO, some of the super bright ones are way too bright, and will leave you blinking after looking into it (and a nice bright blue spot on the ceiling above it!), so you have 100s of choices.

    HTH

    Slick51
     
  5. TubeAmpNut

    TubeAmpNut Member

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    You're going to want an LED with an intesity less than 200. Anything from 10 to 200 is okay. I thought it would be cool to use a purple LED once that had an Iv of 1200mcd or so and it lit my whole living rooom! It was WAAAY too bright.

    BK
     
  6. Normster

    Normster Member

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    Hmmm...dark stage, clean rhythm, hit the footswitch and viola, searing lead AND instant purple spotlight! You may be onto something. :D

    Thanks for all the suggestions. I'll order a few and see which ones I like best.
     

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