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noob question - resistance through a device + LED

Discussion in 'Electronic Basics' started by anonymous, Sep 11, 2004.

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  1. anonymous

    anonymous Guest

    OK - so now Im wondering:

    +9V ----> positive lead ---> device ----> negative lead ----> ground

    seems ok.. yes?

    what if I want to have an led light when the plug is inserted?

    +9v ----> + lead ---> device ----> - lead -----> R1 ----> led ----> ground

    but if the LED needs 2V then I would be running 11 V through my device..
    right? I can figure R1 with V= IR.. but what voltage should I put there?
    will it be 9?

    Since the 2 voltages are in series they are additive. But I don't want more
    than 9 V at my device.

    I think this 2nd diagram isn't correct.
  2. The sum of all voltage around any loop always add to zero. If you
    loop a 9 volt battery with a series combination of an LED and a
    resistor, the battery voltage must equal the sum of the LED voltage
    and the resistor voltage. So you assume that there is about 9-2 volts
    across the resistor, and calculate its value based on the current you
    want to have passing through the LED and resistor while the resistor
    drops 7 volts. For instance, if you want about 10 ma through the led
    and resistor, the value would be about 7v/.01A=700 ohms. A 680 ohm
    would be the next lower common value.
  3. anonymous

    anonymous Guest

    So indeed with my diagram #2 I would be pushing excess voltage through my
    device. I would imagine that it would be damaged by having 11 volts so thats
    not an option.

    I need 2 volts at the LED - lets say 20 mA. This gives me R1 of 2 / .02 =
    100 ohms.

    Whats a better layout for this? The end result should be 9V at the device
    and an LED that turns on when the device is plugged in.
  4. andy

    andy Guest

    no - if anything it would reduce the voltage through the device by 2V or
    more, depending on what the device is.
    In this circuit, the way I'd work it out is to say the voltage across the
    led will be around 2V for most currents, so take that off 9V to leave 7V
    for the rest of the circuit. Then find the resistance of the device, Rd
    (if it has a stable resistance at all), add it to R1, and divide 7V by
    R1+Rd. This gives the current through the whole thing. Then multiply by Rd
    again to get the voltage across the device.
    If you just want the led as an indicator, you'd be better doing it like

    +9v ----> + lead -+-> device -------+--> - lead -----> ground
    | |
    +--> led --> R1 --+

    Then you know that you have 9V across the device, and across the led and
    R1 together, so R1 should be easy to work out.
  5. anonymous

    anonymous Guest

    Ah v fine.

    So by putting the resistor + LED at the + terminal I then have 2 resistances
    in parallel? They would both see 9V and I could adjust the LED resistor to
    get the appropriate voltage?? maybe??

  6. anonymous

    anonymous Guest

    Unf I don't know the resistance across the device. Its a 9v powered
    'accessory' that is turned on/off on the unit. When on it draws 9V at
    anywhere from 20-200mA.

  7. There is no 11 volts? You have 9 volts available, and you get ot
    consume this in parts with all the components connected across that 9
    That calculation is meaningless, since an LED is not a resistance, but
    a very nonlinear (current not proportional to voltage) device. The
    manufacturer warns you that its typical voltage drop is about 2 volts
    when 20 milliamps passes through it. It is also very nearly 2 volts
    when half that current passes through it. Not a resistor.
    You layout is fine.

    Assume that if you get the current about right (near 20 ma) the LED
    will, somehow, drop about 2 volts (that is the given in the circuit,
    based on the LED datasheet). That leaves about 7 extra volts from the
    battery to be wasted in the series resistor, while it also passes the
    same 20 ma of current. Ohm's law applies to resistors (they do drop a
    voltage proportional to their current) so you use ohm's law to
    calculate what resistance will drop (waste) 7 volts when 20 ma is
    passing through. 7/.02=350 ohms. 360 and 390 are standard 5% values,
    so one of these should be easy to find and will pass almost the
    desired 20 ma when the extra 7 volts is impressed across it.
  8. You want to connect the LED+resistor _in parallel_ with your device,
    so that your device and the LED+resistor combination will each see the
    full battery voltage.

    You select the resistor in series with the LED to set the _current_
    through that branch of the circuit. I find that most LEDs are bright
    enough at 10 mA, so I would use a 680 or 750 ohm resistor.

    Peter Bennett, VE7CEI
    peterbb4 (at)
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  9. andy

    andy Guest

  10. I think the problem is that you want some kind of indicator when the
    device is active, meaning its passing some current. You don't want to
    put the LED in series; that'll make the LED brightness depend on how
    much current the device draws, and if the device wants more than 20mA,
    the LED will get fried quickly.

    If the switch is external to your device, then you can just use the
    fact that voltage is applied:

    VCC-o o---o----------,
    | |
    | V =>
    .-------. -
    | your | |
    | device| |
    | | .-.
    | | | | 620R
    '-------' | |
    | '-'
    | |

    created by Andy´s ASCII-Circuit v1.25.250804

    When the switch is closed, your thing turns on, and so does the LED.

    However, I think the real problem is that the switch is inside the box
    labeled "your device" above. If this is the case, the only thing you
    can really do is measure whether current is flowing through the
    device. To do that, you put a resistor in series with it, and measure
    the voltage across that resistor. If its non-zero, then the current
    through it is non-zero.

    Generally, sense resistors are quite small so they don't interfere
    with the circuit they are measuring (ie, 1 ohm or less). This means
    that the voltage across them is also quite small. If your device draws
    10mA, then the voltage across a 1 ohm resistor is 10mV. Thats a pretty
    small difference to sense using your typical transistor circuit, due
    to variations in transistors and resistors.

    You can use a comparator to measure differences, and also power your
    led from the output:

    | | | |
    | | | |
    .----o-----. | | V => (Your LED)
    | your | .-. | -
    | device | | |R1 | |
    | | | | | .-.
    | | '-' | | |R4
    '----o-----' | | | |
    | | | '-'
    | | |\| |
    o-------------|----|-\ |
    | | | >-------------o
    | o--o-|+/ |
    | | | |/| |
    .-. .-. | | ___ |
    | |Rs R2| | '--------|___|-----'
    | | | | | R3
    '-' '-' |
    | | |
    GND | | |

    R1 = 18k
    R2 = 10 ohms
    R3 = 100k
    R4 = 620
    Rs = 1 ohm

    comparator is an LM339 or something like it

    created by Andy´s ASCII-Circuit v1.25.250804

    This circuit measures the current across Rs, and compares it to the
    5mV reference at the V+ input. Thus, if the current through your
    device is more than 5mA, the LED will turn on. When it drops below
    5mA, the LED will turn off. R3 gives you a little bit of hysteresis,
    in case your circuit draw wants to hover around 5mA.

    R1 and R2 are chosen based on the current you want to turn the LED on
    at. Its set up for 5mA, but you can change that if you want. Using a
    22k trimmer in place of R1 might be a good substitution, because then
    you can adjust the point where the current turns on the LED.

    Bob Monsen
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