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Old 6th January 2011, 09:22 AM   #1
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Default power transfo as HT choke

did a search but could not find what i was looking for. what happens if i were to use a 50VA 240V:240V power transfo as a choke (CLC styled) to support a B+ of 500Vdc?

Assuming there is a 20V drop from the windings (using either primary OR secondary only) then the terminal voltages should be 500V on one and 480V on the other. this means the 20V does not fall under a HT application as the 500V earlier was referred to GND.

is this safe HT point of view?

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Old 6th January 2011, 09:30 AM   #2
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Transformer usually has a continuous magnetic core.

Choke has an air gap in its core. If you use Txfmr there is a danger that it will saturate.
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Old 6th January 2011, 09:32 AM   #3
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I've always been told that ordinary power transformers shouldn't be used as chokes since they have no air gap and will saturate.

Andy beat me to it...
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Old 6th January 2011, 10:40 AM   #4
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There is nothing to stop you turning an EI transformer into a choke.

You would need to disassemble the E and I core and refit the core pieces so that all the E's are the same way round as are the I's. Before you clamp it back together place a thin sheet of paper at the junction of the E's and I's to make a small air gap. Someone will respond with the maths that is required to calculate the air gap required. A couple of layers of scotch tape (sellotape) will usually suffice.

Unfortunately a lot of cheap transformers cannot be disassembled.

In a transformer the core pieces alternate E - I then I - E then E - I so that they overlap.
In a choke the core pieces do not alternate so that all the I's are stacked on top of each other as are the E's.
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Old 6th January 2011, 02:24 PM   #5
316a is offline 316a  England
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Quote:
Originally Posted by Andy5112405 View Post
Unfortunately a lot of cheap transformers cannot be disassembled.

You can always get a hacksaw out and make a gap . The main issue I see is a lack of inductance once this is done . Microwave oven transformers' have been used as chokes , on these the secondary rather than primary is used . Better off getting the right tool for the job at the end of the day rather than cutting corners with junk parts

316A
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Old 6th January 2011, 05:28 PM   #6
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Quote:
Originally Posted by 316a View Post
You can always get a hacksaw out and make a gap . The main issue I see is a lack of inductance once this is done . Microwave oven transformers' have been used as chokes , on these the secondary rather than primary is used . Better off getting the right tool for the job at the end of the day rather than cutting corners with junk parts

316A
I'll second that! Lots of people probably make chokes in your neck of the woods or you can get some from China.

There are some vendors here like Twin Audio that would probably make what you need for a nominal fee if you can't find another source. (They make good OPTs from direct experience so I've no doubt they would make equally acceptable chokes.)
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Last edited by kevinkr; 6th January 2011 at 05:31 PM.
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Old 6th January 2011, 06:19 PM   #7
316a is offline 316a  England
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Just measured the primary of a 20VA 240V unit . 5.5H 106 ohms , add the gap and the inductance will drop right off . Not worth it , not enough turns , even if the lams can be restacked . Hammonds have a very cheap U-clamp range , I'd say if funds are tight , look these up . For the skinflints out there , SE output transformers from old radiograms or 'poor purchase decisions' can be used as chokes if the secondary is left flapping .

316A
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Old 6th January 2011, 06:28 PM   #8
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If you refocus on the end function of the choke a bit, and really want to use that particular magnetic part, then a 240:240 could be used as a common-mode choke.

Ciao, Tim
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Old 7th January 2011, 02:11 AM   #9
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thks for the inputs. turner audio puts the choke on the return path (-ve line) of the power supply.

the reason cited for this was its cheaper to source a lower operating voltage device than a HT rated. this makes me think that there could be a operating voltage safety issue here if 240:240 transfo is used (despite the design short falls of not having air gap).

would there be any audiable difference in using a choke or a transfo in CLC?


cheer
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Old 7th January 2011, 08:09 AM   #10
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Quote:
Originally Posted by puginfo405 View Post
would there be any audiable difference in using a choke or a transfo in CLC?
cheer
Using a mains transformer is wishful thinking : it does not have a gap so will saturate when DC is passed through it . I find it highly unlikely that a 240:240V mains transformer will perform well as a smoothing choke . Use something intended for the purpose rather than attempting a bodge-job

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