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Question about chokes

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 09 October, 2013 » Question about chokes « Previous Next »

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bruce
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Username: bruce

Post Number: 983
Registered: 04-2008

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Posted on Monday, 23 September, 2013 - 11:41 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi,
In mains power supplies there is usually a series of components which are placed ahead of the rectifier and reservoir capacitor. These are: a capacitor, X-rated, 0.1 to 0.47uF, not entirely sure why; a varistor to prevent spikes on the line; an NTC inrush limiter; and two in-line chokes. It's the chokes I want to ask about.
The chokes are in pairs: one for L and N. They are inductively coupled as well, not sure why. Lowish inductance, around 10uH, IIRC. What are they for? Is it a noise-suppressor, or another type of inrush limiter?
bruce
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epithumia
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Username: epithumia

Post Number: 896
Registered: 06-2006

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Posted on Monday, 23 September, 2013 - 01:24 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

It's to prevent noise from your circuit getting back onto the mains. (And perhaps vice-versa).

Last one I built (which went through formal EMC testing) had 100nF class X between live-neutral, two 47nF class Ys from live and neutral to earth, and a 25mH common mode choke. All caps on the mains side of the choke.

I leaned to take manufacturers' datasheets with a pinch of salt - the PSU I was using said to use two 220pF class Y caps from live and neutral to earth on the PSU side of the choke.. and the damn thing failed EMC testing until I removed them!

(Now that I know this, the 47nF caps could perhaps have been smaller.)

Epi

(Message edited by Epithumia on 23 September, 2013)
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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basementboy
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Username: basementboy

Post Number: 138
Registered: 05-2012

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Posted on Monday, 23 September, 2013 - 09:06 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I'd second Epi. EMI filters. Always take a box of chokes & caps to a EMC test to make sure it passes the first time!
You often get chokes & caps on IO lines too, plus zeners to make sure you pass the static test.
Generally omitted for hobby work.
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ian_field
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Username: ian_field

Post Number: 138
Registered: 05-2012


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Posted on Sunday, 29 September, 2013 - 09:33 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You can usually make a decent blocking oscillator/inverter from those common mode chokes - once you get the phasing right.

Some have generous window areas so you can thread a secondary through.

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