Full bridge rectifier

Thread Starter

berms03

Joined Sep 24, 2012
18
With the whole thing disconnected from the mains,take a DMM on the ohms range,& check for continuity between the two DC output leads of the bridge & the AC input connections to the bridge.
There should be no continuity.
I did this and there is no continuity between the two DC outputs nor the AC inputs.

If you haven't somehow,got a direct connection between one of the DC outputs & the Neutral line,I would guess that you have one or more of the diodes in the bridge incorrectly oriented.
The diodes are oriented correctly, I have even used multiple four pin Bridge rectifiers to see if i installed the diodes incorrectly, but I still get the incorrect output.


To check this,disconnect the AC input leads to the bridge,& measure with the DMM in diode test position between each bridge AC input in turn,& the +ve DC output connection(with the black lead on that point).
You should see a forward based diode in each position.

Do the same with the -ve Dc output connection,this time with the meter connections reversed.
Again,you should see two forward biased diodes.
I checked each diode with a DMM and they all seem to be working perfectly, I have attached an image of my breadboard.
 

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BSomer

Joined Dec 28, 2011
434
...I am currently using a Variable AC (VAC) so if something is wrong it will blow the fuse in the VAC...
Are you using a variac? As in an adjustable transformer like the one below. This type of transformer does not provide isolation and may be the root cause of your issues.

 

Thread Starter

berms03

Joined Sep 24, 2012
18
I am using a variable AC and that could be my issue, I will change my circuit to accommodate higher voltage, thanks for the reply. Ill let you know how it comes out.
 

Thread Starter

berms03

Joined Sep 24, 2012
18
Are you using a variac? As in an adjustable transformer like the one below. This type of transformer does not provide isolation and may be the root cause of your issues.

Well Unfortunately bypassing the Variac didn't change a thing. Still seeing the same output
 

vk6zgo

Joined Jul 21, 2012
677
I did this and there is no continuity between the two DC outputs nor the AC inputs.
Not quite what I suggested--check for unwanted continuity between each DC output & both AC inputs in turn.


The diodes are oriented correctly, I have even used multiple four pin Bridge rectifiers to see if i installed the diodes incorrectly, but I still get the incorrect output.

I checked each diode with a DMM and they all seem to be working perfectly, I have attached an image of my breadboard.Did you check the diodes in the exact manner I described?
Millions of bridge rectifiers are happily at work throughout the world,so you are obviously doing something wrong.
PLEASE
do not run the bridge direct from the mains!

A variac is not safe,either,as if your mains outlet is reversed,as some inadvertently are,both legs of the output of the variac will be at a high AC potential to earth!

Your breadboard is definitely not safe at mains voltage!!

You should be able to find an old AC wall wart somewhere,& do your testing at a safe voltage.

Try (using a safe AC voltage),wiring your bridge "spiderweb"fashion.
(soldering the connections),applying the (low) AC supply & checking the DC outputs,with no other connections.

I am very suspicious of the breadboard,as it is easy to get a connection one step out.
 

t06afre

Joined May 11, 2009
5,934
Berms03! Small transformers are very cheap. You may pick one out of some old equipment in your junk box. And as many have said I think the variac is the culprit here.
 

Thread Starter

berms03

Joined Sep 24, 2012
18
Many power supplies run bridges directly from mains, they of course are used for low power applications, since the mains are only powering a diode, a few mA, there is no problem with hooking mains directly to the bridge.
 

Thread Starter

berms03

Joined Sep 24, 2012
18
Berms03! Small transformers are very cheap. You may pick one out of some old equipment in your junk box. And as many have said I think the variac is the culprit here.
Unfortunately the products needs to be able to run from mains, whether they are 120VRMS or 220VRMS, 50HZ or 60HZ. I only need the bridge to supply a few mA to a diode, which shouldn't be an issue. I have bypassed the variac getting the same output, so I dont believe the variac is an issue.
 

t06afre

Joined May 11, 2009
5,934
Many power supplies run bridges directly from mains, they of course are used for low power applications, since the mains are only powering a diode, a few mA, there is no problem with hooking mains directly to the bridge.
Both yes and now. For the first it is not legal to connect any equipment to the mains. Unless has all the necessary approvals. And it is not allow to sell this sort of equipment without such approval. It is of course a lot of equipment that use a rectifier directly to the mains voltage. Like most modern switchmode supplies. But they will also be proper fused and tested. What ever you made as a hobbyist will not in any way have such approval. And hence it not legal. And it is also very unsafe for you. If you insist on being stubborn. The mods will put a padlock on this thread. Is it that you want?
 

Thread Starter

berms03

Joined Sep 24, 2012
18
I am not trying to be sturbborn at all. I am new to AC but have been doing DC circuits for years and trying to understand what my mistake is. I have built several devices that have been FCC and UL approved, just not understanding why the bridge rectifier isn't working like it should be. I found a 24V AC transformer in my old junk, hooked it up to my circuit and still i get the same result. I guess the next step is to eliminate the bread board and dead bug the circuit to see if this solves the issue. I do appreciate the help.
 

vk6zgo

Joined Jul 21, 2012
677
Another trick you can do,is to remove the AC input to your bridge,& replace it with a 9V battery.
If your bridge rectifier works,you should be able to measure about 7.6V across the DC output terminals,no matter which way round you connect the battery to the AC input terminals of the rectifier bridge.

If one way round doesn't work,place the DMM "common" on the battery -ve
terminal,& check the voltages present progressively around the circuit,with the battery connection which works.
Write down your results.

Then do the same with the connection which doesn't work.
If the two sets of readings are not the same,you have pinned the fault down.
 
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Thread Starter

berms03

Joined Sep 24, 2012
18
Thanks for the reply, I did you trick and like expected the voltage was the same not mater what polarization of the battery I attached to the bridge. I don't understand why I am not getting the negative portion of the sine wave to show on my o-scope? Do I need to have a load on it? Am i using an incorrect GND for my O-scope?
 

Thread Starter

berms03

Joined Sep 24, 2012
18
WOW okay I feel dumb, after a quick run i realized that my probe on my scope was earth ground shorting out a diode which is why i never saw the negative sine wave. Thanks for all your assistance, sometimes the best way to the solution is do something else and put it to the side and clear your mind.
 

EB255GTX

Joined Apr 30, 2011
62
I found a 24V AC transformer in my old junk, hooked it up to my circuit and still i get the same result.

WOW okay I feel dumb, after a quick run i realized that my probe on my scope was earth ground shorting out a diode which is why i never saw the negative sine wave.
Shouldn't the 24VAC transformer have worked....? It breaks the Neutral to Earth connection from the switchboard that feeds the wall socket.
 
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