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I’m trying to use a half wave rectifier with a capacitor filter and instead of getting a ripple voltage at the output it’s giving a signal that looks like white noise(pic below). Any ideas?

enter image description here

enter image description here (a signal similar to what i'm expecting):

here is the circuit diagram (with a function generator 12vpp/60hz instead of transformer): enter image description here

parts used- capacitor : 2200uF resistor : 10kOhm diode: 1N914

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  • \$\begingroup\$ Please draw a schematic diagram of your circuit, with part values. I think I see what's going on, but it's partly a guess. Also, it is hard to read the scope in the background. Rigol scopes can save a picture of the display, with values for the voltage and time base. \$\endgroup\$
    – JRE
    Commented Feb 6, 2021 at 14:18
  • \$\begingroup\$ apologies, i submitted the wrong image. i added the ltspice of my circuit (i switched it from 12.5u to 2200u just to test). i will include oscilloscope info when i can if needed \$\endgroup\$
    – villaini
    Commented Feb 6, 2021 at 14:30
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    \$\begingroup\$ what is the p-p amplitude of that "noise" and what is its DC level? I'm guessing tiny and about 15V respectively, and that's OK. You probably need to increase the load and/or reduce the C to see what you expected. \$\endgroup\$
    – user16324
    Commented Feb 6, 2021 at 14:32
  • \$\begingroup\$ i added a better image of the scope but im not sure how to obtain the dc level. i will attempt a few different C and R values and see if that works \$\endgroup\$
    – villaini
    Commented Feb 6, 2021 at 14:40
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    \$\begingroup\$ Ahem, your sim circuit says 60 Hz yet your scope says 1.63 MHz. Try slowing your scope timebase to capture a few full cycles of 60 Hz \$\endgroup\$
    – Andy aka
    Commented Feb 6, 2021 at 14:43

2 Answers 2

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You are getting a very good approximation of DC because you have a very large capacitor and a large resistor. What you are seeing is (probably) just garbage picked up from other circuits (and/or the scope itself.)

This is your circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

Here's what the output looks like:

enter image description here

Within a fraction of a second, it reaches a steady state DC that is very clean (no ripple.)

Now here's your circuit with a much heavier load:

schematic

simulate this circuit

Here's the output:

enter image description here

There's the ripple you expected.

The capacitor is there to reduce the ripple. Depending on the size of the capacitor and the current drawn by the load, there will be more (or less) ripple.

The Wikipedia page on "Ripple" goes into some detail about the mathematics.

You need to adjust the scope to show DC rather than AC. Set it for something like 2 volts per division. Also, use a slower sweep time. Something like about 20 milliseconds per division. Your current settings are showing you just the noise on top of the DC, and "zoomed" in to see extremely fast "noise" that really isn't part of your task.

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Your vertical scale is 20mV-per-division, which is FAR too sensitive. Try a scale voltage of 5V-per-division.
Note the zero-volt symbol at lower left. It is pointing down, indicating that zero volts is far below the screen. It should be pointing right. Such a large DC offset as this could very well overload the linear range of the oscilloscope's amplifiers...I wouldn't trust what you see displayed.
scope display

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  • \$\begingroup\$ Also note that the trigger is on rise past 4.66 V which is nearly the highest value displayed. Peak-to-peak voltage (Vpp) is also shown as 70.0 mV @ 1.63 MHz which is fairly respectable. \$\endgroup\$
    – Alex Hajnal
    Commented Feb 6, 2021 at 18:24
  • \$\begingroup\$ @AlexHajnal Good spot on the trigger. Nevertheless, the vertical channel amplifier may not have enough dynamic range to accommodate such small peak-to-peak AC riding on top of a very large DC voltage - I would want to verify that these voltages make sense on a less-sensitive scale (with less gain required). And I'd want to probe the capacitor voltage with a smaller 'scope ground loop too. \$\endgroup\$
    – glen_geek
    Commented Feb 6, 2021 at 18:36
  • \$\begingroup\$ Yea, that breadboard's basically an antenna. \$\endgroup\$
    – Alex Hajnal
    Commented Feb 6, 2021 at 18:41

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