Direct main supply dangerous. Pls explain before flaming

[Ipanema]

I've search in the forum. Found these two threads but unfortunately there are no explanation of why it is dangerous. http://www.diyaudio.com/forums/searc...earchid=202495

I hope that the experts here can provide some explanations of why direct main supply without isolation transformer are dangerous. Felt tempted to try 240Vac direct rectification for tube amp. I've done same googling, surprisingly some ppl consider it safe if done properly. A bit confuse.

[wg_ski]

The only way it is safe is if the input is isolated by a transformer which can withstand several times mains voltage between primary and secondary. The output, obviously is transformer coupled and safe if feedback is taken from the primary.

A lot of old-school radios,TVs, and record players did it this way, and the only accessable 'input' was typically the antenna (which is usually loosely transformer coupled). Even some early solid state stuff. The whole circuit was 'hot chassis' and you REALLY wanted an isolation trafo when working on one.

[wrenchone]

Direct mains supplies were used in the distant past for things like table radios and the cheaper consoles where no contact between the user and the chassis could (theoretically, at least) happen, and no provision was made to connect any other outside circuitry to the equipment in question.

They are based on the assumption that one side of the AC outlet is 0V with respect to earth/safety ground, and the other side of the line is at high AC potential. The chassis of the unit in question is connected to that supposedly quiet side of the line, while the other side is connected to rectifiers, filters, etc., to run the equipment.

In a perfect world, every wall plug in the house is wired properly, and things work ok. That perfect world doesn't exist.

If the wires in the wall plug are reversed, the chassis of the unit in question is live at full AC potential. The rectifiers and filters don't care, as they see the same potential as before, so the equipment keeps on working.
However, if someone touches the chassis of the equipment and something else that is grounded, they get nailed. Same thing happens if you try to plug in a piece of equipment that has its chassis connected to earth/safety ground.

An isolation transformer breaks the direct path between the mains and the chassis so that you can establish a real ground there. That stays at the same (safe) potential no matter the condition of the wall socket. Direct mains supplies are false economy, and very dangerous.

[martin clark]

Also:

The chassis of the direct-fed equipment, and everything connected to it can be raised to full AC potential by a fault elsewhere in the supply network totally outside of your control.

Relying on the output transformer of a valve amp for mains isolation is folly of the very highest order, becasue it exposes interleaved (close-couple) windings to excessively high voltage differences that *will* break down the insulating varnish. Mains transformers are proved to 2- or 4Kv in production using 'hi-pot' tests. Output transformers are NOT.

There is NO WAY to direct-feed valve amps (or any other audio kit using significant power) safely.

[star882]

The main consideration for any mains powered device is to ensure that any externally accessible conductive part is sufficiently isolated from the mains. Note that while mains-rated transformers are one way to do that, mains-rated optoisolators and Y2 capacitors also work. You also need to provide enough clearance to maintain isolation.

[wrenchone]

In addition to Martin's comments - If you have a feedback loop connected back from the transformer secondary, the output transformer will not isolate anything....

[Ipanema]

Very well said. Thanks for the explanation. If I can ensure the correct connection from Live and Neutral, is it safe? BTW, how to determine Live or Neutral from mains socket? but this is with assumption that Neutral is at 0V compared to earth. Pls advise.

Originally Posted by wrenchone

In addition to Martin's comments - If you have a feedback loop connected back from the transformer secondary, the output transformer will not isolate anything....

What is the purpose of this kind of connection? Thankis.

[DougL]

Originally Posted by Ipanema

Very well said. Thanks for the explanation. If I can ensure the correct connection from Live and Neutral, is it safe? BTW, how to determine Live or Neutral from mains socket? but this is with assumption that Neutral is at 0V compared to earth. Pls advise.

What is the purpose of this kind of connection? Thankis.

The point is that you cannot insure that you or someone else will never plug in the amp into a miswired socket. An all-in-one player with double insulated chassis, it may be OK. Anything else, why? Isolation transformers are some of the least expensive transformers available. Why build a death trap if you know better?

[Bakmeel]

Originally Posted by Ipanema

Very well said. Thanks for the explanation. If I can ensure the correct connection from Live and Neutral, is it safe? BTW, how to determine Live or Neutral from mains socket? but this is with assumption that Neutral is at 0V compared to earth. Pls advise.

What is the purpose of this kind of connection? Thankis.

Using an isolation transformer is a must for any DIY equipment. Any DIY enthusiast (on any part of the world) should not attempt to build live mains applications unless they have a professional experience with power electronics. Only if you really understand what you are messing with you may start thinking about direct mains stuff, but judging from the questions you ask here, I don't think you do.

Please do not take that as a flaming attempt... I just like to see forum members come back online after their DIY hobby times

Now, to come back to your questions: Even in a theoretical case where you can ensure live and neutral connected properly (theoretical, because in the real world such connectivity accuracy can only be ensured in large industrial installations) it is still very dangerous because of stray capacitance induced voltages on power lines... For example, if lightning would strike nearby your house, all mains (including neutral) can see transients of several thousands of volts w.r.t. ground. I any case, you can never assume that Neutral will be properly at 0V w.r.t. ground.

The comment on feedback would apply to the output transformers on tube amplifiers I guess.

You were mentioning using the high voltage for tube amps... I would guess then you intend to put the filament on mains? Please don't... You're essentially embarking on an electrified game of russian roulette. One flaw in your tube wil put mains on your speakers... Filament supplies are current limited... for safety.... A fuse is not a current limiter.

Good luck!
Bakmeel

[SY]

surprisingly some ppl consider it safe if done properly

Some ppl are idiots. And if you take them seriously, Darwin will have the last laugh, but unfortunately there may be collateral damage to the innocent.

This is deadly dangerous. Thread closed.