Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!mordor!sri-spam!ames!husc6!bloom-beacon!mgm.mit.edu!wolfgang
From: wolfgang@mgm.mit.edu (Wolfgang Rupprecht)
Newsgroups: sci.electronics
Subject: Re: Running 120V AC equipment off 240V AC.
Message-ID: <2893@bloom-beacon.MIT.EDU>
Date: 11 Feb 88 05:54:13 GMT
References: <204@unh.UUCP> <4591@ecsvax.UUCP>
Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: wolfgang@mgm.mit.edu (Wolfgang Rupprecht)
Organization: Freelance Software Consultant, Boston, Ma.
Lines: 49
Keywords: step-down transformer, resistive/inductive loads ohms-law
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In article <4591@ecsvax.UUCP> hes@ecsvax.UUCP (Henry Schaffer) writes:
>   The problem is that everyone concerned seems to be assuming that a
>240->120 volt converter has to be a transformer.  [...]
>  For much less money and in a much smaller package you can put a
>half-wave rectifier rectifier which will give you the same effect - *for
>loads which don't care about the wave-form*.

Henry you are clearly on the right track, but be careful, you are
missing a power of two.

For resistive devices the power disipated is (E^2)/R. Doubling the
voltage from 120v -> 240v will quadruple your power. If you use a
diode to cut out a half cycle every cycle, then you will only end up
reducing your power by a factor of 2, not the factor of 4 that is
required.

The cheap scr/triac ("light dimmer") type systems that one sees sold
as transformerless 240v adapters actually take a significant chunk out
of each waveform, infact, roughly 3/4 of each waveform gets eaten.
(The actual on time is chosen so that the integral of E^2/R over the
on part of the cycle is exactly 1/4 of the integal over the whole
cycle.)

As a side note, you can actually see 50 hertz flicker from a half vave
rectified light bulb quite clearly, if you observe it out of the
corner of your eye. The center of vision appears to have a bit slower
response, which prevents one from seeing the 50hz flicker. The US line
frequency of 60 hz, is just barely fast enough that a half wave
rectified bulb flickers slightly, but not annoyingly so. (Is this
how 60 hz got chosen as the standard I wonder?)

I have seen 3 way lightbulb sockets wired with a diode between the
extra contact and the normal center contact. This hack lets a normal
single filament bulb act like a "3 way" bulb, with an off, dim, and
full on setting. Problem is, the dim setting flickers a bit.

			       .---.
			      /	    \
      o----------------------( < o > )    three-way socket
        	               | |  /
    120v          +------+-----+ |
     in           |    	 |     	 |
             dim  o      +---|>|-+   <----- this diode added to socket base.
      o--------->  		 |
             full o--------------+

---
Wolfgang Rupprecht	ARPA:  wolfgang@mgm.mit.edu (IP 18.82.0.114)
326 Commonwealth Ave.	UUCP:  mit-eddie!mgm.mit.edu!wolfgang
Boston, Ma. 02115	TEL:   (617) 267-4365