Possible to substitute a lamp for an led?

Discussion in 'The Projects Forum' started by Videodrome, Mar 19, 2010.

  1. Videodrome

    Thread Starter Member

    Nov 12, 2009
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    I am building a guitar pedal which uses an LED to indicate whether its bypassed or not. i have some really nice looking dialco 12v lamps which i would like to use but i'm not sure if they would work in place of the led. i know that replacing a lamp with an led is possible but not sure about vice versa. now on the pedal, there is 1.5k resistor attached to one of the leads of the LED and im not sure as to why. figured id mention that as well. will this work?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    The resistor is there to keep current through the LED to a predetermined level. If the source is 12 volts, then remove that resistor before you wire in the lamp. Its filament is inherently resistive, so it regulates its own current.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Ahh, be careful about replacing an LED with a lamp.

    We do not know what is supplying the current to the LED. If you attempt to replace the LED with an incandescent lamp, you may fry the switching circuit.

    Incandescent lamps have what is known as an "inrush current" which looks like a dead short when power is first applied to them. The filament rapidly heats up, and increases it's resistance while doing so. When at normal operating temperature, the current is considerably less than when the filament is cold.

    I have a few Dialco lamp holders, too - they have a dimming iris and press-to-test function that's pretty cool. However, unless you can find an LED that will fit the socket, I suggest that you stay with an LED for your pedal's safety unless you can figure out what's driving it.

    If it's a CMOS IC, you will kill it almost immediately.

    Why don't you post your circuit, and indicate where the LED is, and tell us about the lamp you're considering?
     
  4. Videodrome

    Thread Starter Member

    Nov 12, 2009
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    thanks alot fellas, that explains alot. the circuit is just a germanium fuzz face circuit, a couple germaniums, an electrolytic and several resistors and a couple ceramics. im actually copying the circuit from another pedal so i can use the copy for live use and im still tracing it out. i know its powered via 9volts and thats about it. ill try and wrap up a schem and get it up here to figure out how to get a lamp running with it, if not its no big deal, just aesthetics.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    An incandescent light bulb uses 90% of its input power making heat and only 10% making light. So its current might be much more than an LED and its current when cold might be much much more than an LED.

    High current kills electronic parts that are not made to deal with it.
     
  6. Videodrome

    Thread Starter Member

    Nov 12, 2009
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    hey again guys. i just took apart one of those dialco's and im now confused. i tested it without its housing and its emitting green. i thought lamps didn't emit any color, are these actually LED's and im being thrown off by them being labeled lamps at the store i bought them from or are they indeed lamps.

    [​IMG]
    [​IMG]
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    If it's powered by a 9v "transistor" battery, you will definitely not want to use an incandescent lamp. It will suck the battery down to nothing really quick. You'll be changing batteries every few minutes, and that is not only annoying but expensive to boot.

    If it's a red LED in the original with a 1.5k resistor, then the Vf (forward voltage) of the LED is probably around 1.7v to 2v @ 15mA. A 9v battery with some load on it might measure around 8.8v when new.

    So, (8.8v-2v)/1.5k = 6.8v/1.5k = 4.53mA current through the LED - that's pretty efficient for a "power on" light. You won't get anywhere near that low of a current with an incandescent bulb and still be able to see it.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    OK, we cross-posted.

    That is indeed an LED. It has a built-in 680 Ohm resistor. Green LEDs usually have a Vf of around 3v-3.2v @ 20mA.

    So, at 14v in (automotive use), that one might have (14v-3v)/680 = 9/680 = 16.2mA current.

    Try adding another 620 Ohms in series with it, and see how bright it is. That should get it down to around 4mA-5mA.

    [eta]
    The Dialco lights I have are practically antiques; they are around 35 years old, and are mil-spec. I didn't realize that Dialco made plastic light assemblies.
     
  9. Kamala

    New Member

    Mar 20, 2010
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    Actually, it's 560 ohms. Not that it makes a big difference.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Yeah, thanks for pointing that out. That's what happens when I post 10 minutes before I hit the sack. :rolleyes: ;)
     
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