Testing diode bridge with a multimeter

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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
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.
Like 3-way switches, don't go by the position of the terminal on the device's body, read the terminal labels. Like shown in all of the pics posted and linked here, the two input terminals are marked " ~ ", the positive output " + ", and the negative " - ".

A continuity tester should show open with one lead on the + to each of the ~ 's in one direction, and continuity in the other direction, and the same results, but with the leads reversed, with the one lead on the - to each of the ~ 's.
 

Besoeker

Senior Member
Location
UK
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 you want to test for a failed element a multimeter is usually adequate.
The most common failure in my experience is a breakdown which measures as a short circuit in both directions.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I hate to point out that in your sketch the polarity of the + and - is backward, the anodes of the diodes are pointing toward the negative terminal and should be toward the positive terminal.
Yep. The "arrow" points toward the + end. The band on a diode body is the cathode, or - end.

It's easy to remember because the end of the symbol with the straight line is the negative end.
 

SG-1

Senior Member
Using the ohm scale on my old Fluke 27 I read OPEN in one direction & 1.4M ohms in the other direction on two different bridge rectifiers. Using the diode function I read OPEN & .448v. Beware that the ohm function may show a false indication in this case.

The bridge I am measuring has the following numbers on it for those interested:
26MB100A
E300359
0808

It is a 25 Amp 400+ volt rectifier
 

SG-1

Senior Member
I have now measured the bridge rectifier with a Fluke 289.
Using the Ohm function I read: 500+MegOhm to OPEN & approx 260 KiloOhms
Using the diode function I read OPEN & .47v

The difference is that the diode function puts out 5+ volts and the Ohm function puts out 2+ volts on the Fluke 289.

I recommend using the diode function if present to test a diode & not the Ohm function.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100710-0605 EST

Hurk27:

In your post labeled #19 and in the paragraph
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.
you have anode and cathode interchanged.

In your post #23 it is correct that tonytonon has the + and - incorrectly located. But you still have anode and cathode backwards.

In a diode symbol the "arrow" points in the direction of conventional "positive" current flow and not electron flow. Thus, the straight line end of the diode is the cathode and the triangular end is the anode. When used as a rectifier and the cathode end as the output the diode produces a + output voltage and thus the labeling on the bridge rectifier. On the other hand the anode has to be more positive than the cathode for current flow. This is logically consistent.



With a 10 megohm load, another DVM, my Fluke 27 has a terminal voltage of about 0.759 V in the ohms position, and 2.54 V in the diode position. The short circuit current in the ohms position is about 0.48 MA, and 0.62 MA in the diode position.

At room temperature a 1N4148 reads 1.12 megohms in the ohms position, and 0.572 in the diode position. In the diode position the reading is the voltage across the diode at a current somewhat less than 0.62 MA, reads about 0.48 MA.

Because of the source impedaance in the ohms position the 1N4148 has a forward voltage across it of about 0.159 V and thus the high resistance reading.

.
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Polarity

Polarity

Thankyou all for this interesting discussion. It goes to show that something that appears simple isn't always that simple. Also, thanks to Hurk27, Larry & Gar for pointing out the polarity error on my sketch. Regargless of this, the readings in the table are correct.;)
See corrected sketch attached.
 
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