Floating a bridge rectifier - Page 1
Forum section: electronics / beginners

author: Aeternam
author_rank: Unknown
timestamp: May 12, 2016, 07:42:02 pm
content: I feed a sine wave into a transformer which is followed by a full-wave bridge rectifier. I expect to see a rectified wave at the output with respect to local ground, but that's not the case. It's flatlining. Why?My signal generator isnotfloating and I suppose that, by connecting the (also mains earth referenced) scope ground pin to the circuit, I create a short. But I was under the impression that the transformer would provide isolation from mains earth -- thus floating the part of the circuit on the right of the transformer and allowing me to probe around the circuit at my hearts content. Am I wrong?

author: Delta
author_rank: Super Contributor
timestamp: May 12, 2016, 07:47:55 pm
content: If your drawing accurately resembles your circuit, your diode bridge is wrong.....

author: Mr.B
author_rank: Unknown
timestamp: May 12, 2016, 07:50:55 pm
content: Quote from: Delta on May 12, 2016, 07:47:55 pmHow?

author: retrolefty
author_rank: Super Contributor
timestamp: May 12, 2016, 08:05:00 pm
content: Quote from: Mr.B on May 12, 2016, 07:50:55 pmIgnore that. Have you taken a DMM reading in case you have a scope/probe/settings problem? Your expectations are correct but your measurement is somehow wrong. If not a measurement problem perhaps component problem(s) ?Your journey of knowledge is simply not complete yet, finish the journey.

author: Delta
author_rank: Super Contributor
timestamp: May 12, 2016, 08:09:33 pm
content: Sorry. It's not. I must stop reading this forum after 8 pints of Guinness...

author: rstofer
author_rank: Super Contributor
timestamp: May 12, 2016, 08:10:27 pm
content: What do you get if you leave the probe ground disconnected?Are you sure there is no conductivity between input and output? There shouldn't be, but who knows?It's worth remembering, and I'm sure you do, that probe ground is directly connected to earth ground.You can also check it by setting your scope for A-B and using two channels. Leave the probe grounds disconnected and put a probe at each location.

author: IanB
author_rank: Super Contributor
timestamp: May 12, 2016, 08:10:42 pm
content: The turns ratio of the transformer will be important. Do you know what voltage you expect on the secondary?Also the 1 k resistor is important. A diode is a current operated switch, and with no load the rectifier will not function properly. You might consider trying a lower value resistor to increase the current flow through the rectifier.

author: Mr.B
author_rank: Unknown
timestamp: May 12, 2016, 08:11:27 pm
content: Quote from: Delta on May 12, 2016, 08:09:33 pm

author: Aeternam
author_rank: Unknown
timestamp: May 12, 2016, 08:16:39 pm
content: Quote from: IanB on May 12, 2016, 08:10:42 pmOf course, OMGIt's a step-down from 230V to 8V and I feed it 5 Vpp. The secondary voltage is completely dropped across the 2 diodes.THANK YOU!

author: alsetalokin4017
author_rank: Super Contributor
timestamp: May 12, 2016, 08:29:57 pm
content: Quote from: Aeternam on May 12, 2016, 07:42:02 pmYes, because the scope probe ground reference clip is connected back through the instruments and their line cords to the grounded side of the FG's output. So you are creating a groundloop short that bypasses the transformer. See the marked-up sketch below.The way to do this measurement is to make a differential measurement using two scope probes with no ground clip connection, and use the scope's Subtract function (or, on many analog scopes, Invert and Add functions) to give the differential measurement on the output side. See the scopeshot below.(And I just saw the transformer ratio. Yes... you'll need a bit more voltage on the output side of the transformer before you see anything sensible!)

author: JimRemington
author_rank: Regular Contributor
timestamp: May 12, 2016, 08:36:15 pm
content: QuoteNot quite true. Expect about 1.4 V drop across two diodes, so about 1.1V across the resistor.

author: IanB
author_rank: Super Contributor
timestamp: May 12, 2016, 08:57:50 pm
content: Quote from: JimRemington on May 12, 2016, 08:36:15 pmYa gotta read the whole thread carefully before firing off a reply...

author: wraper
author_rank: Unknown
timestamp: May 12, 2016, 09:03:23 pm
content: Quote from: alsetalokin4017 on May 12, 2016, 08:29:57 pmI don't see any ground loop there. Yes, both sides of the transformer are connected to the GND, but transformer windings are isolated, therefore no ground loop possible. You cannot create ground loop with only one ground connection to the particular circuit which is otherwise isolated.QuoteNo way this is a correct thing to do

author: JimRemington
author_rank: Regular Contributor
timestamp: May 12, 2016, 09:21:37 pm
content: QuoteI suppose, depending on the interpretation of the word "it".

author: Mr.B
author_rank: Unknown
timestamp: May 12, 2016, 09:26:57 pm
content: Quote from: JimRemington on May 12, 2016, 09:21:37 pmThe OP said the turns ratio was 230:8That is 28.75:1Feeding in 5Vp-p you would get approximately 0.174Vp-p out of the transformer.Not nearly enough for two silicon diodes to start to conduct.Also, assomeoneIanB said above, the 1K resistor is too high to get sufficient current flowing through the diodes. (Even if the p-p V was high enough.)Edit: To acknowledge Ian.

author: danadak
author_rank: Super Contributor
timestamp: May 12, 2016, 09:39:13 pm
content: Differential measurements -http://users.rowan.edu/~mease/resources/Fundamentals%20of%20Floating%20Measurements%20and%20Isolated%20Input%20Oscilloscopes.pdfRegards, Dana.

author: linux-works
author_rank: Super Contributor
timestamp: May 13, 2016, 02:59:36 pm
content: that tek document was nice and to the point. thanks for posting it.

author: onatisul
author_rank: Newbie
timestamp: September 26, 2024, 08:04:45 pm
content: Quote from: wraper on May 12, 2016, 09:03:23 pmSorry to revive this topic, I agree with wraper above but just wanted a sanity check and someone to have a look at what said here:https://download.tek.com/document/Tektronix_Student_Engineering_Lab16_Bridge_Rectifier.pdf?_gl=1*f9fk6p*_gcl_au*OTY2MjA3ODQ2LjE3MjczNzg4MjI.*_ga*MTcyMjE0MDE4Ni4xNzE2MjI5MjAx*_ga_1HMYS1JH9M*MTcyNzM3ODgyMi42LjEuMTcyNzM3OTY3Mi41MS4wLjA.3. Remove the channel 1 probe from the output of the transform to the load resistor (R). Note:because the probe grounds are tied together, the input and out cannot be probed at thesame time without the use of a differential probe. Observe the rectifier output on thescope. Compare to what was shown when the output of the transformer was probed. Thisrectifier is rectifying a 60 Hz input, would this rectifier work at higher frequencies?

author: CaptDon
author_rank: Super Contributor
timestamp: September 27, 2024, 01:13:34 pm
content: I am amazed at how much fouled or completely wrong advice was offered here. To the O.P., your thought process was correct but mainly the transformer turns ratio caught you off guard. If you have any transformers with a pair of equal windings like a 120 / 120 dual primary or perhaps one with a 12 / 12 secondary you could use that as your isolation. Feed 5vac into the 120 primary winding 'A' and see 5vac (apprx) coming out of the other 120vac winding 'B'. Same goes for using the 12 / 12 side. Dual primaries are very common for 120 / 240 operation. Parallel for 120, series for 240 observing the connections having the proper phasing!! 50Hz is o.k. but you may find the signal generator could drive the transformer a bit more efficiently at 100Hz. If you watch the output of the transformer and sweep the frequency you may find a resonance point where the output voltage seems to be about 1.5 to maybe 3 times more than what the turns ratio would suggest. Have fun learning!! Your concern for isolation was right on track. Good work!!