transformers with electrostatic shields

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Just another Moderator
Joined 2003
Paid Member
Well, I've had an ongoing problem with noise in my amp for ages. One of the things I did to try and reduce it was to make a lead case to put over the torroid. I made it out of lead sheet I got from the hardware store.

The result? Well the noise reduced quite substantially (probably about half the level.... haven't actually done measurements) but didn't go away completely. I'm not sure if this is due to poor construction of the lead sheild or whether some of the noise is simply being "induced" into the leads of the transformer.

This is actually the second transformer too. It was better than the first one, but I think maybe I got another dud, after all toroidals are supposed to have very low radiation of noise aren't they?

Tony.
 
According to Lynn Olson the figure is a reduction of 40db of capacative coupling of noise..(Don't know what noise and at what frequency...) This obviously is also dependant on type of transformer...i suppose a toroidal needs a screen more than other types since capacitive coupling is high (i believe) in that type.

Electrostatic screens are almost standard in medical and measuring equipment.

How much this technical advantage translates into good sound is ofcourse debatable...but since the places I buy transformers have them standard...I'm not complaining...and only use transformers WITH screens. I just consider it good engineering practice. I.e. garbage that is not in your psu in the first place won't have to filtered out. But to answer your question ...I have never in the same circuit compared transformers with or without screen.

Cheers,
Bas
 
Doesn't electrostatic shield mean a shieald to protect against electrostatic charges/fields?

What kind of fields do we get from a standard powertransformer?
Isn't it mainly magnetic fields.

I found this at:
http://ourworld.compuserve.com/homepages/g_knott/elect102.htm

"Some transformers have an electrostatic screen between primary and secondary. This is to prevent some types of interference being fed from the equipment down into the mains supply, or in the other direction"

Electrostatic shields is also used in audiotransformers to reduce the capactive coupling.
 
Re: copper tape

jackinnj said:
3M makes copper adhesive tape which you can use for an exterior shield --

it's great stuff to have around anyway -- as I only "do" single sided boards I use the copper tape for ground planes, sometimes for traces.

jack


Where do you get this tape? I've been looking for exactly that, and haven't been able to find any.

Thanks,
Zach
 
Yes the main fields (if not the only fields) are magnetic.

But noise on the primary gets coupled capacitively. (And I suppose are to small to be coupled via electromagnetism)

Just like a capacitor passes high frequencies...So does the transformer....

the electrostatic shield is like adding a shield in between the two windings that cut down on that capacitive coupled noise.

Similar to the electrostatic screen on your monitor.

Cheers,
Bas
 
Just like a capacitor passes high frequencies...So does the transformer....

At the voltage involved, the capacitive current can be quite large, despite the small coupling capacitance in question.
That's why there are sometimes iprovements depending on which way you plug your equipment into the mains sockets.

I once had to learn this the hard way ........ :(

It is a pity that x-formers with a shield between primary and secondary are not that common anymore nowadays.

Regards

Charles
 
As you say, it seems rather difficult to get transformers with
electrostatic shields (I have two, but they were industrial surplus).
However, as I understand, an R-core transformer should also have
a very low capacitive coupling. Although a bit expensive, they seem
to be available nowadays. It has been discussed on the forum
before and I think there is at least one french and one german
company selling R-cores. Can't remember where right now, though.
 
If you can't get a transformer with an ES screen between the primary and secondary it isn't a major problem. Bypass the RF noise to Earth via some very small capacitors. Without checking, I think I remember seeing something around 10nF being used for this, with one from each end of the secondary, effectively making it 20nF to the noise.

Morgan Jones does this in one of this supplies. He also goes a step futher and adds a pair of low value induction coils. I think he only uses this method for the LV supply however. At the front of his supply, before the transformers, he uses Class ?X2?, mains safe, caps and another pair of low value coils to futher remove RF noise.

This method would probably cost less than $10 in the US.

Morgan Jones mentions that the more serious problems are found when there is no ESing between the secondary coils themselves. If this is the cases, common mode noise from the HV supply can be coupled and superimposed on the differential output of the other supplies, such as the bias and heaters.

Best wishes,
John



edit - just got the book, here's the method he suggests. if you don't have his book, buy a copy!

.......................15T 1.6mm on ferite
........................______
Mains L--------- ~~~~~------------ To mains trans
...................|................|
.............class x2.........class x2
............470n cap.......470n cap
...................|................|
Mains N----------~~~~~------------ To mains trans
.........................______
........................15T 1.6mm on ferite

Ignore the dots. The Caps MUST be Class X2 for safety.

The LV filter is the same but remove the caps and wind the coils as 25T of 1mm on a 30mm core each. Attach a 10n (FKP2) cap to the + and - on the right of the coils and tie the end of these caps to Earth.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.