Signal injection with a LAMP or LED

Discussion in 'General Electronics Chat' started by relicmarks, Oct 31, 2008.

  1. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Signal injection:

    To test IC op amps or preamp tubes stages with a sinewave generator and a LAMP or LED not a oscilloscope

    I place the sinewave generator on the input of a IC op amp , than i place a LED from ground to the output of the IC op amp, it burnt out my LED why?
    what can i do to make this work? my LED or LAMP can't take the voltage GAIN of the output voltage of the IC op amps, what can i do to protect the LAMP or LED so i can probe any IC op amps output with alot of current or voltage gain?

    I place a sinewave generator on the input of a tube preamp , than i place a LAMP from ground to the output of the preamp tube, it burnt out my LAMP why? the glass of the LAMP bursts why? what can i do from making the LAMP's glass bursting from the high voltage?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    LED's require a certain voltage to bias them into conduction. After that, some external device like a resistor has to be present to act as a current limiter.

    Your lamp might have been rated for 8 volts at 20 ma. You were feeding it with much more voltage, so the current went up and created enough heat to melt the envelope.
     
  3. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    What can i do to protect them the LED and LAMP so i can really high voltage or current to them ?

    If i use a 400volt or 200volt lamp on the output of a preamp tube stage or a IC chip op-amp , having only a 5 volt input or 12 volt input signal wouldn't the lamp be very dim? because the input signal is very low? and the gain is 1 or 2 not much gain

    My goal is , im trying to use a LAMP and LED to do signal tracing so i don't need a oscilloscope. But how can i do this please?

    Signal tracing "solid state amplifiers" with a LAMP, what voltage rating would i need for the LAMP,wattage and current of the LAMP so i can signal trace a solid state preamp section of 5 stages of gain and the solid state power amp section with power transistors?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    All you get is an indication of voltage being present. Why not use an autoranging DVM? It will at least present a high impedance and not load the circuit. Lamps/LED's are only good as indicators of known voltages.
     
  5. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    1.) Why not use an autoranging DVM? It will at least present a high impedance and not load the circuit.

    Because i'm working on analog mixing consoles and i can't fit a oscilloscope or DVM meter inbetween the channels, plus if i have a sinewave generator on the input the DVM meter jumps the voltages up and down when measuring AC voltages, a DVM meter is good for measure DC voltages but not for AC voltages cause it moves up an down

    2.) Lamps/LED's are only good as indicators of known voltages.

    Yes if i see the LAMP changing its brightness on the output of each stage on a solid state circuit than i know the AC voltages are swinging cycles from looking at the LAMP light throbbing rate


    Plus i can fit a LAMP cause its very small inbetween mixing console channel strips with the boards still in and powered on
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    'Assuming the voltages you are monitoring are mixed AC & DC , you need to measure maximum voltage encountered and calculate a resistor value to give about 20 mA. Also use two LED's ,anode to cathode in parallel to handle AC sig. as LED's are sensitive to reverse voltage.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Most ordinary LEDs have a max allowed current of 30mA.
    Most ordinary opamps have a max output current of 40mA so might blow up an LED if it doesn't have a current-limiting resistor and the output level of the opamp is high.

    An opamp with a positive and negative supply might have an output voltage that is much higher than the max allowed reverse voltage of an LED which is 5V.

    An LED needs 1.8V for a red one to 3.5V for a blue or white one. An audio mixer circuit might not have levels that high.
     
  8. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    So a LAMP bulb would be better than a LED for testing outputs

    What LAMP bulb rating should i get for IC op-amps?


    If i get a 30volt at 1 amp Lamp bulb it should be safe?

    If VCC and VDD is at 28 volts and the IC op-amps is output is 800mA
    it will light/glow the LAMP bulb

    If VCC and VDD is at 6 volts and the IC op-amp is output is 40mA than it will light/glow the LAMP bulb

    IF VCC and VDD is at 12 volts and the IC op-amp is output is 100mA than it will light/glow the LAMP bulb

    If VCC and VDD is at 3 volts and the IC op-amp is output is 800uA than it will light/glow the LAMP bulb
     
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