Full bridge rectifier shows wrong curve on scope - Page 1
Forum section: electronics / beginners

author: petert
author_rank: Regular Contributor
timestamp: January 05, 2020, 08:33:17 pm
content: Hello,I am having a somewhat confusing issue. To make sure I did not make any mistake on the breadboard, when building my circuit, I built another one (slightly different setup) as described here:https://www.instructables.com/id/Make-A-Bridge-Rectifier-From-Diodes/But for some reason the same issue persists.As input to the circuit I have a 50 Hz sine wave, generated by a function generator (which looks correct on the scope). But the output looks like it is only half wave rectified. I am a bit confused what could be going wrong.With a load resistor:Without load resistor:The next test was to simply use a 4.5V battery as input, which shows as positive 4.5V DC line on the scope as expected. If I reverse the polarity, i.e., input -4.5V it also shows a positive 4.5V DC line on the scope, again as expected.Any ideas what could be the issue?

author: StillTrying
author_rank: Super Contributor
timestamp: January 05, 2020, 08:40:11 pm
content: The input or/and output of a bridge rectifier has to be floating, the scope's GND and the function generator GND both being GNDed could explain it.

author: petert
author_rank: Regular Contributor
timestamp: January 05, 2020, 09:32:05 pm
content: That sounds like a good guess. Any way I could make this work using a signal generator and a scope (without using a transformer)?Edit: I made another test: I made sure the Gnd pins of both the scope and function generator are not connected to the wall plug, but there was no change. I disconnected the Gnd output of the signal generator to the circuit, but no change.Finally I tested exchanging the scope positive and gnd leads, which resulted in a horizontally mirrored graph, i.e. all negative instead of all positive:

author: Fenton Bresler
author_rank: Unknown
timestamp: January 05, 2020, 10:13:51 pm
content: Where are you putting the probe and its ground lead in the circuit? They should be either side of the load resistor respectively. My guess is the probe ground is in the wrong place.Full Wave RectifierP.S. Don't disconnect safety earth from your equipment.

author: petert
author_rank: Regular Contributor
timestamp: January 05, 2020, 10:38:02 pm
content: I get that you can do it with the math function, but that's cheating

author: StillTrying
author_rank: Super Contributor
timestamp: January 05, 2020, 11:04:01 pm
content: Are you sure the AC side is connected to the bridge rectifier's "~", I can simulate a similar shape by wrongly connecting the AC to the bridge's +/-, it shorts out the AC for half the cycle.If the signal generator is not floating there's no easy way to do it.There's similar experiments here.https://www.eevblog.com/forum/beginners/fwb-full-wave-bridge-rectify/

author: Mr. Scram
author_rank: Super Contributor
timestamp: January 05, 2020, 11:10:19 pm
content: Quote from: petert on January 05, 2020, 10:38:02 pmThe "proper" way is getting a differential probe.

author: petert
author_rank: Regular Contributor
timestamp: January 05, 2020, 11:21:43 pm
content: Quote from: Fenton Bresler on January 05, 2020, 10:13:51 pmI checked it again (rebuilt it).Attached is a picture of the breadboard and probing, maybe someone sees something I am missing:

author: StillTrying
author_rank: Super Contributor
timestamp: January 05, 2020, 11:28:16 pm
content: It looks like the bottom-right diode is in the wrong row ?

author: wraper
author_rank: Unknown
timestamp: January 05, 2020, 11:32:08 pm
content: Unless you use differential probe or disconnect mains earth from either signal gen or oscilloscope, you are simply shorting 2 out of 4 diodes through earth. It's the same thing why you should be cautious where you connect ground clip when measuring stuff. Of course it does not apply is your signal gen is not earthed (like some old low tech stuff).

author: hermitengineer
author_rank: Regular Contributor
timestamp: January 05, 2020, 11:40:51 pm
content: Quote from: petert on January 05, 2020, 08:33:17 pmThe left vertical diode in that picture is in the wrong orientation.

author: petert
author_rank: Regular Contributor
timestamp: January 06, 2020, 12:24:10 am
content: Using a small transformer with 3 turns on each side (I got it from a CFL lamp) it finally works as it should, but only for higher frequencies around 1 Mhz.Is there a way to make the signal generator floating with a non frequency dependent way?Interestingly, disconnecting the signal gen and scope from earth (as I mentioned above) had no effect.I'll try to generate a signal with a microcontroller, powered by a battery and see if that has the same isolating effect.Quote from: StillTrying on January 05, 2020, 11:28:16 pmIt looks like this on the picture, indeed. I tried to make a reasonably sharp photo, but that part looks odd due to the angle (and closeness to the black "plug" of the red lead). The real circuit is correct in this location, however.

author: Jwillis
author_rank: Super Contributor
timestamp: January 06, 2020, 12:31:29 am
content: Your orientation looks correct. I would check that each connection is seated well in the board and there are no breaks in the board it's self . Did you check each diode was functional.

author: wraper
author_rank: Unknown
timestamp: January 06, 2020, 12:33:27 am
content: Quote from: petert on January 06, 2020, 12:24:10 amDon't you have any data or some other cables connected to them?

author: Jwillis
author_rank: Super Contributor
timestamp: January 06, 2020, 01:26:26 am
content: I did the same experiment and mine does the same thing. It's like the small voltage from the generator doesn't go below zero.So it's not a true AC sine wave.Simplest solution is use a low voltage transformer to test your rectifier.

author: Fenton Bresler
author_rank: Unknown
timestamp: January 06, 2020, 01:31:47 am
content: I'll have another go.Your sig gen is single ended i.e. the entire signal is on one wire. The other is ground.

author: Jwillis
author_rank: Super Contributor
timestamp: January 06, 2020, 01:40:59 am
content: Would over lapping the A channel and B channel work? I don't have a second cord to try that.

author: StillTrying
author_rank: Super Contributor
timestamp: January 06, 2020, 09:28:07 am
content: The OP says he's already floated the signal generator and/or scope so it must be something else.It should be easy enough to trouble shoot itjust the with scope's GND on the generators GND,the +Ve sine peaks should be on the +Ve bridge output and the -Ve sine peaks on it's -Ve output. Simples!

author: mikerj
author_rank: Super Contributor
timestamp: January 06, 2020, 10:47:46 am
content: Quote from: Jwillis on January 06, 2020, 01:26:26 amThat's a pretty terrible signal generator if the output can not swing below ground. Easiest method would be to add a capacitor in series with the sig gen output to remove the DC offset.

author: GreyWoolfe
author_rank: Unknown
timestamp: January 06, 2020, 01:05:33 pm
content: Quote from: StillTrying on January 05, 2020, 11:28:16 pmI see the same thing, the bottom right diode is not connected to anything.

author: petert
author_rank: Regular Contributor
timestamp: January 06, 2020, 01:38:47 pm
content: I used a microcontroller with an integrated DAC to generate a 50 Hz sine wave. I made sure to use a capacitor and checked it was really going from 1.65V to -1.65 V (approximately).Since the µC is powered through a 4.5V battery (and not plugged into USB or anything else), it should be isolated from everything else.Probing is as before, and the result is the same as with the signal generator (with some lightly different distortions). i.e. half the waveform is missing, reversing the probing polarity shows the negative half of the waveform.So far only putting a small transformator between the signal generator and the rectifier input has worked. Odd.

author: petert
author_rank: Regular Contributor
timestamp: January 06, 2020, 01:42:39 pm
content: Quote from: GreyWoolfe on January 06, 2020, 01:05:33 pmI replied to that already, it's just an unfortunate picture issue, but correctly connected in the real circuit.

author: petert
author_rank: Regular Contributor
timestamp: January 06, 2020, 01:44:07 pm
content: Quote from: StillTrying on January 06, 2020, 09:28:07 amConnect both the scope and signal gen ground together, directly (I suppose), and then?Edit: I connected both together, and when I do neither of the bridge rectifier outputs produce any signal (0V DC).If I keep the circuit as in the breadboard picture, and red wire to probe black to scope gnd, then I get V(br+), if I connect the black wire to the scope probe, and the red to scope gnd, then I get V(br-).Not sure what conclusions to draw from that though...

author: petert
author_rank: Regular Contributor
timestamp: January 06, 2020, 02:45:29 pm
content: Okay, another test!I did differential probing (as suggested earlier in this thread).I connected all probe grounds to the signal generator ground. One probe is on the positive bridge rectifier output (red/orange wire), i.e., scope channel 3 (pout). The negative output (black wire) is connected to scope channel 4 (nout).pout - nout (ch3-ch4) is shown as fwb.The differential waveform still does not look what it should like.Maybe that could give a clue?(I also tried different diodes to make sure they are not the problem.)

author: StillTrying
author_rank: Super Contributor
timestamp: January 06, 2020, 04:02:47 pm
content: "One probe is on the positive bridge rectifier output (red/orange wire), i.e., scope channel 3 (pout). The negative output (black wire) is connected to scope channel 4 (nout)."At least the overall shape of those 2 bottom traces look close enough to the bottom 2 on my simulation, and would be even closer if the screen's 0V were in the same place."and checked it was really going from 1.65V to -1.65 V (approximately)."Barely enough voltage for a bridge rectifier.I don't know why the scope's (CH3-CH4) is not working, it looks like it's doing ( Noise-CH4).It looks like you might have one probe on X1 and one on X10, which would help to confuse theMATH.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 07, 2020, 01:51:36 am
content: Your math is trying its best..... even though "it's cheating".But what do you expect the math to look like when one of your signal channels is ten times the amplitude of the other?I'll put this here again so people don't have to look for it on the first page. Consider this an exercise in scoposcopy.(please note that all the numbers have meaning and correspond to what is actually shown by the traces on the graticule)

author: petert
author_rank: Regular Contributor
timestamp: January 07, 2020, 10:15:30 am
content: Thanks, you were both (@StillTrying, @alsetalokin4017) right regarding the scaling of the probes. I was trying out so many things I didn't notice that anymore haha.After I had both probes on the same setting, it got a similar result to yours, even if somewhat noisy.Regarding the "cheating": the reason was that I was trying to find out why the isolation does not work after unplugging the earth lead from both the scope and signal generator, which still puzzles me. (Which was a good example use of the math function, thoughMaybe weird house wiring? A UPS seems like the easiest way to isolate the signal generator (if I had one). Any other ideas?

author: StillTrying
author_rank: Super Contributor
timestamp: January 07, 2020, 11:35:51 am
content: Marvelous, I looked here to see if you'd got there yet.I'm not 100% convinced that your generator's 0V is earth grounded, can't you measure Ohms between the scope's GND clip and the generator's 0V, anything less than about 1k Ohms would mean it is grounded."somewhat noisy."A BR's DC output is often a bit noisy without a few k of load on the DC side to keep the diodes conducting with no non-conducting open circuit time.Even when the input side IS floating you still can't short the input to output of a BR with 2 scope probe GNDs. To capture this one I saved the input sine on screen and then moved the scope's GND and probe to the DC output side.https://www.eevblog.com/forum/beginners/fwb-full-wave-bridge-rectify/msg1117268/#msg1117268

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: January 07, 2020, 02:02:49 pm
content: Just a shot in the blue: To isolate the grounds of your sig gen and scope... you didn't happen to use a multi-outlet extension cord, isolated the ground of that, and plugged both devices into the same extension? That would make both devices basically isolated from mains earth/ground, but the grounds of the scope would still be connected to the ground of the signal generator.This whole "isolation"-Thing was a perfect example that university professors are sometimes dumb as hell, as one of those blew up three brand new scopes before he trusted me when I told him that he is right now shorting the 3 phase mains over the gnd clip... I got a bad grade in the lab report for pointing that out though...

author: Jwillis
author_rank: Super Contributor
timestamp: January 07, 2020, 04:09:50 pm
content: Quote from: mikerj on January 06, 2020, 10:47:46 amIt's an Atten ATF20B .It was all I could afford . Still not sure of many of the functions and how to use it. I'll figure it out eventually.

author: petert
author_rank: Regular Contributor
timestamp: January 07, 2020, 06:40:02 pm
content: Quote from: Ysjoelfir on January 07, 2020, 02:02:49 pmWell, that's embarrassing. It turns out you were guessing right! I didn't think of the "multi-socket shorting independent of house earth". But I could have sworn I tried all possible combinations (isolating the scope plug, isolating the signal gen plug, isolating the plug of the multi-outlet extension cord / multi-outlet power strip), so it should have worked anyways.Maybe I had the right isolation (by "chance"), but the signal didn't look clean enough for some other reason and that threw me off course...But it's good I made the mistake to be aware of my misconception about the multi-outlet power strip.Thanks! Better knowing why you are a fool than staying wrong while clueless haha (sorry about the professor).I was a bit nervous about testing the power plugs with the multi-meter, but I suppose I could have done continuity testing between the scope bnc and signal generator bnc plugs (the outer shells), safely.Doing that indeed confirmed the (now obvious) direct connection between the earth plugs in the multi-outlet power strip (and therefore the scope and signal gen). I somehow assumed that the connection would just be made in the house wiring, later (and therefore not occur if the main's earth is not connected). But since the multi-outlet strip gets connected to the wall outlet by a single power plug, it should have been obvious that this could not be the case (and all individual sockets earth must be joined together). Oops.I'll post a "success scope" image and a summary of the essential points, in case someone else stumbles upon this again.Thanks again everyone for your patience.

author: Mr. Scram
author_rank: Super Contributor
timestamp: January 07, 2020, 10:43:43 pm
content: I actually typed that question and solution a couple of days ago, then deleted it because you mentioned you isolated the signal generator after that. Should have gone with my hunch.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 07, 2020, 11:54:51 pm
content: Quote from: StillTrying on January 05, 2020, 11:28:16 pm+1. It's an answer.

author: petert
author_rank: Regular Contributor
timestamp: January 08, 2020, 12:13:22 am
content: For reference, a related two part video, which talks about full bridge rectifiers and "Oscilloscope Earthing / Isolation Challenges":

author: petert
author_rank: Regular Contributor
timestamp: January 08, 2020, 12:31:26 am
content: Summary/solution:As mentioned earlier, the breadboard circuit was correct, and the isolation was the main issue (multi-socket power outlet connects earth pins, even when the multi-socket as a whole is not connected to household's main earth).Using a 10k load resistor in the bridge rectifier gave the best result, regarding noise and amplitude. Less creates some clipping, more creates distortion without increasing the amplitude. Using a probe with a 1X impedance setting (with the signal generator being isolated from earth ground/the scope's ground) instead of 10x, does not have any effect on distortion (contrary to what viewer comments claimed in the videos I linked right above this post).Bridge rectifier output with 1K load resistor (and 10Vpp 50Hz sine input):Bridge rectifier output with 10K load resistor (and 10Vpp 50Hz sine input):Alternatives to isolating the signal generator's (or the scope's, or both) earth pin from the household's main earth are a UPS (uninterruptible power supply) or an isolation transformer. The former usually runs for some minutes only, the latter will still circumvent the RCD (that protects from dangerous currents through your body).For differential probing using the math function: make sure both probes are set to the same impedance, and ensure the scaling on the scope (the software side) for the channels match the impedance of the attached probes as well.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: January 08, 2020, 11:55:05 am
content: Glad to hear that you solved the problem

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 08, 2020, 11:34:30 pm
content: Sneaky ground loops can be really puzzling. I have an old analog FG (Interstate/Racal F43) which has a very convenient feature: a chassis isolation switch. So it can be "normally" connected through the line cord to mains ground, or floated by simply switching the switch. But ground loops are sneaky: if you connect, say, a BNC patch cord from the isolated FG to the non-isolated oscilloscope so you can monitor the FG's output ... guess what.

author: StillTrying
author_rank: Super Contributor
timestamp: January 09, 2020, 01:39:21 am
content: ... guess what.The BNC socket is connected to the chassis ?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 09, 2020, 11:06:28 am
content: Ah... yes. Or no, depending on which BNC connector and which chassis you are referring to.That is, on this FG, the power supply section _is_ connected normally, that is, the mains line ground is connected to the chassis. But the signal output section sends its output to the world through insulated BNC feedthru connectors that are _not_ electrically connected to the chassis at all.... unless the FG's isolation switch is in the normal "grounded" position. To keep idle fingers away from this switch the FG makers put it on the back panel, not the front.So if you have this switch in the "isolated" position, neither of the signal section's outputs (BNC shield and inner conductor) are connected back through the chassis and mains line cord to the mains ground. Unless... you have connected the FG's output (or ext trigger, synch, whatever) BNC through a patch cable to some other instrument whose BNC connectors _are_ permanently actually grounded to that instrument's chassis which in turn is connected through its line cord to the mains ground and hence to all other stuff that is also connected to that mains ground. Like an oscilloscope probe "ground" reference clip.