Cornerstone Electronics Technology and Robotics Week 21 Electricity & Electronics Section 10.5, Oscilloscope

Transcription

1 Cornerstone Electronics Technology and Robotics Week 21 Electricity & Electronics Section 10.5, Oscilloscope Field trip to Deerhaven Generation Plant: Administration: o Prayer o Turn in quiz Electricity and Electronics, Section 10.5, Oscilloscope, Measuring Voltage with BK 2120B and 2125A: o General Operation: An oscilloscope is like a graphing machine; it plots voltage (y-axis, the vertical axis) as it varies with respect to time (xaxis, the horizontal axis). Oscilloscopes measure voltages only; they do not measure currents or resistances. We will be using the oscilloscope to plot an input signal voltage vs. time. The trace on the screen moves up and down on the y-axis as the input signal voltage increases and decreases. The trace sweeps across the x-axis in a linear motion at a speed set by the operator. If the operator is measuring a direct current (dc) signal, the trace will remain level across the screen at a value equal to input dc voltage. When an alternating current (ac) signal is measured, the trace will follow the voltage level as it changes in time producing a graph of the input signal voltage with respect to time. This displays the form of the voltage wave, or the waveform. See: ml o Types: Analog The voltage amplitude deflects a beam of electrons that strike the oscilloscope screen. The time sweep is generated internally by the oscilloscope. Our oscilloscopes are the analog type. Digital Improved amplitude and time measurements o Safety: See the attached Test Instrument Safety sheet and Oscilloscope safety o Controls: General Controls: POWER button: Turns power on/off. INTENSITY: Adjusts brightness of trace. TRACE ROTATION: Adjusts to maintain trace in a horizontal position. This is a screwdriver-type adjustment. FOCUS: Adjusts trace focus. GND: Oscilloscope chassis ground jack and earth ground via three-wire ac power cord.

2 CAL: Terminal provides 2 volt peak-to-peak, 1 khz square wave signal. Vertical Controls: CH1/CH2 INPUT JACKS: Vertical inputs (signal inputs) for channels 1 and 2. CH1/CH2 AC-GND-DC: o AC: Blocks dc component of the signal, allowing only the ac component through. o GND: Grounds the input signal, which eliminates trace deflections. This provides a zero-volt baseline which can be used as a reference when performing measurements. o DC: Displays both ac and dc components of the input signal. o CH1/CH2 VOLTS DIV: Provides step adjustment of vertical (voltage) sensitivity. When VAR control is set to CAL, the vertical sensitivity is calibrated in 10 steps. CH1/CH2 POS: Adjusts the vertical position of the channels 1 and 2 traces. VERT MODE: o CH1: Displays channel 1 by itself. o CH2: Displays channel 2 by itself. o DUAL: Displays channels 1 and 2 simultaneously. o ADD: Displays the sum of the inputs from channels 1 and 2. Horizontal Controls: TIME DIV: Provides step selection of sweep rate for the main time base (x-axis). VAR SWEEP: Adjusts the vernier sweep rate. If the control is fully clockwise, the sweep rate is calibrated to the setting on the TIME DIV control. POSITION: Adjusts the horizontal position. Triggering Controls: TRIG LEVEL: Stabilizes the waveform on the screen. Automatic-Normal Trigger Control: Set control to auto. This automatically creates a time sweep across the display. Lab work: o Rewire the 555 timer LED brightness circuit from the previous week. Using the oscilloscope, check the wave form across the LED.

3 Pulse Width Modulation to Control LED Brightness o Start-up: See BK PRECISION Oscilloscope Guide, page 8 for initial startup procedure. o Displaying a signal: See BK PRECISION Oscilloscope Guide, page 8. Use the input from the 555 timer circuit. o Wire the 74LS04 below. Use a function generator set at a 1 KHz square wave as the input signal. Now use one of the six inverter inputs, e.g. A1, for channel 1 on the oscilloscope and one of the inverter outputs, e.g. Y1, for channel 2. 74LS04 Hex Inverter

4 Truth Table for an Inverter Input A L H _ Y = A Output Y H L H = High Logic Level L = Low Logic Level o Display dual-trace: See BK PRECISION Oscilloscope Guide, page 8. Use the input from the inverter circuit for channel 1 and the output from the inverter for channel 2. o Now wire the following dual J-K flip-flop circuit. Use a function generator set at a 1 KHz square wave as the clock input. Tie Q1 to channel 1 and Q2 to channel 2 on the oscilloscope. 74LS107 Pin Layout Flip-Flops Generate Binary Count Q1 Q Decimal Binary Q1 and Q2 Outputs Counting to 3 in Binary

5 o Calculating frequency using an oscilloscope: Frequency: The rate of change of an ac voltage or current, measured in cycles per second, or hertz (Hz). Period: The duration of one AC voltage or current cycle, measured in seconds, (s). Formula: Where: f = 1/T f = Frequency in Hz T = Period in s Example: What is the period for typical 60 Hz house current? T = 1/f T = 1/60 Hz T = s See: Therefore, one cycle of 60 Hz house current takes seconds to complete. Calculate the period for a 1 khz signal. Now use the CAL terminal on the oscilloscope to generate a 1 khz signal on the scope. Check to see if your calculated 1 khz period matches the period displayed on your scope.