DC blocking for power xformer

[salomonander]

hey,
can anyone point me towards a circuit that will prevent DC from entering a mains torroid xformer?
i do have some units that hum - with no apparent ground loop issues. i want to see if this may help.

cheers
jakob

 

[radardoug]

Wheres the dc coming from? Do you understand how transformers work?
There is no d.c. on the mains side, and only dc on the secondary side when you rectify the a.c.

 

[salomonander]

Wheres the dc coming from? Do you understand how transformers work?
There is no d.c. on the mains side, and only dc on the secondary side when you rectify the a.c.

thanks. no im not a transformer expert by any means but i did read that sometimes hum can be introduced when DC enters the primary through dirty mains in the first place. is this wrong? cheers

actually when i think about it all of my gear that uses torroids show hum in some way. anything using a "standard" power xformer is quiet.  this is measuring not listening to physical noise

 

[radardoug]

Toroids are generally quieter than conventional cores because the toroid encapsulates the magnetic field.
You need to work out what the hum source is.
1/ Earth loops.
2/ Hum from un filtered supplies.
3/ Hum from transformer fields.

All of these are different. They will give different symptoms.

To do this you need PROPER test equipment. Put the item on the bench and MEASURE.

It is very unlikely that you have d.c. coming from the mains.

 

[salomonander]

Toroids are generally quieter than conventional cores because the toroid encapsulates the magnetic field.
You need to work out what the hum source is.
1/ Earth loops.
2/ Hum from un filtered supplies.
3/ Hum from transformer fields.

All of these are different. They will give different symptoms.

To do this you need PROPER test equipment. Put the item on the bench and MEASURE.

It is very unlikely that you have d.c. coming from the mains.

cheers mate,
i will look into all of these. am i right in that i could rule the dc thing out by simply connecting my multimeter to the ac socket and measure for dc?

 

[salomonander]

ps: are there any other issues that can cause 50hz hum? im just rying to find out what im looking out for. could a direct connection of signal and chasis ground be a cause as well?

ps2: all of these units show hum without any input connected

 

[ricardo]

http://sound.whsites.net/articles/xfmr-dc.htm

I see no one is interested in Rod Elliot who deals with this exact subject in great & practical detail.

 

[radardoug]

Now thats how ******** gets on the internet!  The mains network is an A.C. network! The longterm signal on the mains is ALWAYS a.c. You CANT pass d.c. through a transformer!
While it is possible for the mains to have harmonic distortion of the waveform, it is ALWAYS a.c.

 

[Audio1Man]

A word of caution. NEVER HALF WAVE RECTIFY  THE OUTPUT OF A TOROID. The core will become biased and the transformer will make HUM and even cause a major overload and can look like a short circuit.
Duke

 

[moamps]

While it is possible for the mains to have harmonic distortion of the waveform, it is ALWAYS a.c.

The problem appears if the core of the transformer is non-symmetrically  saturated.
Do you  think that Bryston engineers are so stupid?
http://www.bryston.com/PDF/Schematics/6BSST2_SCHEMATICS.pdf

 

[PRR]

> am i right in that i could rule the dc thing out by simply connecting my multimeter to the ac socket and measure for dc?

On general-purpose meters, the 230V AC will overwhelm the meter's DC input circuits.

I have a distantly related problem trying to measure dirt-rod resistances in the presence of 3V of stray AC around my property.

Follow the wires out to the street and peel-open the power company transformer. (No! Don't!) Your side is a hundred turns of fairly fat wire. The DC resistance is likely to be far-far below one Ohm. You won't have "any" DC across it unless your neighbor has a defective electroplating tank. (Huge unbalanced rectifiers.)

If still looking for trouble--- A >100VA load at 230V is like 500 Ohms. If you have over 40uFd in series, most of the 230V gets through to the transformer. That is a big, high-voltage, non-polar capacitor! It is readily available as a "motor cap", used in large motors and compressors. As a test, a "start cap" will work for many seconds, but will fail in steady use. A "run cap" should hold in all-day use, though these caps are the main reason (after Freon leaks) that refrigerators and air conditioners fail in a few years. 47uFd is a very large Run Cap, would imply a several-HorsePower motor, so may not be in stock at small appliance repair depots.

 

[CJ]

odds of turning off your pwr xfmr at zero crossing are better than the lotto, but not much, try turning the power off and on a few times and see if the hum is reduce, we are trying to degauss the core by applying a spike opposite the bias of the core,

this is also a good test to see if your circuit is bullet proof, 

darn winding machine wants to keep turning even with the variac at zero because of this saturation thing,

 

[ruffrecords]

The problem appears if the core of the transformer is non-symmetrically  saturated.
Do you  think that Bryston engineers are so stupid?
http://www.bryston.com/PDF/Schematics/6BSST2_SCHEMATICS.pdf

Except in the posted schematic schematic, the mains transformer is connected directly to the mains input. The 'DC Filter' connects only to the LED of an opto isolator.

Transformers should be designed such that under normal conditions they don't saturate.

Cheers

Ian

 

[moamps]

The 'DC Filter' connects only to the LED of an opto isolator.

The power transformer IS connected thru DC filter, look a bit closer (yellow PS simbol on right side of the block). Tx 1 is a slave small power transformer used for remote power on.

Transformers should be designed such that under normal conditions they don't saturate.

You should tell that to electric power companies.

 

[My3gger]

A friend and i are arguing from time to time about local winder who seems to be good for small toroids and tries to save material for big ones. So all of my gear which works with small signals is very quiet, while he mostly had problems with high power amps. He changed winder and is using EIs, i'm still very happy with this guy.
Are your power calculations ok, sometimes people forget Iac isn't the same as Idc. Ian Bell has good paper about it, Hammond too.
Like others said toroid could be saturating, i would check for that because they are always quite a bit more quiet than any EI. I only needed to chase lowest hum by turning toroid transformer one time, so it could be ground problem too. This happened a lot more often years ago.

 

[ruffrecords]

The power transformer IS connected thru DC filter, look a bit closer (yellow PS simbol on right side of the block). Tx 1 is a slave small power transformer used for remote power on.

Ah, I see it, via the SCR. The neutral is connected there via the 'dc bocker''. Unfortunately the shorted  diode bridge in the  'dc blocker' will pass dc.

Cheers

Ian