IV Characteristics of passive components

Discussion in 'General Electronics Chat' started by Sci, Jan 21, 2005.

  1. Sci

    Thread Starter New Member

    Jan 21, 2005
    8
    0
    Anyone know where I can get graphs of IV Characteristics of resistors, capcitors, inductors, open-circuit, short-circuit, signal diode, rectifier diode?

    Problem is we are presented with a closed 'mystery box' with one of the components and we have to use equiments where we send the I V from the mystery box to the X-Y of an oscilloscope.

    I know the formula's which relate the charteristics, (like V=IR, i = C dv\dt) but don't know how the graphs for them would look.

    Also what's the diff between a signal diode and rectifier diode? All I found was a signal diode takes small currents and rectifier diodes take large currents.
     
  2. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    You just need to draw the graphs. There are only a couple of them.

    V=IR Choose one of them to be your control varible. Since V is easer to control, rearrange the formula to V/R=I. Pink some constant for R. Make it simple. R=1. So then V=I. Think about basic algebra. y=x. Its a lin, therefore all resistors have a linear response.

    Next we have the cap and inductor. I=C dv/dt, which is Y=A*dx/dt, integrate and you get (.5)Y^2=AX or, Y=sqrt(2*A*X). Sqrt function looks like a parabola that has been put on its side. So now you have another graph.

    2 basic principals of Caps and Inductors which will help is that the current through an inductor can not change instantly just like the voltage across a cap can not change instantly, this will help you determine if you have some form of parabolic/exponential response which one is it for, the Cap or Inductor.

    Now. You can save yourself TONS of time with this. Instead of trying to make the graphs yourself, put an inductor, cap, resistor, diode, etc to a voltage source and look at its response on the scope. Every type of same component will have a similar response. 1 Ohm, 100KOhm are linear. 1 nF ro 100uF cap all have charging times, etc. To tell the different between an inductor or a cap you can use a signal generator and send a oscillating signal at it. A cap will block any DC but let all AC through where as te inductor will permit all DC but begin to block AC as the frequency gets higher.

    Hope I've helped you out solving this.
     
  3. n9xv

    Senior Member

    Jan 18, 2005
    329
    1
    Thats an excellent way to test someone on their knowlage of component characteristics! Also with respect to voltage & current, capacitive current leads the voltage by 90-degrees whereras, inductive current lags the voltage by 90-degrees. With a signal generator and O'scope that would be clearly visable. Of course, voltage and current are in phase in a pure resistance. As far as a signal diode goes, it would have a better response (maintain linearity) at higher frequencies in the RF range. A "signal" diode is just a diode used to rectify smaller currents in the RF (KHz/MHz) range rather than at the 60-Hz power generating frequency.

    good luck :rolleyes:
     
  4. Sci

    Thread Starter New Member

    Jan 21, 2005
    8
    0
    I did the test. Need to write up the report for this friday. I got a ellipse with diameter across the X (Voltage) axis; a skewed ellipse (with an approx. North West or so diameter)
    How do I know which is which? im guessing of the ellipse is the capacitor and one is the inductor, and most importantly why? (and why I can't find info on IV charactersistic on fundamental devices easily anywhere?)
     
  5. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    If you got circles, you had the scope on XY mode. SHould put it in standard mode so you get a voltage in respect to time.

    You can't find them easily because they are very east to come by. V(t) for a cap = e^-(time constant*t) which is discharge or 1-e^-(time constant*t) for charging. Inductor is the exact same graph except your I(t) is now the time constant. ON a cap, current can change instantly, on an inductor voltage can change instantly.

    You can determine a component with just a DC source and a multi meter.

    Put 0.5 v on it. Measure the current. Put 2 v on it, measure the current, put 5 v on it, measure the current.

    Plot voltage vs current.
    If its a line, you have a resistor.
    If current is nearly/equal to 0 all the time you have a capacitor.
    If current starts low and begins to rise then stops, you have an inductor.
    If you have no current around 0.5 v, but get a current spike when you get higher, you have a diode.
     
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