EMI choke before switching power supply

[erachiel]

I have a siemens choke 1A/2x27mH 250v 03.91 that I had cannabalized from a VCR.
Can I put that choke between 220AC plug and commercial switching power supply (IMPUT 240V 50/60 Hz 0.4A Output 14VDC 1A)...

[johansen]

yes

[jackand08]

I don't see any problem with that, but I would be doubtful whether it has much of an effect on a SMPS. They themselves create a lot of EMI and the emissions from the PS itself would probably reduce the effect that the choke has on the output. If you are putting it in to prevent the SMPS from injecting noise onto the mains, I wouldn't be too sure whether the choke would have the desired effect either.

[TechGuy]

27mH is an awful lot of inductance for EMI choke. Such a high inductance is going to limit current input at the zero voltage crossing (maybe for a lot longer depending on how big the core is). Did you mean 27uH?

[johansen]

27mh refers to the common mode inductance, ac currents cancel.

[pmkap]

Its a common mode choke, where that 27mh inductance is not unusual for a small 1a rated choke. Way to improve its common mode efficacy is by following it with Y2 caps to ground. While a measurable improvement, adding more crap to the ground is IMO, rarely a positive. The CMC will also have substantial leakage inductance (100-500uH), useful for filtering normal/differential mode noise. Its well worth the effort to flank the CMC with X2 caps (typically .47uf) and bypass X2 caps, if you see fit (which I do). This will further attenuate normal/differential noise both to and fro.

[kjlonergan]

Also be aware that adding Y caps increases what is called "leakage current" from the AC mains into the safety earth connection. Should this connection ever fail, the effect of leakage current is to raise the chassis to a voltage which now has the potential of becoming leathal.

Y caps are also "special" in that they need to be approved specifically for this use. Y caps are constructed such that their failure mode is always to go open and never to short.

Unless you really have a good reason to play with additional EMI filtering it might be better to assume the designers of the power supply did their job and no additional filtering is needed.

Cheers,
Kevin

[pmkap]

Originally Posted by kjlonergan

.........
Unless you really have a good reason to play with additional EMI filtering it might be better to assume the designers of the power supply did their job and no additional filtering is needed.

Cheers,
Kevin

An assumption that I've never found to be true. Such switching supplies are efforts whose only objective is comply with FCC regs (leastwise here in the States), which to a minor extent addresses crap going out, but never in.

NuForce amps, the poster child of a switching supply switching amp as high end, is unlistenable without proper power conditioning. I've yet to encounter a DVD or CD player under $10,000 that doesn't substantially benefit from appropriate series inductive, parallel capacitive conditioning.

But trying to build anything based upon a found component of unknown (detailed) characteristics, is a waste of everyone's time, including the finders. Use something appropriate for the task(s) where you actually know what you've got.

[Sch3mat1c]

Originally Posted by pmkap

I've yet to encounter a DVD or CD player under $10,000 that doesn't substantially benefit from appropriate series inductive, parallel capacitive conditioning.

But trying to build anything based upon a found component of unknown (detailed) characteristics, is a waste of everyone's time...

Is that based on series of double blind tests? If not, please don't waste everyone's time...

Tim

[erachiel]

First of all thanks for all the answers.
I found the b[IMG]file:///C:/Documents%20and%20Settings/Soldier%20of%20God/Desktop/Filter%20with%20X2%20and%20Y2.jpg[/IMG]est possible s[IMG]file:///C:/Documents%20and%20Settings/Soldier%20of%20God/Desktop/Filter%20with%20X2%20and%20Y2.jpg[/IMG]olution (for me).