Author Topic: Full bridge rectifier shows wrong curve on scope  (Read 5592 times)

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Offline petertTopic starter

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Full bridge rectifier shows wrong curve on scope
« on: January 05, 2020, 08:33:17 pm »
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?
« Last Edit: January 05, 2020, 09:27:04 pm by petert »
 

Offline StillTrying

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #1 on: January 05, 2020, 08:40:11 pm »
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.
.  That took much longer than I thought it would.
 
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Offline petertTopic starter

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #2 on: January 05, 2020, 09:32:05 pm »
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:
« Last Edit: January 05, 2020, 09:46:24 pm by petert »
 

Offline Fenton Bresler

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #3 on: January 05, 2020, 10:13:51 pm »
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 Rectifier

P.S. Don't disconnect safety earth from your equipment.
 

Offline alsetalokin4017

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #4 on: January 05, 2020, 10:24:38 pm »
 8)

The easiest person to fool is yourself. -- Richard Feynman
 

Offline petertTopic starter

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #5 on: January 05, 2020, 10:38:02 pm »
I get that you can do it with the math function, but that's cheating ;)
 

Offline StillTrying

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #6 on: January 05, 2020, 11:04:01 pm »
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/
.  That took much longer than I thought it would.
 

Offline Mr. Scram

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #7 on: January 05, 2020, 11:10:19 pm »
I get that you can do it with the math function, but that's cheating ;)
The "proper" way is getting a differential probe.
 

Offline petertTopic starter

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #8 on: January 05, 2020, 11:21:43 pm »
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.
I checked it again (rebuilt it).

Attached is a picture of the breadboard and probing, maybe someone sees something I am missing:
903590-0
 

Offline StillTrying

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #9 on: January 05, 2020, 11:28:16 pm »
It looks like the bottom-right diode is in the wrong row ?
« Last Edit: January 05, 2020, 11:34:17 pm by StillTrying »
.  That took much longer than I thought it would.
 

Offline wraper

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #10 on: January 05, 2020, 11:32:08 pm »
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).

« Last Edit: January 05, 2020, 11:45:00 pm by wraper »
 
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Offline hermitengineer

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #11 on: January 05, 2020, 11:40:51 pm »
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:
(Attachment Link)
Without load resistor:
(Attachment Link)

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?

The left vertical diode in that picture is in the wrong orientation.
 

Offline petertTopic starter

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #12 on: January 06, 2020, 12:24:10 am »
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.

It looks like the bottom-right diode is in the wrong row ?
It 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.
« Last Edit: January 06, 2020, 12:33:06 am by petert »
 

Offline Jwillis

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #13 on: January 06, 2020, 12:31:29 am »
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.
 
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Offline wraper

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #14 on: January 06, 2020, 12:33:27 am »
Interestingly, disconnecting the signal gen and scope from earth (as I mentioned above) had no effect.
Don't you have any data or some other cables connected to them?
 

Offline Jwillis

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #15 on: January 06, 2020, 01:26:26 am »
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.
 

Offline Fenton Bresler

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #16 on: January 06, 2020, 01:31:47 am »
I'll have another go.

Your sig gen is single ended i.e. the entire signal is on one wire. The other is ground.
 

Offline Jwillis

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #17 on: January 06, 2020, 01:40:59 am »
Would over lapping the A channel  and B channel  work? I don't have a second cord to try that.
 

Offline StillTrying

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #18 on: January 06, 2020, 09:28:07 am »
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 it just 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!

« Last Edit: January 06, 2020, 09:57:13 am by StillTrying »
.  That took much longer than I thought it would.
 
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Online mikerj

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #19 on: January 06, 2020, 10:47:46 am »
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.

That'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.
 

Offline GreyWoolfe

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #20 on: January 06, 2020, 01:05:33 pm »
It looks like the bottom-right diode is in the wrong row ?

I see the same thing, the bottom right diode is not connected to anything.
"Heaven has been described as the place that once you get there all the dogs you ever loved run up to greet you."
 

Offline petertTopic starter

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #21 on: January 06, 2020, 01:38:47 pm »
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.
« Last Edit: January 06, 2020, 01:40:26 pm by petert »
 

Offline petertTopic starter

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #22 on: January 06, 2020, 01:42:39 pm »
It looks like the bottom-right diode is in the wrong row ?

I see the same thing, the bottom right diode is not connected to anything.
I replied to that already, it's just an unfortunate picture issue, but correctly connected in the real circuit.
 

Offline petertTopic starter

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #23 on: January 06, 2020, 01:44:07 pm »
It should be easy enough to trouble shoot it just 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!
Connect 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...
« Last Edit: January 06, 2020, 01:53:29 pm by petert »
 

Offline petertTopic starter

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Re: Full bridge rectifier shows wrong curve on scope
« Reply #24 on: January 06, 2020, 02:45:29 pm »
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?


903934-1

(I also tried different diodes to make sure they are not the problem.)
 


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