Do somebody know why there are large copper strips inside a chopper transformer that I've hacked and dissembled ?

Does anybody know what is their purpose? What's the mathematical theory behind them?

What will happen if I reassemble that chopper transformer without those big strips ?


enter image description here

  • \$\begingroup\$ Could be please be kind to explain the reason for -1? \$\endgroup\$ – Standard Sandun Oct 27 '13 at 10:24

Providing the copper doesn't completely totally surround the wires (forming a shorted turn) it's likely that it is an EM screen intended to reduce emissions. There will be eddy current losses but these are probably acceptable.

The pictures isn't great but it looks like there is a wire soldered to one end of the copper strip and this will further enhance the shielding quality of the copper by Earthing it on the PCB it's mounted on.

  • \$\begingroup\$ Thank you sir. That's everything does have a reason. I don't know why somebody voted -1 here. \$\endgroup\$ – Standard Sandun Oct 27 '13 at 10:23
  • 2
    \$\begingroup\$ I've upvoted your question because it seems a reasonable question to me. Someone might have downvoted because they felt the question didn't indicate you'd done enough research before asking? \$\endgroup\$ – Andy aka Oct 27 '13 at 10:33

The purpose of this screen is to reduce the capacitive link between the primary and secondary windings of the transformer.

The strip forms an open turn and is grounded.

In the low frequency transformers, such a screen is formed by one layer winding with one end grounded.

You can see the difference between a transformer without and with screen on the following schematic:


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Your answer is reasonable too. I would like to read an application note on how to reduce capacitive link. So I could know the math behind that and know how much strip size would be needed there. Thanks for your answer too sir. \$\endgroup\$ – Standard Sandun Oct 27 '13 at 10:40
  • \$\begingroup\$ i would consider this a possible purpose not a definite purpose. \$\endgroup\$ – Andy aka Oct 27 '13 at 16:35
  • \$\begingroup\$ @Andyaka - only if the screen is on top of windings, it can be intended to reduce emission. But it is between the primary and secondary. \$\endgroup\$ – johnfound Oct 27 '13 at 19:27
  • \$\begingroup\$ If it is between primary and secondary it could be a safety earth as per Brian Drummond's answer. It could also be a means of making the core saturate ie it is what is known as a constant voltage transformer. \$\endgroup\$ – Andy aka Oct 27 '13 at 19:57
  • \$\begingroup\$ If the screen is between primary and secondary it can keep secondary side noise out of the primary circuit. This will reduce noise conducted back into the mains supply, which is part of the emissions tests. In that respect johnfound and @Andyaka are making the same point. \$\endgroup\$ – Brian Drummond Nov 10 '13 at 13:14

A third purpose for an interwinding screen - on appliances that have a mains earth connection - is safety.

In the event of the high voltage primary insulation breaking down - perhaps through over-temperature if the supply is heavily loaded - the interwinding screen will short the primary side to earth, thus protecting the secondary side (and you!) from dangerous voltages, and causing fuses or breakers to disconnect the supply.

This obviously does not apply to "double insulated" devices with 2-pin mains connectors and plastic enclosures, which are protected in different ways.

But a desktop PC power supply (with metal case and earth connection) this is an important part of the safety system.


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