Measuring DC Voltages With an Oscilloscope
NMSU-Grants Electronics/Electrical Department.

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Why Measure DC Voltages With an Oscilloscope Instead of a Meter?
Voltmeters can be quite sluggish (unless you're using an old-fashioned analog meter). In some cases, this sluggishness makes it hard to monitor some voltages. An oscilloscope (scope) has the advantage of instant response. Also, most scopes have two or more channels. Thus you can have two (or more) voltages on the scope's screen and see what's going on with both of them.

Setting Up the Oscilloscope For Simultaneously Measuring Two Voltages
  1. Turn the oscilloscope on.
  2. In the TRIGGER section of the front panel, find the switch labeled AUTO, NORM, etc. Set this switch to AUTO.
  3. In the HORIZONTAL section of the front panel, find the MAIN TIME/DIV knob. Set this knob to 0.1 mS.
    • If the knob is set at larger times, the green line on the screen will flicker.
  4. In the VERTICAL section of the front panel, find the switch labeled CH1, CH2, DUAL, ADD. Set this switch to DUAL.
    • This lets you see both traces (channels).
  5. In the CH1 (channel 1) area of the front panel, find the switch labeled AC, GND, DC. Set this switch to DC.
  6. Turn the CH1 VOLT/DIV knob to 1 V.
  7. Adjust the CH1 POSITION KNOB so that the trace (green line on the screen) is lined up with the bottom line of the grid. We now say that the zero point for CH1 is the bottom line of the grid.
  8. Repeat the above steps for the CH2 section of the front panel, making the bottom line of the grid it's zero point.
  9. Find the switches on the scope's probes and set them to x1.
 
Making Voltage Measurements
  1. Each probe has an alligator clip for connecting the scope to ground. Connect one of these clips to your circuit's ground. 
    • It's not necessary to connect the clip on the other probe.
  2. Connect a probe to a point in your circuit that has a voltage you want to measure.
  3. If there's another voltage you'd like to measure, connect the other probe to this second point.
  4. Each square on the screen's grid is one centimeter. Above, you set the VOLT/DIV knobs to 1 V. This means that each centimeter represents 1 volt, and each smaller division represents 0.2 volts.
  5. If the voltage you're measuring is greater than 8 volts, the trace will go off the screen. If this happens, set the VOLT/DIV knob to a larger voltage.
  6. An alternative to having the zero points for both traces on the bottom line of the grid, is to have one zero point on the center grid line, and the other on the bottom line. If the voltages you're working with go negative as well as positive, you'll probably want to have one zero point on the line 2 centimeters down from the top and the other on the line 2 centimeters up from the bottom.

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