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Old 16th December 2011, 03:58 AM   #1
mltube is offline mltube  United States
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Default Chicago power XFMR question.

Recently aquired this transformer which has two HV windings at the same voltage, but the mA differs by 20mA. Can I wire them in parallel to form one winding for the HV, or will it create excess stress on the lower mA section. How would you recommend hooking this up to combine the amperage of the two.
It has been a while, and I value your opinions.
Thank You
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Old 16th December 2011, 04:05 AM   #2
GloBug is offline GloBug  Canada
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My uneducated guess is you should be OK.
If you wanted to be on the safe side then just run it <150ma.

..but in reality I liked to read my own writing and bump my post count.
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Old 16th December 2011, 04:21 AM   #3
mltube is offline mltube  United States
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I was thinking the same, but just attempting to be safe as not to burn up anything. It is a very nice potted unit and would hate to ruin it on guessing. I do appreciate your input here, Thank you.
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Old 16th December 2011, 04:06 PM   #4
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I'd check that they're exactly the same voltage (unloaded) before paralleling. They could be identical except for position on core (outer winding is longer, takes more wire: more DC resistance). It MIGHT be a very conservative rating, depending on DC resistance and transformer size.
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Old 16th December 2011, 04:15 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Tom Bavis View Post
I'd check that they're exactly the same voltage (unloaded) before paralleling. They could be identical except for position on core (outer winding is longer, takes more wire: more DC resistance). It MIGHT be a very conservative rating, depending on DC resistance and transformer size.
Having done this then measure the excitation current of the transformer with no load and the windings not connected, then connect and remeasure the excitation current, if it differs by more than a few % this would be a cause of concern (heating) as it means one winding is driving the other.

If there is a problem you can use separate rectifiers and combine the DC output.
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Old 16th December 2011, 07:34 PM   #6
mltube is offline mltube  United States
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Thank you, Kevin and Tom for your knowledge.

Before going to bed last night I was thinking that it may not hurt if I rectify both sections and then run into separate C filter prior to merging the two, that way the capacitors can help with equalization. (Just a thought, not sure if the theory would be correct.)
In the meantime, I will take the measurments you have mentioned, and then move forward. This is a very robust transformer, and had never been used despite it's age. Just want to make sure I do this right the first time.

Thank you again, I very much appreciate your assistance.

Michael
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Old 16th December 2011, 07:39 PM   #7
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That PTX looks like it was designed for something requiring +/- power. So far as paralleling secondaries, this can be problematic in that you can get some really nasty circulating currents, and that means:

To avoid that, you need to parallel with a center tapped choke for current balancing. This could simply be the secondary of a center tapped heater PTX (with the primary left open of course). If each secondary is providing the same current, there is no differential current in the choke. Unbalanced currents will be forced into balance if differential current appears. If the imbalance isn't too severe, this method doesn't add appreciably to the usual xfmr losses.

Otherwise, simply rectify and filter each secondary separately, then parallel so long as you can assure that the currents will be the same from both sides, and that'll require active regulation.
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Old 16th December 2011, 09:06 PM   #8
mltube is offline mltube  United States
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Default Thank You all for your help here!

Thank you Miles,
The center tapped choke will be given a shot in the test sequence for this XFMR. Just so happens that I have a few of the chokes you describe in my goodies box.
Once I decide on the method of connection, it will be used with a 3c24 SE that I have been wanting to try on the breadboard. Of coarse there will be a stand alone XFMR for the 3c24 filaments as they draw much more current than this XFMR can put out.

Cheers to all,
And thank you,

Michael

Last edited by mltube; 16th December 2011 at 09:09 PM.
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