Secondaries should only be connected in series with the primary, and only if those secondaries are not required for any other purpose and if they have sufficient insulation for mains potential. I would recommend attaching a label to the transformer to warn unsuspecting people that this has been done. In a few years time you may have forgotten what you did.
Never connect two windings in parallel (primary or secondary) except when they are identical.
Never connect two windings in parallel (primary or secondary) except when they are identical.
It is not generally the case: the mains safety isolation barrier is between the primaries and all secondaries.
The secondaries themselves often rely on the wire enamel for sole isolation material.
There are exceptions: transformers for oscilloscopes, variable frequency drives, etc, but no basic general purpose transformer is built like that.
So, the secondary is to only be hooked up in series if this is to be done.
The transformer is in a housing and sealed in an epoxy like substance (I assume to reduce vibration). When hooked up properly to 120V the (unused) secondary has 20V across it. I'm unable to see the actual wire that's wraped around the core, but the lead comming from it is rated for 300V, I don't trust that to be the rating of the secondary.
Is there another way to tell what the secondary's voltage rating is?
The transformer is in a housing and sealed in an epoxy like substance (I assume to reduce vibration). When hooked up properly to 120V the (unused) secondary has 20V across it. I'm unable to see the actual wire that's wraped around the core, but the lead comming from it is rated for 300V, I don't trust that to be the rating of the secondary.
Is there another way to tell what the secondary's voltage rating is?
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I do it all the time, works great when you have unused windings (especially that 5V winding intended for a tube rectifier). When you need to bring that voltage down just a little.
Best bet is to connect the secondary winding at the neutral end of the circuit, but ultimately I don't think there are extreme insulation concerns. If you were using the 5V winding (just as example) on a directly heated rectifier, no one panics about the insulation rating.
Keep in mind if your transformer is equipped with an electrostatic shield, using a secondary winding in the primary essentially negates the action of the shield.
Best bet is to connect the secondary winding at the neutral end of the circuit, but ultimately I don't think there are extreme insulation concerns. If you were using the 5V winding (just as example) on a directly heated rectifier, no one panics about the insulation rating.
Keep in mind if your transformer is equipped with an electrostatic shield, using a secondary winding in the primary essentially negates the action of the shield.
Yes, and that's why it is potentially lethal and definitely illegal. Unless you are absolutely sure it is not.
Most DMM measure voltage. I have never seen one which measures the voltage rating of insulation. For that you need something like a Megger.Tony said:
That winding will have been built for high voltage operaton, because the transformer designer will know that a 5V heater winding is intended for a rectifier. Not all secondaries are.zigzgflux said:
That is quite a large assumption that the manufacturer knows I will be elevating the 5V winding, and designs that specific winding with additional insulation. Along those same lines, what if I want to build a PI or cascode, and intend to elevate one (and only one) of the 6.3V windings to 100, 200, or even 300V. Again, no one panics. Can I assume the mfr knows this, and builds in additional insulation? Of course not. The only real assumptions we can make without getting specifics from the manufacturer is one of two methods:
1. NO secondary winding is built with additional insulation. We should then only use individual xfmrs for DH rectifiers and cascodes (my ideal preference).
2. ALL secondary windings are sufficiently rated to handle a few hundred volts, to accommodate DH rectifiers and cascodes.
We cannot know which is true, but we can minimize the unknowns by connecting the secondary winding in the neutral end.
I would expect that a bi-fillar secondary winding to have low isolation capability.
I would further expect that a transformer for a valve amplifier will always have more isolation capability between the separate secondary windings.
I would not expect the isolation test voltage between primary and secondary to be applied between secondary windings, unless the specification tells the user so.
I would further expect that a transformer for a valve amplifier will always have more isolation capability between the separate secondary windings.
I would not expect the isolation test voltage between primary and secondary to be applied between secondary windings, unless the specification tells the user so.
If you had a megger you could test the insulation break down voltage non destructively.
I was lucky enough to work for one company who were selling a product to the US navy and they would not accept the results done with the 1933 hand cranked megger that the company had. They had to buy a new digital version for over $1000.00 and I bought the old hand cranked one for $5.00. It's also great for bringing up worms for fishing if you put the two leads into the ground about 10 feet apart
I was lucky enough to work for one company who were selling a product to the US navy and they would not accept the results done with the 1933 hand cranked megger that the company had. They had to buy a new digital version for over $1000.00 and I bought the old hand cranked one for $5.00. It's also great for bringing up worms for fishing if you put the two leads into the ground about 10 feet apart
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No. The transformer manufacturer knows that octal rectifiers have 5V heaters, which are either directly heated or at least connected to one side of the cathode. Other valves tend to have 6.3V heaters. So on a valve mains transformer it is quite possible that the 5V secondary has better insulation than the other heater windings.zigzagflux said:
There is a world of difference between a relatively high impedance HT/B+ DC supply and the very low impedance of an AC mains supply, so there are very different safety implications of insulation breakdown.
True, provided that you can always be certain which side is neutral. You can't trust building wiring.
Yes, I would expect that too. It would probably be safer to do this trick with a spare heater winding on an HT transformer, than with a general-purpose low voltage transformer.AndrewT said:
really depends on insulation properties of the magnet wires used.....
the McIntosh MC270 OPT used bifillar windings with no other insulation other than the enameling on the wires themselves......
i also use the 3 volt 3DG4 octal rectifier......can be had for a dollar each at Rogalski's....
Yeah, but then there is no risk to human life.
Quite different when you mix the primaries and secondaries of a mains transformer.
That is part of the fun of DIYaudio forum: make almost any 99.9% true statement and someone will report an exception!Tony said:
Nevertheless, a 5V winding is almost certainly intended for an octal rectifier as most of the commonly used rectifiers had 5V heaters even when the partnering signal valves used 6.3V.
In the first instance, you tell us clearly that there is a Life Hazard.i wonder why we are still having discussions.....
In the second instance you tell us that the capability of wire to wire enamel insulation is up for debate.
The third instance asks why "you" are still discussing the topic?
Rules for some and not for others?
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