Transformer Mystery

[germaniohm]

My problem is this. I have four 12v transformers working and they cause bad humming on an unrelated electronic device that I am using. The four transformers are 24 inches from the device that I am using. But the interesting thing about my problem is that if I remove two transformers, that is to say, if I pull two of them from the single surge protector/power tap that I have all four plugged into, the humming stops completely. If I add a third, the humming returns. If I plug a third transformer into a receptacle across the room, the humming returns. Can anyone please tell me WHY there is this two transfomer limit? I really need to have all four transformers working and my electronic device as well. Thank you.

 

[mvs sarma]

Welcome Germaniohm, to electro-tech-online.

Imagine each transformer having load of different power factor. The currents flowing will be at different phase angles. This causes the additional ripple that you hear, as I suppose. Please try adding a 2µF capacitor/400V ( in case of 230V supply) across each transformer. They would try to compensate for the Power Factor.

 

[germaniohm]

MVS SARMA:
How would I apply the 2 microfarads/400 volt capacitors to the transformer? Thank you.

 

[mvs sarma]

MVS SARMA:
How would I apply the 2 microfarads/400 volt capacitors to the transformer? Thank you.

Across the primary( parallel to primary)it is 2uF/400V AC capacitor and NOT electrolytic cap

 

[germaniohm]

MVS SARMA:
Thank You.
Do you mean:
1.Locate the primary (positive conductor) on said 12v transformer.
2.Sever the primary conductor and strip the insulation off the newly cut ends.
3.Solder the 2microfarads/400AC capacitor onto the primary conductor ends (according to polarity if there is one)
4.Tape the newly soldered wires with pvc tape to reinsulate the pimary conductor/2microfarads-400AC capacitor splice?
Thank You

 

[JimB]

Far too many unknowns here to make any diagnosis or suggested cure.

How big are the transformers, what is their power rating?

What is the electronic device which is humming?
In what way does it hum?
Does it have a loudspeaker which hums, or does it just make a noise?

What is the power supply to this place? Is it on the end of a long overloaded cable?

JimB

 

[Sceadwian]

I would say buy one 12 volt transformer that can supply power to the same devices as the current bank of four would be better. Adding more components to the existing setup is at best a stop gap measure, you're better off taking a broad look at exactly what you're trying to do and simplifying it as much as possible. So the first question I have to ask is why 4 12 volt transformers instead of one? If you need to have different devices on at seperate times run swithches after the transformer.

 

[germaniohm]

Sceadwian:
I was not aware that one 12v transformer could take the place of four 12v transformers. How is this done?

 

[germaniohm]

JimB

Far too many unknowns here to make any diagnosis or suggested cure.

How big are the transformers, what is their power rating?
There are 4 transformers that are 12 volts DC each and range from 300 to 500 milliamps.

What is the electronic device which is humming?
The device is called a "Listen Up" in the United States.

In what way does it hum?
Over my headphones.

Does it have a loudspeaker which hums, or does it just make a noise?
Headphones.

What is the power supply to this place? Is it on the end of a long overloaded cable?

I have a "DigiVue" video-camera card. It has a four camera capacity. I have my computer in my den which is in the rear most part of my house. All four cameras plus the "Listen Up" is located in the front of my house to watch the street as well as listen. My "Listen Up" is attached to a TRS stereo plug and uses the line intended for sound as of course there are three lines, one for the volts, one for the camera and one for the sound. Where the sound, volt and camera lines terminate in my den is where I have attached what is known as an "isolation transfomer" to the sound line to kill the aweful "DC hum" I've read it once described. On the other side of the "isolation transformer" I have attached by solder a TRS plug to plug into the "MICROPHONE" female plug which is on my "Xtreme" sound card. Everything works very well except for the hum.

JimB

I hope this is a sufficient explanation of what I have done.

 

[ecerfoglio]

..................................................
Where the sound, volt and camera lines terminate in my den is where I have attached what is known as an "isolation transfomer"
...................................................
hope this is a sufficient explanation of what I have done.

It may be that your audio isolation transformer is picking up the stray magnetic field from the power transformers.

You may try to increase the distance between the power and audio transformers (magnetic field decays with the square of the distance, so sometimes even a small distance increase - a couple of feet or half a meter - reduces the hum to unnoticeable levels).

Other thngs you may try:

>>> Enclose the isolation transformer in a steel enclosure to act as a "magnetic shield" - it ay even be an used food can, just be sure it is steel and not aluminium)

>>> Verify that the audio signal travels along screened cables, and that the screen is connected to ground at only one point (the ideal setup is a cable with a twisted pair for signal and return - even if the retirn wire is grounded - surrounded by a grounded screen.

 

[ecerfoglio]

Sceadwian:
I was not aware that one 12v transformer could take the place of four 12v transformers. How is this done?

You may use a bigger transformer instead of your four small ones.

If each of your transformers outputs 12 VDC @ 500 mA you may use a 12VDC 2A one to power your four cameras.

 

[ste5442]

This could be caused by many things but if its a PFC issue then you will need to do the math to fix it.
Most devices need to go through radiated emmissions testing before going on sale so radiated noise is 'fairly' unlikely especially if its just a power supply 2 feet away.
Try a filtered multi-plug adaptor since I guess its conducted emmissions.
Alternatively (as mentioned above) try to shield the device to determine whether the interference is radiated or conducted.

S.

 

[germaniohm]

Questions

It may be that your audio isolation transformer is picking up the stray magnetic field from the power transformers.

You may try to increase the distance between the power and audio transformers (magnetic field decays with the square of the distance, so sometimes even a small distance increase - a couple of feet or half a meter - reduces the hum to unnoticeable levels).

Other thngs you may try:

>>> Enclose the isolation transformer in a steel enclosure to act as a "magnetic shield" - it ay even be an used food can, just be sure it is steel and not aluminium)

>>> Verify that the audio signal travels along screened cables, and that the screen is connected to ground at only one point (the ideal setup is a cable with a twisted pair for signal and return - even if the retirn wire is grounded - surrounded by a grounded screen.

May I asked what "screened" cables are? Connect to ground? How might one ground a two wire system? When I rewired my house I "bonded" all the copper water pipes and black iron gas pipes to a 10 foot grounding rod which I pounded into the ground. All my light fixtures and switches and receptacles are grounded. My cameras and "Listen Up" audio device are connected by the standard cable you buy at any electronics store. It's a single black wire that has camera, audio and power cable stuffed into a single wire that terminates in cheap red and white RCA connectors and a coaxial connector for a transformer. I did not mention a very important idiosyncracy and that is the buzzing disappears completely when ANY two transformers are plugged in. When a third transformer is plugged in the buzzing returns.

 

[ecerfoglio]

May I asked what "screened" cables are? Connect to ground? How might one ground a two wire system? When I rewired my house I "bonded" all the copper water pipes and black iron gas pipes to a 10 foot grounding rod which I pounded into the ground. All my light fixtures and switches and receptacles are grounded. My cameras and "Listen Up" audio device are connected by the standard cable you buy at any electronics store. It's a single black wire that has camera, audio and power cable stuffed into a single wire that terminates in cheap red and white RCA connectors and a coaxial connector for a transformer. I did not mention a very important idiosyncracy and that is the buzzing disappears completely when ANY two transformers are plugged in. When a third transformer is plugged in the buzzing returns.

"screened" cables are the ones that have an outer conductor (or shield, usually a wire mesh or else aluminium foil), arround the "active" conductors. Audio or video signal cables are usially screened ones.

Ground** is a relative term, if you have a 2 wire system your ground may be your computer case - you may connect the cable screens to the outer conductor of the RCA (or phone) plugs, or else to one of the screws that close the PC's case.

** (EDIT) I'm speaking of a signal ground, not a safety ground.

A safety ground must comply to your local codes.

 

[germaniohm]

Attenuation?

"screened" cables are the ones that have an outer conductor (or shield, usually a wire mesh or else aluminium foil), arround the "active" conductors. Audio or video signal cables are usially screened ones.

Ground** is a relative term, if you have a 2 wire system your ground may be your computer case - you may connect the cable screens to the outer conductor of the RCA (or phone) plugs, or else to one of the screws that close the PC's case.

** (EDIT) I'm speaking of a signal ground, not a safety ground.

A safety ground must comply to your local codes.

Thanks for your answers. Are you familiar with the term "ATTENUATED"?? Would this be what you are referring to? I've never seen anything screened or aluminum foiled at the Radio Shack where I shop for my electronic hardware. But I have seen a few cables with the term "attenuated" printed on the label.

 

[germaniohm]

Last Question

You may use a bigger transformer instead of your four small ones.

If each of your transformers outputs 12 VDC @ 500 mA you may use a 12VDC 2A one to power your four cameras.

I was wondering if a 12vdc 2 amp transformer would "blow up" or overwhelm the camera circuitry. Is this possible?

 

[RODALCO]

Should be ok, it pays to put a 2 or 2.5 Amps slow blow fuse in the secondary leads of the TX.
Twisting the latter may reduce hum too.

 

[Hero999]

It depends, if the regulation is poor the output voltage could be as high as 20V. If your camara is designed for 12V operation the absolute maximum voltage rating is probably between 13.8V and 16V (depending on the sort of battery its designed for) in which case yes, it will damage the camera.

 

[germaniohm]

Questions

Should be ok, it pays to put a 2 or 2.5 Amps slow blow fuse in the secondary leads of the TX.
Twisting the latter may reduce hum too.

About the camera blow out paranoia, I wasn't thinking. I was thinking how I plugged each transformer in one at a time. I totally forgot that I used the power switch to shut off the cameras when I not at home. So each camera will get the 2 amp jolt anyway. Amateurs like me can be a pain. Anyway, could you please tell me what "the secondary leads of the TX" would be?? Would it be the negative wire that brings AC to the transformer? Is "twisting" as in a helix? For example, take two straight parallel wires and literally twist at one end and continue until you have a double helix? Thank you.

 

[germaniohm]

To one and all. Many thanks and apologies. I just found out that the hum was caused by the volts/amps going through the line that carried the video and THE AUDIO! I was too blind to see it. Too stupid to see it. But when I began unplugging transformers again I realized that the transformer that energized the cable carrying the "Listen Up" audio was the culprit. I just couldn't see it. I should have.