Re: Resistor vs transformer

On 10 Feb 2006 00:53:21 -0800, "lemonjuice" <exskimos@xxxxxxxxxxxx>
wrote:

On Thu, 09 Feb 2006 13:08:32 +0100, Weinberger Hans
<weinberger@xxxxxxx> wrote:

On 8 Feb 2006 06:27:15 -0800, "Ancient_Hacker" <grg@xxxxxxx> wrote:

I would first open up one of the units and peek at the power
transformer. There's maybe a 45% chance the transformer has 120/240V
primaries, they're just not configured that way from the factory.
Follow the line input, past any switches and fuses and trace the wires
to the power transformer. If there are two sets of two wires each
going into the transformer from the line, those are two 120V primaries
hooked up in parallel. To wire them for 220V, you just have to put
them in series instead of parallel. The phase is important-- you'll
have to snip one wire from the line, then the opposite windin'g wire
from the other line and hook those two together.
Thanks for the advice. It certainly is the cheapest. I will check
when all the units are here . I think they will most likely be
SMPS inside, but even then your idea should still work .

Hans

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.

---
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!
---

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

---
And that 240V going into the 120V primary isn't going to make the
transformer unhappy?
---

so X2 = X

---
Only for a little while...
---

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

---
Ancient Hacker was right, but you're not, so stop trying to lend
credence to your specious prattle by association.

--
John Fields
Professional Circuit Designer
.

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