Resistor vs transformer

[John Fields]

Cut that out. Its very funny.

Another error in your post is you don't seem to realize that its NOT
RESISTANCE that is cutting down the voltage BUT inductance. Write an
equation for that .

---
One would think that even an oaf like you would realize that for a
given wire size wound on a given core, inductance increases with the
number of turns, and if the number of turns increases, so does the,
um... resistance?
---

Another error is the resistance depends also on the geometry of the
transformer.

---
Really? Where are you getting these esoteric concepts from???
I thought that all primaries and secondaries, whether wound on
toroids, E-I laminations, sintered ferrite or powdered iron cores,
or whatever, all had to have precisely the same primary and
secondary winding resistances. Hmmm... who would have though that
geometry played a role in it? Gosh, I guess ya learn something new
every day, silly me.
---

Another error is the OP was talking about a different type of
transformer from Slomans .

---
So what? I was talking about the way Sloman suggested that the OP
consider solving his problem, which is perfectly acceptable and
within the realm of possibilities that the OP should consider.

Besides, the OP didn't have anything specific in mind, he was
looking for a better solution than a series resistor and said so a
couple of times. Go read the thread.
---

etc ... want another error listing so we can all laugh.

---
I think at this point we're all pretty much laughing at _you_, so if
you want to continue, be my guest.

---
Well, I see from the paucity of information in the rest of your
posts as well as your generally flippant and unwarrantedly
argumentative attitude, that you're more interested in playing games
than you are in a serious technical discussion of any type.

Such being the case, I have better use for my time than to squander
it on trying to teach pigs to fly.

Goodbye, unless you learn to behave yourself in other than the
childish manner you've displayed so far.

 

[John Fields]

Goodbye, unless you learn to behave yourself in other than the
childish manner you've displayed so far.

 

[lemonjuice]

On Wed, 08 Feb 2006 06:09:44 -0600, John Fields

Winding resistence doesn't depend ONLY on input voltage range. You can
get dual 115V's with higher resistence then dual 230V's
as far as I know neither does regulation ... unless you have some good
reasoning to prove that.

For the same core, to get the same flux density with a 240V input
would require twice the number of turns as for a 120V input.


The voltage on the secondary is related to the voltage on the
primary by:

Es Ns
---- = ---- (1)
Ep Np

Where Es and Ep are the primary and secondary voltages,
respectively, and Ns and Np are the number of turns wound on the
secondary and on the primary, respectively.

Since the transformer is tranferring power, the current in the
secondary has to be related to the current in the primary by:

Is Np
---- = ---- (2)
Ip Ns


So, for, say:


120V>--+ || +-->120V>--+
P || S |
R || E [120R]
I || C |
120V>--+ || +-->120V>--+


With 120V out of the secondary and a 120 ohm load, we'll have a load

------>>>>current (in the secondary) of:

Es 120V
Is = ---- = ------ = 1 ampere
Rl 120R

Now you are tapping current from the secondary ... wheras the secondary
remains unconnected !!!!!!

Now, since the voltage across the primary and the voltage across the
primary, and:


Np Ep 120V
---- = ---- = ------,
Ns Es 120V

--------------->>>>>>>then there must be the same number of turns on
the primary as there

are on the secondary.

The transformer Sloman had in consideration was a step down => number
of turns on the primary NOT equal to turns on secondary

Rearranging (2) to solve for the primary current will give us:


Ns
Ip = Is ---- = 1A * 1 = 1.0 ampere
Np

Irrelevant as your completely off topic

Now, with that behind us, let's take a look at a transformer with
dual 120V primaries and connect them in series so we can use it as
an autotransformer with a 240V input and a 120V output and see what
happens:


240V>------+
|
||P
||R
||1
|
+-->120V>--+
| |
||P |
||R [120R]
||2 |
| |
240V>------+-->120V>--+

Since we have twice the voltage across the inductance of the
primary, we'll need twice the number of turns to keep the flux
density in the core the same as it was for the 120V case, and that
criterion is satisfied with the two windings in series.

Now, just assume, for the sake of the argument, that our first
transformer:

120V>--+ || +-->120V>--+
P || S |
R || E [120R]
I || C |
120V>--+ || +-->120V>--+

-------------->>>>>>>>>Really had two 120V primaries which were
connected in parallel:
???????????????????????????????????????????????????????????????
Parallel primaries would give you 240V on the primaries NOT 120V
!!!!!!!

120V>--+---+ ||
| P ||
| R ||
| 1 ||
120V>--|-+-+ || +-->120V>--+
| | || S |
| | || E [120R]
| | || C |
+-|-+ || +-->120V>--+
| P ||
| R ||
| 2 ||
+-+ ||

In which case each primary would be rated for 500mA and, in
parallel, the combination could carry 1 amp.

Now, though, since we're not using the secondary and taking
advantage of the current sharing we'd get with both primaries in
parallel feeding the load on the secondary, we're forcing one of the
primaries to supply the entire load, which is going to cause twice
the voltage drop across it than would occur with with the windings
in parallel. That, in and of itself, will cause the regulation to
be poorer than it would be with a conventional transformer with the
same amount of iron in the core.

You talk about voltage regulation with a load connected to the
secondary . This isn't happen.
Man you give a bad name to this newsgroup!!!!!!!!!!!!!!!!!!!

Finally, consider another autotransformer wound on the same core,
but with two _240V_ primaries wired in series with 240V across the
ends. 120V will still appear from the center tap to either end, but
because the primaries were wound with smaller diameter (higher
resistance) wire, and exhibit a higher resistance than the other

why should they necessarily be wound with smaller wire???????????
Grow up man . You don't know what at Transformer is!

autotransformer, the drop across the 240V primary with the 120 ohm
load connected across it will be even greater than across the other
autotransformer, resulting in even poorer regulation.

I REPEAT .......Go ask your granny she'll tell you thet : You talk
about voltage regulation with a load connected to the secondary . This
isn't happening in the case we are considering.
LOL ....

 

[lemonjuice]

Who said the wire size was constant ? You of course ... because you
don't have a clue about transformers at which point you should shut up.

inductance increases with the
number of turns, and if the number of turns increases, so does the,
um... resistance?
---

So its the resistance of the wire thats cutting down the voltage???!!!!
LOL
This is the stupidest thing I've heard in years.

---
Really? Where are you getting these esoteric concepts from???
I thought that all primaries and secondaries, whether wound on
toroids, E-I laminations, sintered ferrite or powdered iron cores,
or whatever, all had to have precisely the same primary and
secondary winding resistances.

No ... You have a bad assumption. Even your high school equations
don't suggest that .

Hmmm... who would have though that
geometry played a role in it? Gosh, I guess ya learn something new
every day, silly me.
---


---
So what? I was talking about the way Sloman suggested that the OP
consider solving his problem, which is perfectly acceptable and
within the realm of possibilities that the OP should consider.

Besides, the OP didn't have anything specific in mind, he was
looking for a better solution than a series resistor and said so a
couple of times. Go read the thread.

You said the OPs suggestion of using a transformer with 2 primaries
rated for 240V was wrong . Refresh your aging memory .Go on vacation it may help you.

lemonjuice

 

[John Fields]

Sloman's trick of using the 120V primaries in series and having your
device connected in parallel across the primaries causes the
transformer to become an autotransformer, which would work.

Using the scheme with a transformer with dual 240V secondaries
likely wouldn't work well because of the much higher winding
resistance you'll encounter causing,
Winding resistence doesn't depend ONLY on input voltage range. You can
get dual 115V's with higher resistence then dual 230V's
I would expect, _very_ poor
regulation.
as far as I know neither does regulation ... unless you have some good
reasoning to prove that.

For the same core, to get the same flux density with a 240V input
would require twice the number of turns as for a 120V input.


The voltage on the secondary is related to the voltage on the
primary by:

Es Ns
---- = ---- (1)
Ep Np

Where Es and Ep are the primary and secondary voltages,
respectively, and Ns and Np are the number of turns wound on the
secondary and on the primary, respectively.

Since the transformer is tranferring power, the current in the
secondary has to be related to the current in the primary by:

Is Np
---- = ---- (2)
Ip Ns


So, for, say:


120V>--+ || +-->120V>--+
P || S |
R || E [120R]
I || C |
120V>--+ || +-->120V>--+


With 120V out of the secondary and a 120 ohm load, we'll have a load

------>>>>current (in the secondary) of:

Es 120V
Is = ---- = ------ = 1 ampere
Rl 120R

Now you are tapping current from the secondary ... wheras the secondary
remains unconnected !!!!!!

---
I don't know what you're looking at, but from my point of view what
I drew was:

120V>--+ || +-->120V>--+
P || S |
R || E [120R]
I || C |
120V>--+ || +-->120V>--+

where we have a transformer with a primary connected to 120V mains
and a secondary with a 120V output clearly connected to a 120 ohm
load.

Perhaps you're not viewing the ASCII schematic with a fixed-pitch
font, like Courier?
---
\
--- \
Oops... typo. Should read 'secondary'.
---

--------------->>>>>>>then there must be the same number of turns on
the primary as there

The transformer Sloman had in consideration was a step down => number
of turns on the primary NOT equal to turns on secondary

---
As I suspected, you haven't read the thread through.

The transformer I refer to later on was not the step-down
transformer which Sloman refererred to at first, but one which he
referred to later on, which was a transformer with dual 115
primaries which he suggested be wired in series to make the
transformer an autotransformer. He also stated that the secondary
(ies) be ignored.

The transformer I referred to above was used just to illustrate a
few principles and was certainly not one which anyone would suggest
that the OP seriously consider, considering that the OP need a 2:1
stepdown in voltage from 240 to 120, and enough current out of the
secondary to feed a 6 watt (or, maybe, VA) load.

Had you been paying attention, you might have noticed that the
transformer I referred to, above, givea a 1:1 voltage ratio from
input to output and is capable of sypplying 120 watts into a
resistive load. Neither one of which the OP asked for, but
perfectly fine for illustrating a point.
---

Irrelevant as your completely off topic

---
LOL, You really don't even know that you have no clue.
---

Now, with that behind us, let's take a look at a transformer with
dual 120V primaries and connect them in series so we can use it as
an autotransformer with a 240V input and a 120V output and see what
happens:


240V>------+
|
||P
||R
||1
|
+-->120V>--+
| |
||P |
||R [120R]
||2 |
| |
240V>------+-->120V>--+

Since we have twice the voltage across the inductance of the
primary, we'll need twice the number of turns to keep the flux
density in the core the same as it was for the 120V case, and that
criterion is satisfied with the two windings in series.

Now, just assume, for the sake of the argument, that our first
transformer:

120V>--+ || +-->120V>--+
P || S |
R || E [120R]
I || C |
120V>--+ || +-->120V>--+

-------------->>>>>>>>>Really had two 120V primaries which were
connected in parallel:
???????????????????????????????????????????????????????????????
Parallel primaries would give you 240V on the primaries NOT 120V
!!!!!!!

---
Really? with two 120V primaries connected to each other in parallel,
and in phase, across 120V mains, what makes you think that would
magically put 240V across the primaries?

Take a look at the next ASCII diagram, and if you can't see what an
asinine statement it was that you made, then I don't think there's a
lot of hope for you. I think Wendy's may pay a little better than
BK, but you might have trouble getting in.

120V>--+---+ ||
| P ||
| R ||
| 1 ||
120V>--|-+-+ || +-->120V>--+
| | || S |
| | || E [120R]
| | || C |
+-|-+ || +-->120V>--+
| P ||
| R ||
| 2 ||
+-+ ||

In which case each primary would be rated for 500mA and, in
parallel, the combination could carry 1 amp.

Now, though, since we're not using the secondary and taking
advantage of the current sharing we'd get with both primaries in
parallel feeding the load on the secondary, we're forcing one of the
primaries to supply the entire load, which is going to cause twice
the voltage drop across it than would occur with with the windings
in parallel. That, in and of itself, will cause the regulation to
be poorer than it would be with a conventional transformer with the
same amount of iron in the core.

You talk about voltage regulation with a load connected to the
secondary . This isn't happen.
Man you give a bad name to this newsgroup!!!!!!!!!!!!!!!!!!!

---
Wrong again. I give a bad name to people like you who come here to
play stupid games. Actually, _several_ bad names! In your case,
the first one which springs to mind is "moron".
---

why should they necessarily be wound with smaller wire???????????

---
Same core, same winding window, more voltage, more turns, less
current, smaller diameter wire.
---

Grow up man . You don't know what at Transformer is!

---
You may know what one is, but you sure don't know how they work!

Speaking of work, what is it you claim you get paid to do?
---

I REPEAT .......Go ask your granny she'll tell you thet : You talk
about voltage regulation with a load connected to the secondary . This
isn't happening in the case we are considering.
LOL ....

---
You're right, it's not a case _we're_ considering, because the
concepts presented, even though somewhat elementary, seem to be
beyond _your_ grasp.

Here's a clue for you:

If we take this:

240V>------+
|
||P
||R
||1
|
+-->120V>--+
| |
||P |
||R [120R]
||2 |
| |
240V>------+-->120V>--+


and redraw and relabel it like this:


240V>-----------+
|
240V>------+ +-->120V>--+
| | |
P || S |
R || E [120R]
I || C |
| | |
+----+-->120V>--+

Does that make it any clearer?


No? Then here's one even you might be able to figure out:


240V----+
|
[120R]
|
+--->MYSTERY VOLTAGE
|
[120R]
|
GND-----+--->GND


What is the mystery voltage with reference to GND?

 

[John Fields]

Who said the wire size was constant ? You of course ... because you
don't have a clue about transformers at which point you should shut up.

So its the resistance of the wire thats cutting down the voltage???!!!!
LOL
This is the stupidest thing I've heard in years.

---
Then your education is _sorely_ laking. Whether or not you're so
stupid that you can't understand it, it _is_ the resistance of the
wire (among other things) which cuts down the voltage available to
the load, and if you ask me politely I'll explain it to you in a way
even you can understand it.
---

No ... You have a bad assumption. Even your high school equations
don't suggest that .
You said the OPs suggestion of using a transformer with 2 primaries
rated for 240V was wrong .

---
It was, and it still is, and if you ask politely I'll explain why,
again, but at a level you can comprehend this time.

That is, I'll try. I'm not sure I can reach that low.
---

 

[Phil Allison]

"lemonjuice" = a sourfaced fuckwit

Winding resistence doesn't depend ONLY on input voltage range.

** The actual resistance of the primary of a mains power transformer depends
on the particular unit's voltage rating, VA rating (size) and regulation
percentage.

Eg: a 12VA transformer with a 230 volt rated primary has a primary
resistance of about 250 ohms. The same transformer may have a split primary
( 115 +115) with each winding half the above resistance. So, when connected
in parallel for 115 volt use, the primary resistance will be 63 ohms (at
room temp).

as far as I know neither does regulation ... unless you have some good
reasoning to prove that.

** The voltage regulation spec of a mains transformer is almost *entirely *
due to winding resistance.

Load current causes a voltage drop ( IR in each case) to appear across the
primary and secondary windings which act to *reduce* the voltage seen at the
secondary. At the main frequencies of 50 or 60 Hz there is no significant
contribution from *leakage inductance* to the voltage drop under
*resistive* load.

Taking the 12VA, 230 volt example again, primary full load current is 12/230
= 52 mA ( neglecting I mag)

250 ohms x 52 mA = 13 volts drop or 5.6 % of the supply voltage.

The secondary winding contributes another similar IR drop so the overall
regulation is about 11%.

The windings get hot in use and copper resistance rises by 0.4% for each
degree C.

The final regulation figure for operation at 75C ( 50 degree rise above
ambient) is then 13.2%.



........ Phil

 

[Phil Allison]

"Ancient_Hacker" <

I would first open up one of the units and peek at the power
transformer.

** The Kraut OP is such a pathetic bloody imbecile he refuses to do even
that.

Or even just connect an ohm meter to the units AC plug's pins.

Makes you suspect he has NOT got one in his possession and arrogantly will
NOT admit that.

Typical Kraut.



.......... Phil

 

[lemonjuice]

"lemonjuice" = a sourfaced fuckwit




** The actual resistance of the primary of a mains power transformer depends
on the particular unit's voltage rating, VA rating (size) and regulation
percentage.

Eg: a 12VA transformer with a 230 volt rated primary has a primary
resistance of about 250 ohms. The same transformer may have a split primary
( 115 +115) with each winding half the above resistance. So, when connected
in parallel for 115 volt use, the primary resistance will be 63 ohms (at
room temp).

Why ARE YOU aren't connecting the primaries in parallel ? OP/Sloman
were talking about a series connection. In parallel you are NOT
solving anything. Do you know the equivalent impedance of a RL parallel
connection?

** The voltage regulation spec of a mains transformer is almost *entirely *
due to winding resistance.

Load current causes a voltage drop ( IR in each case) to appear across the

Bloody you... CAN'T YOU READ THE SECONDARY IS UNCONNECTED SO YOU AREN'T
GETTING ANY VOLTAGE FROM THERE!!!!!!!!!!!!!! SO DON'T MENTION VOLTAGE
REGULATION.
This has been written over and over again but neither you nor Field
seem to be able to read

At the main frequencies of 50 or 60 Hz there is no significant
*resistive* load.

WHO CARES ABOUT LEAKAGE INDUCTION .... YOU ARE TAPPING THE OUTPUT FROM
THE PRIMARIES .....

Taking the 12VA, 230 volt example again, primary full load current is 12/230
= 52 mA ( neglecting I mag)

250 ohms x 52 mA = 13 volts drop or 5.6 % of the supply voltage.

regulation is about 11%.
READ ABOVE at bloody you...
The windings get hot in use and copper resistance rises by 0.4% for each
degree C.

ambient) is then 13.2%.

Irrelevant Write the formula for regulation AND I'll try to help you.

....... Phil

Nice try but ALL totally irrelevant/offtopic to the case the OP/Sloman
brought up.


lemonjuice

 

[Phil Allison]

"lemonjuice"


** You are clearly a schizophrenic psychopath.

Go blow you brains out - anytime.





......... Phil

 

[lemonjuice]

---

\
--- \
Oops... typo. Should read 'secondary'.
---


---
As I suspected, you haven't read the thread through.

The transformer I refer to later on was not the step-down
transformer which Sloman refererred to at first, but one which he
referred to later on, which was a transformer with dual 115
primaries which he suggested be wired in series to make the
transformer an autotransformer.

Reread his post. Whether he denies it or NOT, BOTH were stepdown
transformers.

He also stated that the secondary
(ies) be ignored.

Thats what I've been repeating in all my posts, huh you finally
understood that .

The transformer I referred to above was used just to illustrate a
few principles and was certainly not one which anyone would suggest
that the OP seriously consider, considering that the OP need a 2:1
stepdown in voltage from 240 to 120, and enough current out of the
secondary to feed a 6 watt (or, maybe, VA) load.

Had you been paying attention, you might have noticed that the
transformer I referred to, above, givea a 1:1 voltage ratio from
input to output and is capable of sypplying 120 watts into a
resistive load. Neither one of which the OP asked for, but
perfectly fine for illustrating a point.
---

The point is you failed to answer my question. ASCII drawings of
transformers may be impressive to your clients but as far as I'm
concerned ...

Irrelevant as your completely off topic

---
LOL, You really don't even know that you have no clue.
---

Now, with that behind us, let's take a look at a transformer with
dual 120V primaries and connect them in series so we can use it as
an autotransformer with a 240V input and a 120V output and see what
happens:


240V>------+
|
||P
||R
||1
|
+-->120V>--+
| |
||P |
||R [120R]
||2 |
| |
240V>------+-->120V>--+

Since we have twice the voltage across the inductance of the
primary, we'll need twice the number of turns to keep the flux
density in the core the same as it was for the 120V case, and that
criterion is satisfied with the two windings in series.

Now, just assume, for the sake of the argument, that our first
transformer:

120V>--+ || +-->120V>--+
P || S |
R || E [120R]
I || C |
120V>--+ || +-->120V>--+

-------------->>>>>>>>>Really had two 120V primaries which were
connected in parallel:
???????????????????????????????????????????????????????????????
Parallel primaries would give you 240V on the primaries NOT 120V
!!!!!!!

The OP uses 240V. 240V applied to a 2 primaries connected in parallel
gives you 240V at the Primaries. If you can't understand something so
elementary ... I think I will have to stop wasting my time here.

Take a look at the next ASCII diagram, and if you can't see what an
asinine statement it was that you made, then I don't think there's a
lot of hope for you. I think Wendy's may pay a little better than
BK, but you might have trouble getting in.

120V>--+---+ ||
| P ||
| R ||
| 1 ||
120V>--|-+-+ || +-->120V>--+
| | || S |
| | || E [120R]
| | || C |
+-|-+ || +-->120V>--+
| P ||
| R ||
| 2 ||
+-+ ||

In which case each primary would be rated for 500mA and, in
parallel, the combination could carry 1 amp.

Now, though, since we're not using the secondary and taking
advantage of the current sharing we'd get with both primaries in
parallel feeding the load on the secondary, we're forcing one of the
primaries to supply the entire load, which is going to cause twice
the voltage drop across it than would occur with with the windings
in parallel. That, in and of itself, will cause the regulation to
be poorer than it would be with a conventional transformer with the
same amount of iron in the core.

You talk about voltage regulation with a load connected to the
secondary . This isn't happen.
Man you give a bad name to this newsgroup!!!!!!!!!!!!!!!!!!!

You still didn't answer my criticism above . So I'll paste it here
again. ---Lets get this argument settled then we can talk about personal matters.

I REPEAT .......Go ask your granny she'll tell you thet : You talk
about voltage regulation with a load connected to the secondary . This
isn't happening in the case we are considering.
LOL ....

---
You're right, it's not a case _we're_ considering, because the
concepts presented, even though somewhat elementary, seem to be
beyond _your_ grasp. The truth is quite obvious

Here's a clue for you:

If we take this:

240V>------+
|
||P
||R
||1
|
+-->120V>--+
| |
||P |
||R [120R]
||2 |
| |
240V>------+-->120V>--+


and redraw and relabel it like this:


240V>-----------+
|
240V>------+ +-->120V>--+
| | |
P || S |
R || E [120R]
I || C |
| | |
+----+-->120V>--+

Does that make it any clearer?


No? Then here's one even you might be able to figure out:


240V----+
|
[120R]
|
+--->MYSTERY VOLTAGE
|
[120R]
|
GND-----+--->GND


What is the mystery voltage with reference to GND?

Wow ... I already answered that question zillions of times in my posts
.. Read carefully man.

lemonjuice

 

[Phil Allison]

"lemonjuice"


** You are clearly a schizophrenic psychopath.

Go blow you brains out - anytime.





......... Phil

 

[Fred Bloggs]

No. Get a transformer with a dual 115V+115V primary, connect the
primaries in seires, and hook your fire alarm across one of the
primaries. Ignore the secondary windings.

Cost goes by size, but 6VA transformers have rotten regulation. The
2002 Farnell catalogue lists a 12VA part (stock number 159-591) whcih
cost 6.58 euro and would presumably do the job, You'd have to put the
transfomer in a box to protect the outside world.

Farnell have a whole range of boxes - I'd probably go for the 525-625
(which cost another 5.65 euro back in 2002), and mount the transformer
on the lid. You might be able to get cute and mount the fire-detector
on the other side of the same lid (leaving it outside the box).

This ought to work - the transformer will run a bit warm, but it would
run warm without any load at all.

It makes no sense to use a transformer this way. If he can buy a
transformer then he should get a 230->115 step down.

 

[Phil Allison]

"Fred Bloggs"

It makes no sense to use a transformer this way. If he can buy a
transformer then he should get a 230->115 step down.

** Of course.

Sizes start from about 20 VA in wall wart form - eg :

http://www.dse.com.au/cgi-bin/dse.storefront/43eb0fc307c2afea2740c0a87f9c075e/Product/View/M1155


Just beware if the device draws power on one half cycle only - OR its
internal transformer fries running on 50 Hz instead of 60Hz.




......... Phil

 

[[email protected]]

It makes no sense to use a transformer this way. If he can buy a
transformer then he should get a 230->115 step down.

It's a question of price and availability. Split primary transformers
are a commodity product available off the shelf in a vast range of
sizes. Auto-transformers are a niche product, and you are likely to
have to buy a bigger transformer than you need at a consumer price.

Phil Allison's example cost some $A59 (including tax) equivalne to
$43.60 or 36.5 euro which is a couple of times more than you'd have to
spend on a regular transformer and a box to put it in.

 

[Weinberger Hans]

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

 

[Phil Allison]

<[email protected]

It's a question of price and availability. Split primary transformers
are a commodity product available off the shelf in a vast range of
sizes.

** None of which are intended for use as 2:1 mains auto-transformers.

Auto-transformers are a niche product, and you are likely to
have to buy a bigger transformer than you need at a consumer price.


Phil Allison's example....

** Is not an "auto-transformer" - but has full safety isolation.

Check out the symbols including the "double square" symbol.

cost some $A59 (including tax) equivalne to
$43.60 or 36.5 euro

** From a chain of retail, consumer electronics stores.

Buy 10 from the wholesaler and them get for half price.

Much better, much safer & ready to use with no effort.


Da Slow Man's mad ideas are just as whacko as ever.





............ Phil

 

[lemonjuice]

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

Man wake up. You are risking blowing up your companies equipment, plus
starting/not preventing a dangerous fire.

Look carefully. If you hook up a series connection at the primaries for
240V you are doubling the turns ratio! Which means you get twice the
desired voltage at the secondary.

If you want to follow that way, I'd say check if you can also get a
SERIES connection at the secondary . Otherwise don't touch/change
anything.

Can't you offer a coffee to a secretary(ies) to do a Google search for
you on CHEAP STEP DOWN TRANSFORMER 230 240 120 110 ?

Otherwise for 8-9 dollars (transport excluded) each I could get
supply you 50 of those in plastic casing rated at 45 watts ! Email me
if you're interested.

lemonjuice

 

[John Fields]

It makes no sense to use a transformer this way. If he can buy a
transformer then he should get a 230->115 step down.

 

[John Fields]

Man wake up. You are risking blowing up your companies equipment, plus
starting/not preventing a dangerous fire.

Look carefully. If you hook up a series connection at the primaries for
240V you are doubling the turns ratio! Which means you get twice the
desired voltage at the secondary.

---
Well, it seems that with every post you write the hole you're
digging around yourself gets deeper and you're getting deservedly
closer and closer to being admitted into PA's Asylum for the
Criminally Insane.

Here is a schematic of a transformer with, say, a 100 turn, 120V
primary connected to 120V mains, and a single 120V secondary
connected to a 120 ohm load


120V>--+ || +-->120V>--+
P || S |
R || E [120R]
I || C |
120V>--+ || +-->120V>--+

what is the turns ratio, primary to secondary?

Np Ep 120V
---- = ---- = ------ = 1
Ns Es 120V

So the secondary will also be wound with 100 turns.


Here is a schematic of a transformer with two 120V primaries wired
in parallel and connected to 120V mains, and a single 120V secondary
connected to a 120 ohm load


120V>--+---+ ||
| P ||
| R ||
| 1 ||
120V>--|-+-+ || +-->120V>--+
| | || S |
| | || E [120R]
| | || C |
+-|-+ || +-->120V>--+
| P ||
| R ||
| 2 ||
+-+ ||

Since the primaries are wired in parallel, they will each, and both,
have 120V across them and, since the secondary will also have 120
across it, the turns ratio will be:

Np Ep 120V
---- = ---- = ------ = 1
Ns Es 120V

Assuming that the secondary has 100 turns on it, just like the
transformer used in the previous example, means that since each
primary has 120V on it, the only difference between the transformers
is that the single-primary unit will have the entire load current
circulating in the primary, while the dual-primary unit will have
half of the load current circulating in each primary.

Now, looking at the same dual-primary transformer with the primaries
connected in series:

120V>------+ ||
P ||
R ||
1 ||
+-+ || +-->120V>--+
| || S |
| || E [120R]
| || C |
+-+ || +-->120V>--+
P ||
R ||
2 ||
120V>------+ ||

It should be easy to see that it if there are 100 turns on each
primary, then there will be, effectively, a 200 turn primary wound
on the same core with a 100 turn secondary.

That means that with a turns ratio of 2:1, primary to secondary, the
output voltage from the secondary will be half of the primary
voltage.

That being the case, for a 120V output, the input would have to be
240V.

So, your advice to:

"Look carefully. If you hook up a series connection at the primaries
for 240V you are doubling the turns ratio! Which means you get
twice the desired voltage at the secondary."

Is totally flawed because doubling the turns ratio
(primary-to-secondary) halves the secondary voltage.

---

If you want to follow that way, I'd say check if you can also get a
SERIES connection at the secondary . Otherwise don't touch/change
anything.

---
No. With a series connection on the secondary as well as on the
primary, the output voltage will doubleas the input doubles, since
the turns _ratio_ between the primary(ies) and secondary(ies) will
not have changed
---