Bridge rectifier

[sabre504]

Hi all can you test a bridge rectifier without putting mains voltage through it ?

I have a dell 177 that had blown a fuse put in new one and it has blown again but also has blown a chunk out of the thermistor.

I did an audible cont test on the bridge pins and found i think a short as i have tested others same way but found no short on known good ones (when i say short i mean am getting continuity between 2 of the pins)

Am i right in what i am thinking or does anyone know a better way ,

thanks in advance

[370forlife]

Re: Bridge rectifier

Use the diode test function found on most multimeters. The two inner legs should be for AC. The outer leg that is below the corner with the notch missing is usually positive. Should only conduct from the AC legs to the outer legs.

[Dgtech]

Re: Bridge rectifier

This is a diagram of the bridge rectifier. Without getting too technical about what it does, to best answer your question, yes you will get continuity depending on how you test it.

[Dgtech]

Re: Bridge rectifier

This component attatched here is also a bridge rectifier. It's internal circuitry is in my the previous post. Observe the markings on the side of it. the (+) will be the positive DC voltage output. The (-) will be the DC ground. The two pins in the middle both have the same symbol (~). This is the sine wave symbol and it's used for the AC input.

If I thought this part was failing, I'd desolder it and test it out of circuit. Obviously, be aware of it's in circuit orientation - very important.

[sabre504]

Re: Bridge rectifier

Measured as pin + 1 ~2 ~3 -4

Red probe on + 2=1 3=1 4=1

Red probe on 2 3=667 4=001 +1=594

Red probe on 3 4=1 +1=597 2=1

Red probe on 4 +1=597 2=001 3=665

But am getting continuity bleep between pins 2 and 4
If this makes sense

[sabre504]

Re: Bridge rectifier

Oh i understand that it takes in ac volt and converts to dc to the main cap
but as it blows fuse i cant test volts to see if they are correct.

So if the bridge is failing then it may be letting ac voltage thro or too high a dc voltage to blow the thermistor or am i barking up the wrong tree.

Sorry still not 100% on some of the inner workings of smps done lots of caps and transistors, just the first time i have come across this problem

[Th3_uN1Qu3]

Re: Bridge rectifier

Well the 001 you get between 2 and 4 and between 4 and 2 indicates the same thing as the continuity beep - the bridge is shorted.

Diodes only conduct in one direction. If after getting a reading on a diode you swap the leads around and find your meter reading anything else than "over", the diode is bad, or there is another component in the circuit that influences the reading. That's why you always test diodes with one lead lifted from the circuit.

[Dgtech]

Re: Bridge rectifier

Treat it like 4 diodes. Use the supplied diagram for which pin is which (Anode - Cathode). One way of one diode should conduct and the other way should not. Test all 4 diodes and see what you got. If any should conduct in both directions, failed test.

What is the part number? I will start looking for a replacement.

[sabre504]

Re: Bridge rectifier

Thanks guys for all the info and help i have solved it ,
took out the bridge and still had same results dug around in my spare boards box and found one the same and it had same thermistor .

Took these off tested ok so thought nothing to lose so soldered them in and hey presto all working and yes before you say it i know i am going to order new parts.

Just one point i cant find specs for the thermistor markings are
SCK
054 assume sck is the make ?
Found a spec sheet but there are more numbers on the lists oh and it is green in colour

[Th3_uN1Qu3]

Re: Bridge rectifier

I never understood thermistor markings either. If you can figure it out pleas let me know.

[shovenose]

Re: Bridge rectifier

Thanks guys for all the info and help i have solved it ,
took out the bridge and still had same results dug around in my spare boards box and found one the same and it had same thermistor .

Took these off tested ok so thought nothing to lose so soldered them in and hey presto all working and yes before you say it i know i am going to order new parts.

Just one point i cant find specs for the thermistor markings are
SCK
054 assume sck is the make ?
Found a spec sheet but there are more numbers on the lists oh and it is green in colour

if its working fine why buy a new one?

[sabre504]

Re: Bridge rectifier

Just on the strength that they are second hand bits and may not last.

But then again they may outlast something else may go pop

[toyin]

Re: Bridge rectifier

Good Day Guys,
I have a similar problem with a power board that keeps blowing a fuse. and I have followed your recommendation thus far and I have this from the bridge rectifier readings out of the circuit: please is it good or bad
+1 ~2 ~3 -4 (pin)
Red probe on + 2= 1 3= 1 4= 1
Red probe on 2 3= 1 4= 1 1= 655
Red probe on 3 4= 1 1= 656 2= 1
Red probe on 4 1=1433 2= 652 3= 653

thanks I would appreciate this

[selldoor]

Re: Bridge rectifier

OK I never did like that way of setting it out I prefer this which clearly shows the two diode results

1 2 3 4
+ ~ ~ -


1 to 2 = 1 ??? OHMS OR "1" (Meaning over scale)
2 to 1 = 655
3 to 4= 1
4 to 3 = 653
1 to 4= 1
2 to 3= 1

IF all the 1s are "1" meaning overscale then the bridge is OK

Rather than risk blowing components perhaps you can use a 75w filament bulb
in series with the positive mains input. This explains it but doesnt need to be that complex.
http://antiqueradio.org/dimbulb.htm