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Unused secondary on a transformer

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shortbus=

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I have a 'theoretical' question about a multi secondary transformer. Say we have a transformer with one primary and three secondaries, one a step up output and two of them step down output. We only want to use the step up out put. Will the step down ones be a problem if left 'floating' and unconnected to anything? Or should they be removed? Is or would there be a problem with "inter winding capacitance" or anything else?
 

ronsimpson

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No problem. (I am assuming you are talking about power line 60/50hz transformers)
Just cut off the wires you don't want. Or tape over the ends of the wires so they don't connect to something. I have zip-ties to hold the wires in place.
Floating is OK.
 

KeepItSimpleStupid

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I would not snip the wires too close though. Unconnected is fine. I know there are some issues with say dual 120 V primaries where it's used at 120 V and the other is left floating. I don;t know what the issue is, but it has been seen on some manufacturer's data sheets. They want you to use the in parallel as intended.
 

crutschow

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I know there are some issues with say dual 120 V primaries where it's used at 120 V and the other is left floating. I don;t know what the issue is, but it has been seen on some manufacturer's data sheets. They want you to use the in parallel as intended.
The reason for that is to cut the effective output winding resistance in half to improve voltage regulation and reduce power loss in the winding resistance.
If you draw the rated transformer power from only one winding it could result in overheating of the transformer.
 

dr pepper

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If there is high frequency harmonics on any used windings theres a possibility of ringing, but thats probably not going to harm anything.
If you were a little paranoid you could put a resistor across the unused ones drawing a ma or so just to keep them damped.
Cruts is right, the va rating of the tranny will no longer apply, the trans will only output the current rating for the winding your using not the total Va.
 

shortbus=

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Thanks to all of you for answering. It is a high frequency transformer, a LOPT from a CRT TV I only need the high voltage but was told the auxiliary winding must be removed. But I didn't know for sure, the transformer works with them connected and can't see the harm if they were left floating not supplying an output. Does this make a difference in the answer?
 

dr pepper

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I've played with lopty's a few times with & without using some secondaries, I had no problems, unless you have sensitive Rf or analogue stuff very close I dont think theres any need for special measures, just leave them open.
You making an ion lifter or something?, last time I played with a lopty I made a high voltage cap tester.
 

shortbus=

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I've played with lopty's a few times with & without using some secondaries, I had no problems, unless you have sensitive Rf or analogue stuff very close I dont think theres any need for special measures, just leave them open.
You making an ion lifter or something?, last time I played with a lopty I made a high voltage cap tester.
No this is all just theoretical/hypothetical. It stems from a disagreement on another forum. where the other party is an "expert", and me just a hobbyist. My research on "inter winding capacitance" has lead me to the conclusion that it is limited to the winding's of a single coil on the transformer, not to all winding's on the same transformer. But again I'm just a hobbyist and not an "expert". Since I trust the opinions of many here more than the "expert", I thought this would be a more unbiased source of information. Thank you all for you help.
 

AnalogKid

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I would not snip the wires too close though. Unconnected is fine. I know there are some issues with say dual 120 V primaries where it's used at 120 V and the other is left floating. I don;t know what the issue is, but it has been seen on some manufacturer's data sheets. They want you to use the in parallel as intended.
If you don't connect the dual primaries in parallel, you cannot get the full rated power through the transformer without overheating the one primary winding being used. Also, the unused primary leads/pins/contacts are at a lethal voltage; isolated from the input power line, but still lethal.

ak
 

dr pepper

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I dont profess to be 'expert', more of a seasoned throw it together & see.
Unused windings on a lopty I dont htink would bear much effect on ones that are being used, so long as they dont arc over or something.
If the trans were auto wound however then there is a possibility interwinding c would have some effect esp with high voltage, as interwinding cap has a bearing on the practical max voltage.
I pulled a couple of lopty's apart, the higher current windings like the filament supply are at the bottom against the pcb and the stacked Hv windings at the top, not having the windings over each other and side by side will probably hekp to reduce interwinding c.
 
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