What to do with unused transformer windings...

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Ian MacGregor

Well-known member
Joined
Jun 3, 2004
Messages
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Echo Park, Los Angeles, CA, USA
Hi,
A quick search didn't turn up anything too interesting...

What's the best thing to do with unused transformer windings, both for audio transformers and for power transformers. My instinct tells me that an unused winding needs to have some sort of load, or else the transformer windings will break down (like a tube gtr amp without speaker load?)

Power example:
Multi tap power toroid with but at least one of the multiple windings will not be used. If the proper approach is to just load the unused windings resistively, is there a good amount of current to draw? 1%, 10%, etc of the max winding current?

Audio example:
Quad filar transformer being driven 1:2 (3 windings used). What to do with the 4th winding? Parallel it with the primary? Load it down?



 
For the audio, if you can do a sweep, check loaded vs unloaded and see if anything significant happens to the curve. 

Loading will most likely change the reflected impedance.  I have an output transformer with what appears to be an auxiliary speaker or feedback winding on the secondary.  Unloaded gives one impedance - loaded with ~8-10ohms it changes the primary impedance by 60%.

Do you have a spec sheet or winding diagram for yours?
 
lassoharp said:
Do you have a spec sheet or winding diagram for yours?

Not really, it's more of a general question that pops up for me from time to time. The audio transformers I'm usually working with are similar to the API quad filar output type. The power transformers are usually toroids, with a winding for +/-24V, +250V, and +12V. Sometimes I just need to use the power transformer for +/-24V and don't need the tube stuff (250 & 12). Curious what to do with those unused windings...

ian
 
> transformer windings will break down (like a tube gtr amp without speaker load?)

WHY does that happen?

Over-driven output stage pulls-down to near-short then flies-back to near-open.

If you understand a choke, or knew how an ignition coil worked, you'd see that the voltage goes to "infinity" for ideal parts and to >10X supply voltage for good real parts. (12V battery kicks to 400V on coil primary.)

Taking a 350V tube amp, smacking the snot out of it, will kick-up 3,500V on the end of the primary. Standard fishpaper is super reliable at 500V, will stand a wild weekend at 1,500V, but has short life at ~~3,000V.

A power transformer has NO such abuse. The flux is always coupled to a low impedance (power company plus eveything in the neighborhood).

Audio transformers -other- than hi-volt tube outputs generally do NOT have "dangerous" voltages. +27dBm or 70V line, kicked-up 10X, is well within safety margin for enamel and fishpaper.

Lightning-strike always does whatever it wants, laughs at fishpaper, and no load you might add will really change things.

> just put tape over them.

If I can find my shrink-tube, I use that. Pulled too much old loose tape out of shorted amps; and worked so long in one place that "someday problems" came around to bite me. But exact same idea: insulate the ends as good as you think necessary.
 
+1 insulate them.

A power amp is different; you're trying to avoid making your output devices dissipate too much heat by keeping your impedance within spec.  You don't have the same isse with a line output stage;  it's really not dissipating anywhere near it's max heat dissipation.

Your open secondary winding will attempt to reflect infinite ohms back to the primary at whatever ratio your transformer is - whether it steps it up or down is kinda not an issue!  ;-)
 
Perhaps for a tube amp (I don't know), but for solid state amps you can leave windings unconnected. I put a lot of output transformers in fixed install 70v/100v systems, and they would be very unhappy if every output tap was loaded down, and there is no problem with leaving them all floating.

JR
 
doesent it make sense to parallel whenever possible?  (assuming they are equal dual pri or sec)  You increase the current capability with no downside i can think of.
 
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