Bench testing a full wave bridge rect. Can they leak? Is is common?

[fastline]

I am fighting some really odd problem in an audio D class amp that is emitting a loud hum from the driver with only power going to it. Many have indicated the PS is highly suspect but I can find nothing wrong so far. Using a cap meter on my filter caps, they read about 15% higher than stamped.

My next thought was to take a closer look at the bridge. Diode testing shows it good but I am wondering if powering it could show a different story? Is that unlikely? Just quickly applying 120V to it, I get 107V of DC with an RMS DMM which is in the ballpark.

Should I do some other testing to prove it out or look at something else?

 

[Reloadron]

My next thought was to take a closer look at the bridge. Diode testing shows it good but I am wondering if powering it could show a different story? Is that unlikely? Just quickly applying 120V to it, I get 107V of DC with an RMS DMM which is in the ballpark.

Yes, you could have a diode or diodes breaking down. This is where a scope would come in handy. However, your quote here is a little confusing as you say 107 VDC with an RMS DMM? Assume the AC functions of the DMM are True RMS but you measured DC correct? You could measure the bridge output using the AC function and see what you get.

Ron

 

[fastline]

I guess I was referring to the AC ripple that you still have on the DC side of a rect without caps. When I tested for AC on the DC side, I was a little confused because sometimes I was able to get 30-40VAC, and if I switch the probes around and such I would have .5VAC which is probably OK.

I do have a scope but just not sure what I should expect to see in the DC side and what I need to do to definitively test out the component.

 

[Reloadron]

Just so we are on the same channel, this system is rectifying 120 VAC mains power? There is no transformer prior to the bridge? Typically you would use the scope to measure ripple content of the DC level after the filtering capacitor(s). You would AC couple the scope and see what you get for a waveform. Something seems a little strange about the numbers you are seeing.

Ron

 

[fastline]

Yes, this is connected directly to the AC 120V and is rectified from there. Remember that I did not have any filter caps on the rectifier. I assumption is that my meter is picking up on the ripple right now. My Fluke RMS DMM is indicating 48VAC on the DC side. I have a known good one here some probably try something there. I am not sure how much voltage my scope is good for so did not want to try just yet. I also learned the hard way that the neg side of the test leads is direct to circuit ground and will bow fuses every time...

 

[crutschow]

Yes, this is connected directly to the AC 120V and is rectified from there. Remember that I did not have any filter caps on the rectifier.

No filter caps? In your first post you said you were measuring across the filter caps. Where are those located?

You should not be operating an amplifier directly off the line without an isolation transformer. Accidentally touching the circuit when grounded can be lethal. The connections to the speaker/load can also be lethal.

 

[Nigel Goodwin]

From a practical point of view, from MANY years experience, bridge rectifiers go S/C, VERY occasionally you may get one go O/C - but I've never seen one go leaky.

As in your other threads, hum is typically causes by the amplifier drawing too much current, particularly in this case as the capacitors are puny.