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Discussion in 'Electronic Basics' started by luc, Feb 20, 2004.

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  1. luc

    luc Guest

    I have a circuit with 3 resistors.
    R1=5500 ohms, R2= 2200 ohms, R3=?
    The only other thing that is known is the voltage over R3, which is 4.5V.
    How can one derive from this, the current running in this circuit and R3.
    What do you have to look for first and why?
     
  2. More input !!
    Are the resistors in series or parallel or other combination. Perhaps you
    should have paid more attention to your tutor with regard to ohms law etc ??
     
  3. luc

    luc Guest

    This is a simple series circuit.
     
  4. Hmmmm...A homework question...You're supposed to work it out yourself.
     
  5. luc

    luc Guest

    I'm not a student, just someone who wants to learn more about electronics.
     
  6. Greg Neill

    Greg Neill Guest

    Even more input !!
    Do you know the total voltage across the resistor string?
     
  7. First off, if this is a series circuit (as you posted below) then you
    are still missing some data. All we can do at this point is lump R1 and R2
    and use that as a single 7700 ohm resistor.
    So, we have a single known resistance (7700 ohms), a voltage drop across
    an unknown resistor (R3), and no idea what the source voltage is. From
    that, we can determine only that a) the circuit has at least 7700 ohms of
    resistance, b) the supply voltage is at least 4.5 volts, and c) we do not
    know enough to figure out anything else.
    If R3 is an open circuit, then the supply voltage would indeed be 4.5
    volts, and you could not draw more than (4.5V / 7700 ohms) or 584.4
    microamps through it. And clearly, R3 cannot be a short or the voltage drop
    across it would not be 4.5 volts.
    The best you can do is to create a graph showing the relationship
    between R3 and the supply voltage, and eliminate many values as being
    impossible at either end of the graph. Values of the supply voltage can
    only range from 4.5 to infinity at a first approximation.
    And, on the resistance axis of the plot, the values of R3 can only range
    from some non-zero value to infinity (open circuit). So you have two
    definite limits to the graph in any case. Do it on log graph paper, using
    sample value of R3 and Ohm's formula to determine the required source
    voltage in each test case. The yield will be a curve that will show you for
    each assumed value of R3 what the resulting supply voltage will be.
    Once you have that value, you can easily determine the current through
    the circuit through the formula (Vsupply / (7700+R3)). In fact, just assume
    the 7700 ohms as being in series to start with and your graph will yield
    your current for any given supply voltage and usable value of R3.

    Cheers!

    Chip Shults
     
  8. If you have a circuit made up of only 3 resistors, your meter is
    broken if you read 4.5 volts across any two nodes of the circuit. If
    you actually have 4.5 volts across R3, there has to also be a power
    source somewhere in the circuit. How the 3 resistors are connected to
    that power source affects the voltages and currents.

    For instance, if all 3 resistors are in parallel across the power
    source, then they all share the 4.5 volts across R3, and you can
    easily calculate the currents through R1 and 2, because you know their
    resistances. But you can't calculate the current through the unknown
    resistance, R3. If all resistors are in series across the power
    source, You know that all share the same current, but without knowing
    either the value of R3 or the source voltage, you can't know what that
    current is. Etc.
     
  9. Bill Bowden

    Bill Bowden Guest

    You have to look for another known value, there is
    not enough information. Or, you can just plug in something
    for R3 and work out the rest. Suppose R3 is 4.5K. That would
    make the current equal to 1 mA which would drop 5.5 volts on
    R1 and 2.2 volts on R2 so the total supply voltage would
    be 2.2 + 5.5 + 4.5 = 12.2 volts. So, you can see there are
    many answers to the problem depending on the value of R3.

    -Bill
     
  10. There are at least two unknowns in your equation, the total voltage across
    the string, and R3. Unless you can eliminate one of them somehow, you can't
    figure out what either are without another independent equation.

    Regards,
    Bob Monsen
     
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