Oscilloscope Measurements

This page shows oscilloscope measurements for the circuit I want to use to run a DC ornament from a string of AC Christmas lights.

Cathy working

Tom snapped this photo of me while I was taking measurements. My left hand is in my lap; I found that was more convenient than behind my back!

The lights are plugged into house AC, as is the oscilloscope, so the "ground" clips for the probes are not connected. By connecting to the last bulb in the string, I was able to get small values for the measured voltages (relative to the oscilloscope's ground), enabling me to use a scale that also worked well for the waveform showing the difference between V+ and V-.
  Test Circuit
The graphs below show measurements for the test circuit on the right, with various zener diodes and capacitors. The 40 Ohm value is because the ornament will have a (minimum) effective resistance of about 40 Ohms at the target 3 volts.
In all cases:
  • Vertical scale: 2 V per gridline
  • (Horizontal scale: 5 ms / 2.5 ms per gridline,
    as specified by column header)
  • channel 1 [ch1] (yellow): V+ output from bridge
  • channel 2 [ch2] (blue): V- output from bridge
  • channel 3 [ch3] (violet): input to bridge (AC)
  • channel 4 [ch4] (green): input to bridge (AC)
  • math (red): channel 1 - channel 2 (voltage for ornament),
    unless otherwise specified

Circuit5 ms horiz. grid2.5 ms horiz. grid
  • 4.3 V zener
  • 680 uF cap
  • DC: ~4 V
 4.3 V zener, 680 uF, 5 ms 4.3 V zener, 680 uF, 2.5 ms
  • 4.3 V zener
  • 680 uF cap
  • (including AC
    measurement)
  • DC: ~4 V
 4.3 V zener, 680 uF, 5 ms, 4 channels, ch1-ch2 4.3 V zener, 680 uF, 2.5 ms, 4 channels, ch1-ch2
  • 3.3 V zener
  • 680 uF cap
  • (including AC
    measurement)
  • DC: ~2.7 V
 3.3 V zener, 680 uF, 5 ms 3.3 V zener, 680 uF, 2.5 ms
  • 4.3 V zener
  • 680 uF cap
  • math: ch3-ch4
    (AC V drop)
  • DC: ~4 V
  • AC: ~6 V
   4.3 V zener, 680 uF, 2.5 ms, 4 channels, ch3-ch4
  • 4.3 V zener
  • 47 uF cap
   4.3 V zener, 47 uF, 2.5 ms

The following graphs show startup voltage measurements using a 4.3 V zener diode and 680 uF capacitor.
Vertical scale is 2 V per gridline; horizontal scale is 10 ms per gridline.

4.3 V zener, 680 uF, startup, 4 channel
4.3 V zener, 680 uF, startup, 2 channel
4.3 V zener, 680 uF, startup, math only