[sdiy] diy autotansformer?

[Magnus Danielson]

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