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Very basic question about placing an LED in a circuit

Discussion in 'Electronic Basics' started by Thot, Apr 29, 2005.

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  1. Thot

    Thot Guest

    Hello all.
    I have been loorking at this group for a while.

    I am an absolute beginner just now experimenting with hobby electronics on
    my own.

    I am building a simple ADC from a project that I found on a magazine
    (Popular Electronics).

    My question is:
    I would like to improve it by adding an LED so that I know when the ADC is
    powered.
    The circuit is powered at 5V (9V battery through a 5V stabilizer IC).
    Do I just connect the leads of the LED to any + and ground points, in series
    with a proper resistor?
    Or will this affect the behaviour of the rest of the circuit?
    If so, is there any better strategic point to connect the LED?
    Perhaps to the 9V source, before the 5V IC?

    Sorry if the question appears trivial.

    Any hint will be appreciated


    Luca
     
  2. You have several choices. LEDs run on current (their light output is
    roughly proportional to their current) and drop a certain voltage
    while this current passed through them. For instance, red LEDs
    typically drop about 2 volts when they are operating somewhere near
    their rated current. Yellow and green LEDs drop 2.5 to 3 volts and
    blue ones may need 3.5 (shorter wavelengths need more voltage across
    the junction to produce the higher energy photons).

    You could operate the LED from the regulated 5 volt output. For
    instance, a red LED would need a resistor in series to control its
    current that wastes the about 3 volts extra. If you decide that 10
    milliamps provides the needed light, that resistor would be about
    3V/0.01A=300 ohms.
    But this additional load will make the regulator hotter and it will
    drop out of regulating well with a bit higher battery voltage. But
    the LED will shine steadily till the regulator sags.

    If you run the LED directly from the 9 volt battery, you will need a
    higher value of resistance to limit the current to the same peak
    value. In this case, the 9 volt battery has 7 volts more than the LED
    needs, so you will need a 7V/.01A=700 ohm resistor. I would probably
    pick 680 ohms because it is the nearest 5% standard value. Now, the
    LED will dim smoothly as the battery sags, but the regulator will
    operate to a slightly lower battery voltage because it doesn't have to
    pass the LED current.

    A stranger possibility would be to operate the LED in series with the
    regulator input. This would use no extra power from the battery, but
    would transfer power that would have been burnt up in the regulator to
    the LED. This case is practical only if the 5 volt load current
    happened to be about right for the LED to operate upon. It also
    raises by a couple volts, the battery voltage that allows the
    regulator to begin to sag.

    I recently built a fourth version based on the LP2950CZ5.0 low drop
    out regulator. Since this regulator works with an input voltage below
    5.5 volts, I connected a PNP transistor, emitter to the battery,
    resistor from base to the regulated output (keeping the value high
    enough that it didn't pull the regulated voltage above 5 volts) and
    put an LED and current limiting resistor between the collector and
    ground. As long as the regulator has more than .6 volts across it,
    the transistor is on and the LED lit. This makes the LED go out just
    just before the sagging battery drops low enough to sag the regulator
    output. A lit LED indicates not only power on, but regulator
    regulating. It draws the same power from the battery that connecting
    the LED directly across the battery does. But I was able to achieve
    acceptable brightness with only a milliamp or 2.
     
  3. John Smith

    John Smith Guest

    Put the LED (with a suitable resistor in series) across the +5V and GND.
    That way you know that the ADC has power applied to it.

    Putting it (the LED and resistor) across the 9V only tells you that you have
    9V: it doesn't tell you if the 5V regulator is operational.
     
  4. Andrew Holme

    Andrew Holme Guest

    Yes, you may connect them between any convenient points.
    No, it won't if you choose points with a low resistance path to +5V and GND.
    The worst you can do is cause a voltage drop by drawing the LED current
    through the distributed resistance of your power supply wiring. This will
    be a VERY small and constant drop so it can't do much harm.
     
  5. Thot

    Thot Guest

    Thank you for the replies, they were very helpful!

    Luca
     
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