An off the wall transformer question/problem * * * SOLVED * * *

[99tito99]

Hi Folk:  Thanks for all the input, it got me thinking a bit more clearly/analytically.

It turns out it was a short, but on the mains side.  I lucked out last week when I turned on the transformer with my clamp-on amp-meter in hand and it started humming.  So I clamp-on to one of the secondary output lines and during the humming the current dropped (from ~3.5A to ~3.0A).  Ah ha, so it’s not loading-up on the load side, it could still be a bad/intermittent connection on the secondary, but my thought was the problem was more up-stream.  I started jiggling connections in the plug-box when a light went on in my head ---> This line has a GFI on it.  The GFI is in another exterior plug-box that all other exterior outlets are connected to.  So, I opened the plug cover and all I did was brush my finger across the front of the GFI and when it brushed across the reset button (Note: not pressed just brushed across) the GFI stated to buzz.  And to quote Dave, "Ta-Da."  A dodgy GFI that got some water into it, as the teardown revealed obvious corrosion inside.

Again, to quote Dave, “WRONG.”  After changing the GRI, same problem.

Anyway to make a long story a bit shorter, it turned out to be the mains breaker.  The problem was that the spring clap/connector that engages one phase of the 120v bus was spread apart and apparently making intermediate contact.  I bent them back and all is one.

Thanks again. Cheers, Mark

PS
My Wife laughed at the sell the Dog comment; surprising since it is her Dog and she is a Veterinarian.

PPS
The Dog also barks at my laser printer (Brother 9325CW), which I think has flybacks to generate the high voltage that make a high frequency sound.

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I have outdoor lights that are powered by a 12 volt transformer.  It is a Malibu brand with two big transformer in a metal weather proof box and a motor driven clock timer.  It is rated at 600 watts, each transformer is 300 watts. When it is on it gives the normal 60 cycle hum. I only have a few lights on it so it is only pulling about 60 watts.

So, on a “semi-random” basis this transformer produces a very loud hum (easily heard 50 feet away).  When it is running (usually at night) this may occur once every few hours, but when it does it usually occurs several times (3 to 4) over the course of a few minutes. The individual “hums” last about 4 or 5 seconds and then back to the normal hum.  I took it apart and the transformers look OK, the laminations are not loose or separated, the spools and windings are secure, tight and do not appear to move. I even brushed into the windings and laminations some automotive lacquer thinking to glue down anything loose. But no change, still gives the big hum. So what, what’s some extra hum? The main problem is when this occurs even though the transformer is outside on the far end of the house our Dog go nuts, and at 3AM this is not good.

So, any ideas why this is happening (I don’t mean the Dog barking a 3AM ). Cheers, Mark

[kolbep]

Sounds like there is an intermittent short on the circuit on the load side of the transformer.
Do the lights on thecload side flicker, or go dim, when it buzzes?

And, sell the dogs, and get a cat, that will sort out the other problem 

[Wytnucls]

Since they are two transformers close to each other, every now and then, they may get into a resonance mode with the metal box, producing some loud noises.
Putting some dampers on the metal box may solve the problem.
Mounting the transformers on rubber blocks could do the trick too.
Hope that helps.

[G7PSK]

The timing could well be a clue as to what is going on. As stated by Wytnucles there could be a resonance problem, the frequency  of the mains alters during the day dropping under load so the utilities run the frequency up at night in order to keep synchronous clocks correct , it could be that this is triggering the resonance as the frequency is either rising or falling it may well occur during the day but there is too much background noise to hear it, the only cure would be to change the resonant frequency of the case and transformer cores, try coating the box with bitumen and the transformers with epoxy paint.

[NiHaoMike]

Since you're only using 60W, what happens if you disconnect one of the transformers?

[mmagin]

Perhaps this is just some mechanical problem in the motor-driven timer mechanism.  Did you look inside that?

[TMM]

+1 for mechanical resonance in the metal case. I've had transformers in audio amplifiers do this. You could try adding some mass to the large flat sections of the metal case to dampen them, and add foam window sealer between parts of the case so they can't rattle against each other.

A current probe and a data logging multimeter would also be handy to see if there is any electrical correlation with the mechanical noise or if it is instead just a mechanical resonance that builds over time.