Stray capacitances in transformers

[0xFFF0]

I measured the stray capacitance of a few transformers. This goes from 20pF (3VA) to 300pF (180VA). A good transformer with separate chambers (30VA) has 40pF only. Does anyone know in which range the measured values are, where the transformers have shielding between primary and secondary?

[julian1]

For large mains transformers, Fluke state that with a shield between primary/secondary one can get to 1pF or below. 


> The guard shield also surrounds the secondary winding of the DVM power transformer utilizing a foil type shield with the secondary winding wound completely separately from the primary windings and other secondary windings that feed cligital circuits outside of the guard. By doing this, the capacitance from the guarded secondary winding to the primary winding and the transformer core can be kept typically below one picofarad, This causes a reduction in the common mode current.

p98, Fluke Calibration Philosophy In Practice

[profdc9]

If you're looking for low stray capacitance transformers, some of the toroidal transformers used for audio applications are quite low.  Some have electrostatic shields to achieve low stray capacitance.

[jonpaul]

The Ultra Isolation mains trsf can acheve 0.001 pF in shielded guarded measurement.

Use a trsf with an internal Faraday shield if you need low capacitance.

That afftects ONLY CMRR not DMRR.

J

[moffy]

I always wonder where the best place to ground the shield is, since both the primary and secondary are coupled to it.

[Kleinstein]

It is normal that a larger tranformer will have more capacitance. Similar to cables the capacitance scales linear with the dimensions and than a factor dependending on the shape / distance ratio.

One has to distingush between the simple 2 wire measured capacitance and the coupling capacitance with a grounded shield / core. The 2 wire capacitance can't get very low. With shields the coupling capacitance can get very small, but there is than also capacitance between the windings and the shield(s) and if present between the 2 shields.  For some effects the capacitance to the shield is also relevant, not just the winding to winding coupling.


Ideally one would have 2 shields. The more relevant is usually the shlied at the primary (e.g. connected to protective earth).
The purpose of a secondary shield is to avoid capacitance from different parts of the secondary windings. The voltage of the windings and local capacitance can cause some common mode current. The 2nd shield gets more relevant with a relatively large secondary voltage.  In some cases (some DMMs) the 2 nd shield is connected to circuit shielding and an external connection.

[jonpaul]

Hello again: To clarify the issues raised, a moment please!


Topaz and other made many sizes and types of "ultra-Isolation" transformers, with single, double and even triple shields to 0.001 pF.
(first pix) We used many up to 5 kVa.

Small signal level double and triple shielded transformers are used in sensitive applications like EEG, EKG,  seismic amplifiers, mic preamplifiers input

For theory of shields see 1960s..1970s classic  texts on noise reduction and EMI shielding, eg
Noise Reduction Techniques in Electronic Systems, 
by Henry W. Ott  for shields in transformers, multiple shields and wiring for grounds.

(second pix)

Read, learn and enjoy!

Jon

[Andy Chee]

Here's a snippet from Keith Billings' Switchmode Power Supply Handbook (page 1.45)



https://dl.icdst.org/pdfs/files/e0dcfc20dbf61d43f9f0285c2b0de6e5.pdf

[moffy]

Hello again: To clarify the issues raised, a moment please!


Topaz and other made many sizes and types of "ultra-Isolation" transformers, with single, double and even triple shields to 0.001 pF.
(first pix) We used many up to 5 kVa.

Small signal level double and triple shielded transformers are used in sensitive applications like EEG, EKG,  seismic amplifiers, mic preamplifiers input

For theory of shields see 1960s..1970s classic  texts on noise reduction and EMI shielding, eg
Noise Reduction Techniques in Electronic Systems, 
by Henry W. Ott  for shields in transformers, multiple shields and wiring for grounds.

(second pix)

Read, learn and enjoy!

Jon

I found your diagram confirming what I thought about the couplings, I guess the shields ensure that the noise becomes  CM rather than DM across the secondary, maintaining a common potential that is coupling across all of the secondary equally. I will have to look up some of your references.

[jonpaul]

Moffy:

The shields ONLY affect CM noise.

DM noise is faithfully transmitted by the P-S coupling to the limit f the transformer BW.

DM to CM conversion is another issue.

Another good source on interfetrence and noise (for audio) is SYN AUD COM of Pat Brown and Don Davis. See the AES papers.

Jon

[moffy]

Moffy:

The shields ONLY affect CM noise.

DM noise is faithfully transmitted by the P-S coupling to the limit f the transformer BW.

DM to CM conversion is another issue.

Another good source on interfetrence and noise (for audio) is SYN AUD COM of Pat Brown and Don Davis. See the AES papers.

Jon

Yes, a good point, I was thinking that the primary and secondary are not equally connected elctrostatically across all the windings so there could be some DM, electrostatically without a shield, and that the shield provides an equipotential point for the secondary coupling. But that would be relatively minor, as the primary shield should be connected to a low noise point on the primary side to catch the electrostatic part of the switching noise and provide a low radiation potential for coupling to the secondary.