# Why am I getting this oscilloscope reading? (Half wave rectifier)

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?

(a signal similar to what i'm expecting):

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

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

• 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.
– JRE
Commented Feb 6, 2021 at 14:18
• 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 Commented Feb 6, 2021 at 14:30
• 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.
– user16324
Commented Feb 6, 2021 at 14:32
• 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 Commented Feb 6, 2021 at 14:40
• 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 Commented Feb 6, 2021 at 14:43

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.)

simulate this circuit – Schematic created using CircuitLab

Here's what the output looks like:

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

simulate this circuit

Here's the output:

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.