Resistor vs transformer

So what?

Which makes it an even more extravagant over-kill.

If you don't know what you are doing.

Phil Allison's pretensions to competence are just as comical as ever,

He doesn't seem to understand that the centre tap of a 2:1 autotransformer strung across 230V AC mains will deliver a slightly safer voltage (with respect to the neutral line, not that that exists in Germany) than a direct connection to 110V AC mains.

--
Bill Sloman, Nijmegen.

--
Not likely.  If they\'re SMPS then there will likely be a direct
connection between the mains, a rectifier, and a reservoir
capacitor,  with the DC on the capacitor being chopped and sent
through a small, high frequency transformer down stream for
isolation.

Be careful.  It sounds like you don\'t know what you\'re doing, and if
you\'re messing around with 240V mains you could easily injure or
kill yourself or someone else.

If there are 240V units available from your supplier, then by far
the best solution is to exchange what you have for them and use the
ammunition you\'ve gotten from us to explain to your boss that that\'s
the only sensible way to go.

If you think that he\'s going to think you\'re a fool for doing that,
explain to him what the consequences could be if you _don\'t_ do it
that way, and he may just start to see you in a different light.

I think I've wasted enough time on you without getting anywhere. If there is something wrong there I'm listening to someone else.

Well, not exactly-- most SMPS don't have an input transformer, but about 90% of the ones I've seen come with an input circuit that can be switched from 120 to 240. For 120 volt input, they use a voltage-doubler to generate about 300VDC peak. For 240V input, you move a jumper that disables half of the doubler, turning 240VAC into again about 300VDC.

Even if it doesnt come with a jumper, it's nearly trivial to disable half of the voltage doubler.

--- It seems you're referring to me, from the message you're replying to, even though you didn't quote context. Trying to get tricky, huh?;

The only reason you're not getting anywhere is either because you refuse to admit that you're wrong, while being pelted with irrefutable evidence, from _everyone_, that you are, or because you're incapable of understanding the subject matter to which this thread relates.

-- John Fields Professional Circuit Designer

** Like "Da Slow Man" FUCKWIT is constantly *proving* about himself. ** Da Slow Man is a posturing, f****ng idiot.

On his good days that is.

** Da Slow Man is a public MENACE !!! ---------------------------------------------

Auto-transformer 230v to 120v step-downs are * INHERENTLY * unsafe items !!!

They *cannot* be legally type approved in Australia or any other country AFAIK for general sale or use by the public.

Only *isolation* type step-downs are safe to use with arbitrary 120 volt appliances.

For a clue as to why this is so - imagine that *only* the 230 volt active is connected to an auto-transformer stepdown OR that the supply active and neutral connections are reversed going to the unit.

All way over Da Slow Man's pointy, autistic head.

....... Phil

So what. This is a forum for electronic engineers.

It isn't going to make much difference in Germany, where the power plugs are symmetrical, so there is no distinguishable live or neutral connection, and regular sockets are protected by earth-leakage trips.

The output end of the OP's device would have to isolate the user from

240V AC mains potentials, but the device was almost certainly originally designed to stand off 240V AC safely (there may be some intensely parochial Japanese and US products that were only designed for a nominal 110V, but even those have to be designed to stand off lightning-induced surge voltages, which don't depend on the human-specified mains voltage).

Phil does seem to have the belt and braces approach to safety that you find in people who don't actually understand what's going on.

If you say so, Phil.

--
Bill Sloman, Nijmegen

** Far too much snipping !!!!!!

What a criminal liar Sloman is.

** Proves the point re lack of safety - f****it.

** It is a PUBLIC FORUM with all kind of folk asking dumb questions.

The OP here is total moron.

Like you.

** Da Slow Man Menace has no idea what the *hell* he is crapping on about.

On this or any other topic.

** You crass stupidity has no limits - Slow Man.

As does your gross recklessness with facts.

Kindly drop dead, ASAP.

........ Phil

Yawn... Fuck, YAWN......

Another way you may get the desired output operating at 240V is by connecting in parallel the wire connections at the secondary in case they are in series. So either go for Ancient Hackers advise IN CASE their is a parallel connection at the Primaries or go for MY advise IN CASE there isn't one at the PRIMARIES BUT there is a series connection at the secondaries. Its quite simple to see as: iF X is the unknown voltage at the transformer secondary before you tamper with it then

X = Nsec./Nprim. * 120

putting the secondaries in parallel Nsec 2= Nsec./2

substituting

X2 = (Nsec/2)/Nprim. *240 = Nsec/Nprim *120

so X2 = X as you see your Voltage at the secondaries is unchanged by the modifications! In a similar way you can prove Ancient Hackers advice to be right. Just substitute Nprim2 =2*Nprim

lemonjuice

So Hans check on the rectifier circuit if its SMPS . If you have 4 diodes there then "most likely" ... it can work on both 120/240 .Its easy to do the correction even without a jumper there as the jumper Ancient mentions is usually used as a connection between 2 connected diodes (in the rectifier section) linked to a smoothing capacitor(s) (which are in parallel to the diode connection) in case of 120V operation. That backbiases the 2 series diodes throughout the whole Voltage cycle. For 240V you would have to have them working for half the cycle or in other words break the connection to the smoothing capacitor(s).

lemonjuice

This from the man who says I'm a moron - IQ 51-70

Phil - of course - hasn't argued with any of the facts that I adduced, thus conceding the real argument. He's being gracious about it (for him) - or maybe his shallow resources of invective don't allow hin to be any ruder than he has already been in the course of the argument.

--
Bill Sloman, Nijmegen

Hans, in case you havent figured it out yourself, Senor lemonjuice may be well-intentioned but his advice is mostly useless and often harmful. There's never a series connection at the secondary, and you can't apply

240V to the input if it's designed for 120V.

You *can*, but it's generally not a very good idea.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

--
Have you forgotten already that the OP has 240V mains but 120V
receivers? 

Your "advice" would have him disconnect the parallelled secondaries
and reconnect them in series, a really idiotic thing to do.

Just think, here he has a 120V receiver with, let\'s say, just for
grins, dual 12V secondaries wired in parallel (Which BTW isn\'t at
all likely, since there would really be no point in using a
transformer with dual secondaries on a production unit, due to the
extra expense and total lack of necessity.)

With the thing designed to be plugged into 120V mains, plugging it
into 240V mains (if you were daft enough to try it) would result in
an immediate doubling of the transformer\'s output voltage.  To add
insult to injury, your suggestion to disconnect the parallelled
secondaries and reconnect them in series would result in another
doubling of the output voltage, to 48V.

Somehow, I don\'t think your advice is sound, LOL!

--
Hans seems to have some difficulty grasping even the most basic
ideas presented here, and you\'d have him rooting around in the guts
of the receiver trying to make modifications based on your "input"?

Truly, that\'s another step you\'ve taken which will bring you closer
to being admitted into the Asylum for the Criminally Insane.

See

and for step-down autotransformers with CSA and UL approvals. The 175 is step-down, 230->115, and the 176 is step-up.

--
Peter Bennett VE7CEI 
email: peterbb4 (at) interchange.ubc.ca        
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html 
Newsgroup new user info: http://vancouver-webpages.com/nnq

"Peter Bennett" "Phil Allison"

** From the bareness of unit pictured and the makers application notes - those transformers are NOT intended to be sold direct to the public. Rather, they are intended to be incorporated into an installation or particular item of equipment.

General purpose ( ie with US 2 or 3 pin outlet) auto-transformers that convert 230v to 115v are " PROHIBITED ITEMS " under the *legally binding* Australian wiring regulations ( AS3000 ).

Aussie regulations very closely follow European regulations - particularly the UK ones.

Step-downs, similar to the ones from Hammond but with carry handles, were recently withdrawn from all *retail* sale here due to the safety hazard they can create and because liability insurers excluded them from coverage.

........ Phil

"Spehro Pefhany"

** HUH ????

Its "not a very good idea " in the same league as jumping from an aeroplane in flight with no parachute !!!

When a 120 volt primary mains transformer is subjected to 240 volts, the result is sudden and dramatic.The iron core becomes completely saturated eliminating nearly all primary inductance and leaving only the winding resistance to limit current flow.

Eg:

A 12VA, 120 volt transformer has a primary R of about 65 ohms. If connected to 240 volts, the rms current will be about 3.5 amps initially - rapidly falling in the next few seconds as the copper wire heats.

Next, the wire's insulation will burn under the onslaught of circa 500 times normal dissipation. The current draw will then start rising again as shorted turns have less overall resistance - plus of course lots of smoke is being generated by now.

If fitted, a suitable supply fuse would simply immediately blow and likely save the tranny from damage.

Larger VA transformers ( over about 200VA) draw enough current in this situation to trip the breaker in the AC power board.

........ Phil