Half-Wave Rectifier showing negative polarity on scope?  - Page 1
Forum section: electronics / testgear

author: otpowell
author_rank: Regular Contributor
timestamp: March 07, 2015, 11:59:56 pm
content: I'm pretty sure this is supposed to rectify all the negative polarity, for some reason, the bottom of the waveform is going below the zero line. Am I just reading this wrong or does something need to be calibrated on my scope? I'm a sophomore EE student and just bought a Owon SDS7102. I'm not really too sure how to calibrate it yet have some reading to do. In my circut, I have a 2.2k ohm resistor after my diode, 7 Vpp and 60Hz. Any suggestions would be much appreciated.

author: max666
author_rank: Frequent Contributor
timestamp: March 08, 2015, 12:09:00 am
content: You are measuring the voltage drop across the diode, this is exactly how it's supposed to look. The part that goes negative, clipping steady at -0.7 V, is the forward voltage drop of a silicone diode, when the diode is conducting. The positive sine peak is the block voltage dropping at the diode when the diode is ... well ... blocking.Nothing to calibrate, just to learn and understandYou are measuring this:Saucebut you thought you were measuring this:Sauce

author: otpowell
author_rank: Regular Contributor
timestamp: March 08, 2015, 12:18:07 am
content: Yeah I just noticed the negative part is about -700 mV. Makes sense thanks brother. All the pictures on the web of oscilloscopes showing half wave rectifiers show the half wave starting at zero so that's why I thought I was doing something wrong. . How come other scope pics of half wave rectifiers don't show it going below the zero line? Is it simply because they moved their vertical position? Seems like I would have noticed that their vertical position was manually changed.

author: otpowell
author_rank: Regular Contributor
timestamp: March 08, 2015, 12:19:58 am
content: Ok I see your pics now I'm trying to figure out the difference lol

author: max666
author_rank: Frequent Contributor
timestamp: March 08, 2015, 12:23:12 am
content: You're measuring voltage across the diode, but that's not the output voltage of a rectifier. The output is across the resistor.

author: otpowell
author_rank: Regular Contributor
timestamp: March 08, 2015, 12:24:59 am
content: No I swear I'm measuring across the resistor. Let me take some pics

author: otpowell
author_rank: Regular Contributor
timestamp: March 08, 2015, 12:29:20 am
content: Here's exactly where my probe is and my circuit. The diode is a 1N4001

author: otpowell
author_rank: Regular Contributor
timestamp: March 08, 2015, 12:33:24 am
content: My probe attenuation is correct, and my probes are setup for AC coupling. I did read my quick start guide to my scope and ran through a auto calibration type thing. The full wave with the scope across the function generator, looks perfectly centered between positive and negative axis.

author: otpowell
author_rank: Regular Contributor
timestamp: March 08, 2015, 12:35:43 am
content: Perhaps it's because I'm using a function generator and hot a center tapped transformer? I don't see why that would matter, it's AC current.

author: max666
author_rank: Frequent Contributor
timestamp: March 08, 2015, 12:47:34 am
content: When I look at the breadboard, it looks like the diode and the resistor aren’t on the same row? Are they connected?Although I would be surprised to see such a clean waveform if that resistor is just hanging in the air, only capacitive coupled.

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 08, 2015, 01:04:30 am
content: Your probe should be DC coupled in this case since you are interested in measuring a (changing) DC voltage.

author: otpowell
author_rank: Regular Contributor
timestamp: March 08, 2015, 01:08:55 am
content: No I'm measuring a changing polarity sine wave out of my function generator so it should be AC? Yes the diode is connected to the resistor.

author: otpowell
author_rank: Regular Contributor
timestamp: March 08, 2015, 01:10:02 am
content: Quote from: TerminalJack505 on March 08, 2015, 01:04:30 amI get it! It should be DC COUPLED!

author: otpowell
author_rank: Regular Contributor
timestamp: March 08, 2015, 01:11:39 am
content: I'm gonna try changing the coupling and post my results

author: max666
author_rank: Frequent Contributor
timestamp: March 08, 2015, 01:22:31 am
content: D'oh! Of course, the couplingWas lead astray by the 0.7 V.

author: otpowell
author_rank: Regular Contributor
timestamp: March 08, 2015, 03:09:17 am
content: Ok I changed it and it turned out perfect. Thanks guys!