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Center Tapped Mains Transformer as Dual Channel PSU Choke
Center Tapped Mains Transformer as Dual Channel PSU Choke
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Old 9th September 2016, 03:45 AM   #1
ballpencil is offline ballpencil  Indonesia
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Default Center Tapped Mains Transformer as Dual Channel PSU Choke

I'm just curious.. will the attached plan work?
I have a small power transformer with dual 110V primaries and dual 12V secondaries. Paying attention to the winding direction (the dot sign), can i use it as a choke? Both ends will supply the same amount of current so as to prevent core saturation (DC current cancellation).
Center Tapped Mains Transformer as Dual Channel PSU Choke-jpg

This is a "better than nothing" situation so i'm aware inductance maybe lacking but is there any problem to this plan? Despite the lack of inductance, is it still worth the trouble? It's for a tube amplifier supply so the current is in the mA.
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Old 9th September 2016, 07:40 AM   #2
AllenB is offline AllenB  Australia
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Center Tapped Mains Transformer as Dual Channel PSU Choke
What about the AC flux?
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Old 9th September 2016, 08:19 AM   #3
ballpencil is offline ballpencil  Indonesia
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Quote:
Originally Posted by AllenB View Post
What about the AC flux?
What about it? Anything i miss? Perhaps you're referring to the case where there's a difference in current draw between channels when music is playing? Will this be an issue? Perhaps the plan would be possible only for Class A amps?

If there's a current draw imbalance when music is playing, causing (maybe) some ripples to escape, will this matter? I mean perhaps the music itself will mask the ripple.. and when the amp idles again, current returns to balance and less ripple escape.

Sorry for the barrage of questions.
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Old 9th September 2016, 08:29 AM   #4
stevenrotterdam is offline stevenrotterdam  Netherlands
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The AC fluxes will interact so this won't work well. Maybe you have a power relay lying around that will work better for milliamps.
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Old 9th September 2016, 09:05 AM   #5
AllenB is offline AllenB  Australia
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Center Tapped Mains Transformer as Dual Channel PSU Choke
If you were to run the same DC and ripple currents through each leg then the choke would be ineffective for both, notwithstanding a small leakage inductance. To get the effect you need to keep the windings in phase.

You are in the unique position here that you have the 12V windings at your service. Maybe you can cancel your DC with a separate supply connected there. Then again you might not need it if the core is already a good size.
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Old 9th September 2016, 09:07 AM   #6
DF96 is offline DF96  England
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That is not a choke. It is an autotransformer.

However, if equal currents are drawn from each side (both AC and DC) then you can consider what happens if the two outputs are connected together - by symmetry this should not change anything. The only choking action you get is due to leakage inductance - i.e. you are relying on the transformer being sufficiently poor in quality to have significant leakager inductance. You won't get much smoothing this way.

If unequal currents are drawn (i.e. real life) then the two supply rails will be coupled to each other so disturbance or signals on one get transferred across in antiphase to the other. Might have some purpose in an effects box or electronic instrument?

This question (can I use a CM choke for smoothing) seems to crop up every few weeks. The answer is always the same.
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Old 9th September 2016, 09:18 AM   #7
AllenB is offline AllenB  Australia
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Center Tapped Mains Transformer as Dual Channel PSU Choke
Thinking about this some more you should know what inductance you need and measure the inductance you have. You could saturate the core with the 12V windings and measure again.

If it were me I'd try to use it earlier in the supply (as a single combined winding). Sometimes closed cores are less unsuited to choke input supplies.
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Old 9th September 2016, 10:06 AM   #8
ballpencil is offline ballpencil  Indonesia
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Quote:
Originally Posted by DF96 View Post
That is not a choke. It is an autotransformer.

However, if equal currents are drawn from each side (both AC and DC) then you can consider what happens if the two outputs are connected together - by symmetry this should not change anything.
You're right. Why didn't i think of it that way..
Thank you. I will see if i can use the primaries in series and cancel the DC current through the secondaries as AllenB suggested.
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