FWB - Full wave bridge rectify - Page 1
Forum section: electronics / beginners

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: January 19, 2017, 06:59:56 pm
content: I have a simple test circuit (see attached).An RC205 full wave bridge rectifier, connected to an oscilloscope, and to a signal generator.The sim correctly shows the rectification, however, in my real circuit, I do not see it rectified.I suspect this may have to do with my grounding.Should I connect common ground to both sides of the rectifier?Or how should I be grounding this test circuit? (I know both the units are plugged into grounded wall outlets, and I know the neg terminal of the scope is connected to the wall ground).FYI: my plans are to eventually use some 110v to 24v transformers which i came across, in some real circuits

author: DmitryL
author_rank: Regular Contributor
timestamp: January 19, 2017, 07:08:53 pm
content: Do you have your a load after your rectifier ?If not, load your rectifier with a resistor to have at least a couple of ma current thorough it.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: January 19, 2017, 07:40:12 pm
content: Quote from: mikeinfodoc on January 19, 2017, 06:59:56 pmAre you saying your real function generator doesn't have its negative output tied to wall ground?What voltage and frequency are you testing with? Same as what's in the simulator?

author: Vtile
author_rank: Super Contributor
timestamp: January 19, 2017, 08:10:44 pm
content: Are you sure the you are connecting the bridge correctly AC is where AC input should and scope is measuring DC output pins. If you have high enough input impedance in scope and no other load in circuit the leakage current in bridge could show up in different ways or wrongly installed bridge. I'm not played with these enough to say top of my head, but like DmitryL said, put some Resistor over the bridge output (parallel to scope) to load the output and see if the measurement changes.What is the non-rectified output voltage from bridge shown in oscilloscope btw.?? Not happen to be some mV range?Quote from: Galaxyrise on January 19, 2017, 07:40:12 pmThere might be equipment with floating this and floating that depending the age and designed use. Just saying. Ie. my arbitrary USB gen. is floating entirely when I use it with laptop from laptops battery and grounded (not actually tested, I need to some day) when using the wall power.

author: StillTrying
author_rank: Super Contributor
timestamp: January 19, 2017, 09:50:09 pm
content: "Should I connect common ground to both sides of the rectifier?"Only if you want to blow things up.As you're connecting the scope and real GND to the BR's output side, you need the input side of the BR to be completely floating.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: January 19, 2017, 10:47:48 pm
content: ok thank, you.I had the sig gen set to sine wave with 7v amplitudeI put an LED resistor in for the load, and now I get full wave rectification.It looks very noisy however. (not a clean McDonalds "sign")

author: shteii01
author_rank: Frequent Contributor
timestamp: January 19, 2017, 11:15:25 pm
content: LED IS NOT a resistor.Quote from: mikeinfodoc on January 19, 2017, 10:47:48 pm

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: January 19, 2017, 11:19:36 pm
content: True, I figured the .7 v drop with 20ma would be enough load. I suspect the right answer is that a resistor would make a cleaner output signal due to the diode not conducting under .7positive

author: Codebird
author_rank: Regular Contributor
timestamp: January 19, 2017, 11:24:02 pm
content: QuoteThere are a few causes that can be behind this.- If you're running a signal in the kilohertz, some rectifier diodes are not made for it.- Don't forget you'll see flat bits around the crossing due to diode drop.- You may have a ground loop with both gen and scope grounded, which will lead to an absolute ton of noise.- Your LED won't draw anything below Vfd, then increases sharply with voltage, which may give a distorted waveform.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: January 19, 2017, 11:29:56 pm
content: I believe the RC205 is rated for a frequency of 60Hz (middle of page 1 on the left)http://www.mouser.com/ds/2/345/RC205-243263.pdfBut my sig gen defaults to 10k when powered on. (So that may have been part of the issue earlier)Thank you for the added feedback.All of this will be added to my checklist going forward!

author: StillTrying
author_rank: Super Contributor
timestamp: January 19, 2017, 11:41:09 pm
content: The diodes in a BR have capacitance, leakage and are slow. Assuming about 60Hz 7V AC input you'll need a load of about 1k on the DC side to see the mmmmm on a scope, an LED as a load is not good.

author: tpowell1830
author_rank: Frequent Contributor
timestamp: January 20, 2017, 12:28:58 am
content: You're running this at 5 Hz., is your function generator capable/rated for this low frequency? Try at 50 or 60 Hz.PEACE===>T

author: StillTrying
author_rank: Super Contributor
timestamp: January 20, 2017, 03:12:37 am
content: As I only needed the BR, and knew where they where, I braved the cold dark junk room, and did the experiment for you.AC input +/- 6V peaks, 1k5 load. Without the load resistor the BR's outputwasvery noisy - like mains noise, still looked OK at 500Hz.

author: Vtile
author_rank: Super Contributor
timestamp: January 20, 2017, 10:02:44 am
content: Quote from: mikeinfodoc on January 19, 2017, 11:19:36 pmLike said LED is NOT a resistor, period. It is rectifier (while poor for such application) like diodes in your rectifier bridge, L.E.D = Light EmittingDIODE.Also if it do have only 0.7 volts over it, then there is some other component that do have the rest U_ilimiter=7-0.7=6.3V or the current through led is BIG.Ohms law:U=R*IU/R=IU/I=RDo you have the concept of R.M.S familiar as now you have the test setup in hand to verify that for sinewave the RMS is U_peak / 1.414. Where 1.414 is aproximation forEdit. Changed ? to squareroot symbol if it this time stays ..aand corrected the typo in aprox value..

author: tpowell1830
author_rank: Frequent Contributor
timestamp: January 20, 2017, 05:14:32 pm
content: Quote from: Vtile on January 20, 2017, 10:02:44 amYes, VTile is right about ohms law, except here in states we use V instead of U (V for volts) and the 1.414 is approximation for the square root of 2 for RMS (root means square) voltage.What everyone is trying to tell you is to put a resistive load, not an LED, which will burn up without a resistor in series with it.PEACE===>T