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AC Circuits

  • K7-14: RC CIRCUIT - 100 MICROSECOND TIME CONSTANT

    K7-14
    Demonstrate a reasonably fast RC circuit.
    Using a decade resistance box and a decade capacitor box a series RC circuit is produced. (1) It can be driven by a slow square wave (approximately 500Hz) and the voltage across the capacitor as the capacitor charges can be observed using the dual trace scope. (2) It can be driven by a sine wave (approximately 5kHz) and the phase shift between the voltage across the capacitor and the sine wave from the oscillator can be seen on the dual trace scope. See circuit above.

  • K7-21: RLC CIRCUIT - 10 KHZ - RESONANCE

    K7-21
    Demonstrate resonance in an RLC circuit.
    Using the circuit above, the frequency of the oscillator is swept to find the resonance. Both the signal from the oscillator and the signal across the resistor are displayed on the dual trace scope. The capacitor (0-300 picofarads) and the resistor (0-100 kilohms) in the circuit box are variable. The increase in amplitude of the signal across the resistor and the phase shift at resonance are both easily seen.
    K7, ME2, ME3

  • K7-22: RLC CIRCUIT - 10 KHZ - DAMPED OSCILLATIONS

    K7-22
    Demonstrate damped oscillations in an RLC circuit.
    Using the circuit above with a 500-Hertz square wave, damped oscillations are shown on the dual trace scope. The upper trace is the applied square wave and the lower trace shows the damped oscillations produced each time the square wave changes. The circuit can be adjusted to obtain either underdamped, overdamped, or critically damped oscillations by changing the capacitance or the resistance. In the photograph above the horizontal scale is 250 microseconds per centimeter, the frequency of the square wave is 500 Hertz (period of 2 milliseconds), and the frequency of the damped oscillations is about 8000 Hertz (period of 125 microseconds). The capacitor is set to its maximum value and the resistor is set to about 20% of its maximum (20 kilohms).
    K7, ME2, ME3

  • K7-23: RLC CIRCUIT - 60 HZ

    K7-23
    Show phase relationship between components of a 60 Hertz RLC circuit.
    This setup allows you to see the phase relationship between various components of a series RLC circuit operating at 60 Hz. An isolation transformer allows grounding of the series RLC circuit at a point which can be chosen by the demonstrator. The circuit above is for comparison of the current between the capacitor and the resistor in the RLC circuit.

    Please be aware that this is plugged into a wall socket! Other RLC circuit demonstrations may be safer and easier to use in many circumstances.

                 

  • K7-24: RLC CIRCUIT - 60 HZ WITH LIGHT BULB LOAD

    K7-24
    Demonstrate a series RLC circuit in a graphic way.
    A series RLC circuit, shown in the diagram above, consists of a variable inductor, a fixed capacitor, and a light bulb serving as the resistor. The capacitor and the inductor can be removed from the circuit using parallel switches. When the capacitor is in the circuit, the inductance can be adjusted so that the bulb is brighter, dimmer, or the same intensity as when the capacitor is out of the circuit. The photograph above shows the inductance in its resonance position. The photographs above show what happens when the inductor is shorted out (left), the capacitor is shorted out (center), and both are shorted out (right), so that the bulb is simply wired across the 110VAC power source.

  • K7-26: RLC CIRCUIT - 0.3 HZ RESONANCE

    K7-26
    Plot a graph of resonance behavior in a very low frequency resonant circuit.
    A series RLC circuit, containing a very large inductor, a 0-50 kilohm resistor, and a 100 microfarad capacitor is driven by a sine-wave oscillator as shown in the circuit above. As the frequency of the oscillator is varied between about 0.1 Hz and 0.5 Hz, the resonance in the system is observed to be about 0.3 Hz. The voltage across the capacitor is shown at various frequencies on an oscilloscope. The scope shows both the signal from the oscillator and the signal across the capacitor.

  • K7-27: RLC CIRCUIT - COMPLETE

    K7-27
    Show the phase shift between components in a series RLC circuit.
    An RLC circuit has been constructed with linear isolation transformers to eliminate grounding when the circuit is attached to a four-trace oscilloscope. Using this device, the signals across the input, R, L, and C can be viewed simultaneously as the oscillator frequency is swept through resonance. The complete circuit diagram is shown above.
    K7, ME2, ME3

  • K7-29: RLC CIRCUIT - 0.6 HZ TRANSIENTS

    K7-29
    Demonstrate transients using a circuit with a very long time constant.
    The circuit above is used to show transients of long duration. Closing the switch charges the capacitor, and opening the switch discharges it. The voltage across the capacitor is viewed using a storage scope. (Note: This demonstration is quite old and not always reliable.)

  • K7-41: RC CIRCUIT - DIFFERENTIATION AND INTEGRATION

    K7-41
    Demonstrate differentiation and integration using RC circuits.
    A series RC circuit is used to obtain the derivative or the integral of a periodic electronic signal. For differentiation the time constant of the series RC circuit must be very small compared to the period of the wave. The derivative is sensed as the voltage across the resistor (current in the circuit). For integration the time constant of the series RC circuit must be very large compared to the period of the wave. The integral is sensed as the voltage across the capacitor. Waves from a signal generator are input into the circuit, including sine wave, triangular wave, sawtooth, and square wave. The appropriate circuits are shown above.

    Note that these circuit elements are very small, and hard to see in a classroom. A camera may be requested to display them on screen in the large lecture halls.
    K7, ME2, ME3

  • K7-44: RLC CIRCUIT - 10KHZ - DIFFERENTIATION AND INTEGRATION

    K7-44
    Demonstrate differentiation and integration using an RC circuit.
    The resistance and capacitance from the packaged RLC resonant circuit experiment are used to demonstrate differentiation and integration by an RC circuit. Shown above are the circuits for differentiation (at left) and integration (at right). The photograph above shows a triangular wave (top trace) and its derivative, a square wave (bottom trace). Setting the capacitance at full scale (about 300 picofarads) and the resistance at half scale (about 50 kilohms), frequencies of 500 Hz for differentiation and 100 kHz for integration can be conveniently used.

  • K7-45: LOW AND HIGH PASS FILTERS

    K7-45
    Demonstrate use of a series RL circuit as a low-pass filter and a series RC circuit as a high-pass filter.
    The circuits above have been mounted on a plastic base for use with an overhead projector. Values of circuit components are shown on the projectual. In each case the input signal is input into the upper trace of the scope and the filtered signal into the lower trace. Complex waves can also be input to see how removal of some harmonics affects the wave shape.

    I: Low-pass filter with cutoff frequency f=R/L: The signal is input to the series RL circuit and the output is taken across the resistor.

    II: High-pass filter with cutoff frequency f=1/RC: The signal is input into the series RC circuit and the output is taken across the resistor.

    In this demonstration the experimental crossover frequency is about 5 kHz, so the biggest change in the amplitude of the signal is between 1 kHz and 10 kHz. In the sequences of photographs above the effect of the low pass filter (first sequence) and the high pass filter (second sequence) are shown at 1 kHz and 10 kHz respectively.
    K7, ME2, ME3

  • K7-61: TESLA COIL

    K7-61
    Demonstrate a tesla coil, including how magnetic induction and a resonant RLC circuit is used in the production of high-voltage high-frequency sparks.
    Our Tesla coil, circuit above, uses a 5000 volt transformer to charge a large oil capacitor. When the potential across the capacitor reaches the breakdown potential of the spark gap, breakdown across the gap occurs. The spark gap then becomes a conducting part of the RLC circuit, which resonates at a frequency of about 200 kilohertz. The large coil in the resonant circuit is the primary of the final transformer and the very fine coil is the secondary, producing about 200,000 volts at 200 kilohertz.
    Identify the components of the coil from the close-up of the figure at the right above.
    In the photograph the wires known as JACOB'S LADDER have been attached to the output terminals of the Tesla coil. A fluorescent light held by one end with the other end near the secondary coil will light by induction.
    DANGER: THE LOW VOLTAGE SECTIONS OF THIS DEVICE HAVE LETHAL CURRENTS.

  • L1-05: PERSISTENCE OF A FILAMENT

    L1-05
    Demonstrate that high-frequency AC looks like DC.
    A sealed beam car headlamp is run by an oscillator and audio amplifier. Below about 20 Hz the light flickers, but above 20 Hz it appears continuous. This persistence is caused partly by your eye and partly by the heating of the filament remaining relatively constant over the period of the applied AC voltage.