Testing diode bridge with a multimeter

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Pitt123

Senior Member
I want to test a diode bridge with a multimeter or megger. I know if you test a single diode with a multimeter (ohm settings) then with your leads in one arrangement you will get continuity however if you reverse the leads the diode will block current and your meter will read open.

If I am testing a full diode bridge then if I connect my meter leads to the two input of the bridge then I will get current flowing through the bridge in one direction on half of the bridge and will read continuity. When I reverse my leads I should still see contunitly (the same as first arrangement) on my meter as current has reversed direction and is flowing through the bridge on the other half of the bridge. If I do not get continuity from one of these tests and get a high ohm reading then its safe to say that one of the diodes are bad.

Is it a good idea to disconnect the secondary circuit and just jumper the bridge's output terminals so that you are strictly testing only the brige and not the external circuit?
 
SG-1

SG-1

Senior Member
Location
Ware Shoals, South Carolina
Yes, it would be best to disconnect all four terminals to test the bridge. This way you will not get any interferrence from other devices & you will not damage any other devices. There is no need to short out any terminals. Use the diode setting on your meter and work your way around the bridge.

On each set of terminals take a reading in both directions, for a total of 8 readings ( 2 reading on each diode ). The diodes will read OPEN in one direction & the breakdown voltage will be shown on the meter in the other direction ( typically .4-.6volts ).

Another test would be to apply an AC voltage to the input terminals, the DC output will be 10% less than the input. ( no capacitor on output )

Another test would be to apply DC voltage to the input terminals, it should pass through the bridge.
 
ATSman

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Bridge Rectifier Elementary

Bridge Rectifier Elementary

Pitt123 said:
I want to test a diode bridge with a multimeter or megger. I know if you test a single diode with a multimeter (ohm settings) then with your leads in one arrangement you will get continuity however if you reverse the leads the diode will block current and your meter will read open.

If I am testing a full diode bridge then if I connect my meter leads to the two input of the bridge then I will get current flowing through the bridge in one direction on half of the bridge and will read continuity. When I reverse my leads I should still see contunitly (the same as first arrangement) on my meter as current has reversed direction and is flowing through the bridge on the other half of the bridge. If I do not get continuity from one of these tests and get a high ohm reading then its safe to say that one of the diodes are bad.

Is it a good idea to disconnect the secondary circuit and just jumper the bridge's output terminals so that you are strictly testing only the brige and not the external circuit?
Click to expand...

SG stated it well. See attached sketch. The attachment limits of this site caused me to squeeze it down quite a bit. If you cannot read it send me your email and I will send the full size image.:)

Tony
 
SG-1

SG-1

Senior Member
Location
Ware Shoals, South Carolina
I would like to add that it does not make any difference to which AC input terminals the DC is applied. The + DC output will remain positive no matter which way the positive & negative are connected to the AC input.
 
SG-1

SG-1

Senior Member
Location
Ware Shoals, South Carolina
76nemo said:
Just for my own sake, how or what good do you think testing a full bridge with a megger do:-?
Click to expand...

If you suppected the rectifier was breaking down to the chassis then you could short all four terminals together and apply voltage between them & ground ( the chassis ). The bridge may not be breaking down until 50 volts or more is applied. An ohm meter would probably show an open in this case.
 
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hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
SG-1 said:
I would like to add that it does not make any difference to which AC input terminals the DC is applied. The + DC output will remain positive no matter which way the positive & negative are connected to the AC input.
Click to expand...

It was a great way to stop truckers from reversing the polarity to their CB radios, I use to install them in every CB that came into my shop years ago, stopped a lot of headaches.
 
76nemo

76nemo

Senior Member
Location
Ogdensburg, NY
SG-1 said:
If you suppected the rectifier was breaking down to the chassis then you could short all four terminals together and apply voltage between them & ground ( the chassis ). The bridge may not be breaking down until 50 volts or more is applied. An ohm meter would probably show an open in this case.
Click to expand...


Alright SG-1, I really like your input. I have to ask, without isolating the output of the rectifier itself, how can you tell the bridge itself is breaking down with that method?
 
H

hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
LarryFine said:
It opened for me. It was a picture of a polar bear in a blizzard.
Click to expand...

attachment.php
 
SG-1

SG-1

Senior Member
Location
Ware Shoals, South Carolina
Nemo,
If you know the circuit after the bridge can withstand the test voltage, then you could lift the bridge off the chassis if a breakdown occurs. If the breakdown no longer occurs then it is the bridge.

The bridge is shorted out ( all four terminals ) to prevent any voltage from appearing across a diode junction.

And what Hurk said !
 
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Pitt123

Senior Member
When I am testing the brigde but putting meter leads across the input of the bridge (opposide sides of diamond) I am seeing a low impedance one way and high impedance when the meter leads are swapped. Shouldn't this be a low impedance path both ways? Speaking to someone else here they said that in one direction the diodes will conduct and in the other they will not. But since its a full bridge should't we see it conduct both ways.

Now we are only on the input of the bridge and the output of the bridge is not connected to an external circuit. Doesn't current need to flow through the external circuit in order to read continuity through the bridge. Could the fact that the external circuit is not completing the connection result in what i am seeing? Can I tie the two outputs of the bridge together to complete this circuit?
 
H

hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
Pitt123 said:
When I am testing the brigde but putting meter leads across the input of the bridge (opposide sides of diamond) I am seeing a low impedance one way and high impedance when the meter leads are swapped. Shouldn't this be a low impedance path both ways? Speaking to someone else here they said that in one direction the diodes will conduct and in the other they will not. But since its a full bridge should't we see it conduct both ways.

Now we are only on the input of the bridge and the output of the bridge is not connected to an external circuit. Doesn't current need to flow through the external circuit in order to read continuity through the bridge. Could the fact that the external circuit is not completing the connection result in what i am seeing? Can I tie the two outputs of the bridge together to complete this circuit?
Click to expand...

To properly test a bridge diode with a ohm meter you must start by removing all external connections to it, as these loads or windings (transformer) will give you unwanted readings, start at one of the AC leads/ terminals, by placing one of the probes on it, touch the other probe to the + terminal reverse the leads, do this again to the - terminal and same AC terminal, then move on to the other AC terminal and repete the above, checking between both AC terminals or between both + and - will not result in the proper readings your looking for. look at the link I posted in post 12 as it has more info into checking these.

A bridge diode/rectifier is nothing but 4 diodes aranged in a dimond so that two will have anodes connected to the + terminal, each of these will go to each AC terminal, and two more diodes will have the cathodes connected to the - terminal with again each going to each AC terminal.
 
Last edited:
ATSman

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Bridge Rectifier Elementary

Bridge Rectifier Elementary

Pitt123 said:
I want to test a diode bridge with a multimeter or megger. I know if you test a single diode with a multimeter (ohm settings) then with your leads in one arrangement you will get continuity however if you reverse the leads the diode will block current and your meter will read open.

If I am testing a full diode bridge then if I connect my meter leads to the two input of the bridge then I will get current flowing through the bridge in one direction on half of the bridge and will read continuity. When I reverse my leads I should still see contunitly (the same as first arrangement) on my meter as current has reversed direction and is flowing through the bridge on the other half of the bridge. If I do not get continuity from one of these tests and get a high ohm reading then its safe to say that one of the diodes are bad.

Is it a good idea to disconnect the secondary circuit and just jumper the bridge's output terminals so that you are strictly testing only the brige and not the external circuit?
Click to expand...

See attached sketch for expected results of a good rectifier.

TT
 
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