- Make sure the power supply is off.
- For one of the 0 to 30 volt supplies, turn the voltage adjustment knob to zero (fully counterclockwise).
- Turn the current adjustment knob to maximum (fully clockwise).
- Using alligator clips, put two 10 ohm, 10-watt
resistors in parallel and connect them to the 0 to 30 volt
terminals on the power supply.
- Our power supplies are of fairly high quality. To make ripple visible, we need to draw a lot of current.
- These resistors will be getting hot, so set them on top something to avoid damaging the tabletop.
- Connect an oscilloscope to the resistors, being sure to have the probe and the alligator clip at least 1/4 inch away from the resistor body.
- This will keep the resistor's heat from melting the plastic.
- Set the scope for automatic triggering, and have it trigger
off of the line (instead of off of channel 1, channel 2, or the
external trigger input).
- "Line" refers to the 120 VAC, 60 Hz line.
- Triggering on this will avoid the problem of trying to trigger the scope off of a signal in the millivolt range.
- Set the vertical input to DC mode.
- The DC mode allows both DC and AC to be displayed on the scope. Think of it as the DC & AC mode.
- Turn the power supply on.
- Get a line on the scope's screen and align it with the bottom line on the graticule (grid).
- Increase the power supply's voltage to 10 volts.
- How much total power is being dissipated by the two resistors? ____________
- Set the volts/div knob for smaller and smaller voltages,
until the line goes above the top line on the screen where you can't
see it.
- Use the vertical position knob to bring the line back down where you can see it.
- Continue setting the volts/div knob for smaller and smaller
voltages, and using the vertical position knob to bring the line back
down onto the screen.
- When you can no longer set the volts/div knob to smaller
voltages (without the line disappearing off of the top of the screen),
look to see if there is any ripple on the trace you have on the screen.
Was there any visible ripple? _____________ . What was the smallest
voltage setting you were able to use? _____________.
- Change the vertical input to AC mode.
- This will discard the DC portion of the voltage the scope probe is connected to.
- Because only the AC part is kept, you will no longer have
the problem of the trace going beyond the top of the screen when you
set the volts/div knob to low voltages.
- Adjust the vertical position knob to bring the trace into the middle of the screen.
- Turn the volts/div knob to the lowest voltage setting.
- Adjust the time/div knob to 5 ms.
- What is the ripple voltage (peak-to-peak)? ______________
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