[sdiy] diy autotansformer?

[harrybissell]

Hello Magnus et al

I'm not sure that you are correct... or better to say that in the USA you are
probably wrong...
but int Europe you might be right.

Each musical instrument has its own internal transformer... this will provide the
galvanic isolation
that you need.  Changing the 230 to 220 by autotransformer will be no problem. Once
you have
wired the phase of the transformer windings correctly (fixed wiring one time!) you
cannot connect it
wrong after that.

If you have any gear without galvanic isolation (an amplifier with a hot chassis
perhaps ? not common
in the USA but not completely unheard of either)... you might get into a problem by
connecting two
disparate 'hot leads' (phases) together.  This can happen even now with old guitar
amps etc.  A similar
problem can occur with two amplifiers running on separate main feeds... such as
guitar on one circuit
and PA (microphone) on another.  In the USA (120V) this is harmful, in Europe
(240V) it is often lethal.

If this is for a studio installation JH should be fine in any case.   I use
balanced power in my studio
(+/-60VAC)... if I have an amplifier with a 'hot' ground... running on the normal
120V and I connect it
to the +/-60 system I expect the sparks to fly (big time).  The trick here is that
I know the system and can
prevent anyone from hooking into it in my absence.

I don't think the autoformer causes any safety issue, the problem is with the hot
chassis devices which
I have banned from my studio and my life !!!

H^) harry

Magnus Danielson wrote:

> From: "JH." <jhaible at debitel.net>
> Subject: [sdiy] diy autotansformer?
> Date: Wed, 29 Dec 2004 18:44:20 +0100
> Message-ID: <000501c4edce$0966e280$0200a8c0 at jhsilent>
>
> > As you might know, I have replaced a lot of transformers in old synths and
> > FX to reduce
> > the mechanical hum, caused by undersized transformers and increased mains
> > voltage:
> > When vintage synths were new, we had 220V, now we have 230V.
> >
> > I still have some synths where replacement would be very difficult (many
> > secondary
> > windings etc.).
> >
> > I think I should try to reduce the mains voltage for these - build a box
> > with 230V
> > input and 220V output. As no isolation is needed, an autotransformer (one
> > winding
> > with a tap at the right place) would be fine. Actually, there are even
> > variable
> > toroidal transformers that work like this.
>
> Nooooooooooooooo!!!!!!!!!!!!!!!!!!!!!!!!!!!
>
> Gasp! Glurgl... urp! No!
>
> Just don't do it! Forget it! I understand why it may sound like a good idea,
> but it isn't. Trust me!
>
> Here are a few issues just from the top of my head:
>
> * Security - you *don't* know the polarity between the zero and phase. This
>   means that the "signal ground" can now become on the hot side of things.
>   You could end up seing a potential difference between two boxes as great as
>   210-220 maybe even 230 VAC, full current!
>   Really, if you *think* it is a great idea you have not learned about
>   electric safety. You can get electric shock, your gear could be burnt up,
>   etc. You don't *ever* want it, trust me!
>   In some countries you can know about the "polarity" (i.e. which conductor is
>   "zero" and which is "phase), but it is a mixed curse and as soon as you see
>   a non-polarity-driving outlet you end up not knowing what you deal with.
>
>   I have friends who suffered personal dammage from autotransformers. Let's
>   just say that it is *NOT* a good thing to have 230VAC arm-to-arm.
>
> * Besides security (which is a real issue, trust me), you could run into deep
>   troubles of hum canceling, since the CMRR range (if any) is being grossly
>   overloaded.
>
> > But I want the voltage fixed (no accidential change when a knob is touched),
> > and I want it
> > as unexpensive as possible. If I were to wind it myself (or have it custom
> > wound), an
> > autotransformer would certainly be the best solution.
>
> It is not the best solution. Time to relearn my friend, time to relearn.
> Whenever I have encountered them, eventually I have been sorry they where
> there! Many others have the same experience.
>
> > But buying a stock
> > product,
> > the easiest way might be this:
> >
> > Use a 230V primary and 12 V secondary transformer, connect pri and sec in
> > series,
> > connect the mains to both windings and connect to synth to the primary only.
> > This configuration could stand 242V at its input (good to have some
> > "headroom"),
> > and the output would be 230V * 230/242 = 219V.
> >
> > Will this work as intended, and be foolproof, or is there some catch?
>
> There is a catch, and however you try to work around it, it will eventually
> bite you.
>
> *IF* you are to wound your own mains transformers, here are my recommendation:
>
> * Wire one primary and (at least) one secondary winding.
> * The primary and secondary winding should be isolated by an isolating wound.
> * Use different wire gauge for primary and secondary, the wire gauge of each
>   needs to be adapted to the maximum current (under packed and high temperature
>   condition).
> * Calculate saturation limits for the core, you *don't* want the core
>   saturated. Too few primary turns (less wire) give you higher saturation!
> * Make sure the core is a good core. Toroid is a good choice, but a bit more
>   tedious to wound yourself.
>
> I really wonder if it is worth the time and money. There are good transformers
> available, you save alot of time and effort and most of the time they will
> solve your problems and do that well. Only if you do tube stuff I would
> understand it better.
>
> If you didn't get it, I think single-wound transformers should be extint. Do
> yourself a favour and learn yourself to view it as a historic artifact that you
> *may* encounter but should be moved over to the museum and be showed off as an
> example of how wrong things was done in the old days.
>
> Even as I just came home from the pub I react strongly to this. This is how
> wrong it is! Did I bang the message in so it stuck? Let me know and I keep
> banging till you learned!
>
> Hopefully keeping you on the safe side,
> Magnus