Transformer newbie question

M

Mark

Guest
[Preamble]
Many years ago at school, transformers were simple things: you pop an AC on
one side (primary) and out pops a different AC (the secondary) on the other
side. I leant that this was a good way to step down (or up) voltages.
However, reading the FAQ and various pages I find that they're far from the
simple beasts that I once knew.
[/Preamble]

So here's the question:
I need a 9v 6vA AC output (input 240V UK mains). So I've purchased a
transformer that meets exactly that spec. What arrived is kind of this, but
not quite... At the top of the transfomer are two termoinals marked 230-240V
50/60Hz. So far so good. However, at the bottom, I have 4 terminals in a
horizintal line in 2 pairs. Each pair has 9v 3vA written above it. Now
before I set my house on fire wiring this up incorrectly (the input will be
fused :) ), can someone enlighten this newbie as to how I get the correct
output from this transformer? (Combine the two outputs? Send it back and ask
for the _correct_ transformer this time?)

Sorry for the noddy question!

TIA
Mark
 
In article <3f6825df$0$242$cc9e4d1f@news.dial.pipex.com>,
Mark <mark@ReMoVeThIsBiTmossywell.com> wrote:
However, at the bottom, I have 4 terminals in a
horizintal line in 2 pairs. Each pair has 9v 3vA written above it. Now
before I set my house on fire wiring this up incorrectly (the input will be
fused :) ), can someone enlighten this newbie as to how I get the correct
output from this transformer? (Combine the two outputs? Send it back and ask
for the _correct_ transformer this time?)
Click to expand...
This transformer has two separate secondary windings, each giving 9V at 3VA.
This is a flexible implementation as it allows the designer to use the
transformer for more applications.

A Use the two secondaries separately; this gives you "two" transformer
each for 9V 3VA (0.333A). View picture with a fixed width font.
_____ _____
X:X*
X:X 9V
X:X 3VA
X:X_____
230V X: _____
X:X*
X:X 9V
X:X 3VA
_____X:X_____

B Connect the two secondaries in series; this gives you a transformer
for 18V 6VA (0.333A). Primary and core not shown again.
_______________
X*
X 9V
X 3VA
X________ 18V
________| 6VA
X*
X 9V
X 3VA
X_______________

C Connect the two secondaries in series with center used; this gives
you a transformer 2 x 9V 3VA (0.333A) suitable for a power supply
producing an unregulated +12V and -12V.
_______________
X*
X 9V
X 3VA
X________
________|______
X*
X 9V
X 3VA
X_______________

D Connect the two secondaries in parallel; this gives you a transformer
for 9V 6VA (0.667A).
___________.___
X* |
X 9V |
X 3VA |
X_______ | 9V
_______|___| 6VA
X* U
X 9V |
X 3VA |
X_______|_______

For your application you need to use example D.

It is important to connect the secondaries correctly. If they are
connected so that the induced voltages in the secondaries tries to
cancel out each other then a series configuration gives 0V and the
parallel configuration gives a very hot transformer.

The secondaries are often marked with a dot on one end. This is indicated
by the * in the pictures above. Use this dot as you would use a + on a
battery when connecting batteries together.

--
Göran Larsson http://www.mitt-eget.com/
 
In article <3f6825df$0$242$cc9e4d1f@news.dial.pipex.com>,
mark@ReMoVeThIsBiTmossywell.com says...
| Each pair has 9v 3vA written above it. Now
| before I set my house on fire wiring this up incorrectly (the input will be
| fused :) ), can someone enlighten this newbie as to how I get the correct
| output from this transformer? (Combine the two outputs? Send it back and ask
| for the _correct_ transformer this time?)
|
|


primary
| |
xxxxxxxxx
---------
xxxx xxxx
| | | |
A B C D
created by Andy?s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de



The above transformer allows you three options.

1. The obvious 2 x separate 9v 3vA supplies (A, B + C, D)
2. 18v 3vA, if you connect the middle two pins together (B and C) and
take the supply from the outer pair (A and D).

3. 9v at 6vA if you connect one LH term to the other LH term (A to C)
and likewise with the RH ones (B to D).

3 assumes that the two winding exactly match each other for voltage and
that the manufacturer has not crossed over the terminals. The latter
effect would be the equivalent of a dead short across the secondary. It
is rare for a manufacturer to make a mistake, but it does sometimes
happen. To check for the prior, link A to C, power up, then check for
any AC voltage difference between B and D. If there is none, then it is
safe to link the two. A difference of the order of 18v, would indicate
a reversed winding operating as 2.


--
Regards,
Harry (M1BYT)...

Remove the 'NOSPAM' in my email address to reply.

Free Amateur Radio Courses:-
http://www.ukradioamateur.org
 
"Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> writes:
You can safely operate the 2 secondaries in parallel to achieve the
required VA rating. There is no danger in this because the transformer
will automatically split the power to each winding. The only precaution
is to match their phases.
Click to expand...
Careful. This assumes they are identical with the same number of turns.
Any difference will show up as wasted power flowing throught he two
windings determined by the voltage difference divided by the winding
resistance.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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"Mark" bravely wrote to "All" (17 Sep 03 10:14:06)
--- on the heady topic of "Transformer newbie question"

You can safely operate the 2 secondaries in parallel to achieve the
required VA rating. There is no danger in this because the transformer
will automatically split the power to each winding. The only precaution
is to match their phases.

One way to do this is to temporarily connect 2 "like colour or marking"
terminals and measure the voltage across the 2 remaining terminals. If
the AC voltage is zero or nearly so then the phase is correct. If
instead the voltage is twice the rated voltage then the phase is wrong
and you need to invert one winding connection. (i.e. marks in error)


Ma> From: "Mark" <mark@ReMoVeThIsBiTmossywell.com>

Ma> [Preamble]
Ma> Many years ago at school, transformers were simple things: you pop an
Ma> AC on one side (primary) and out pops a different AC (the secondary) on
Ma> the other side. I leant that this was a good way to step down (or up)
Ma> voltages. However, reading the FAQ and various pages I find that
Ma> they're far from the simple beasts that I once knew.
Ma> [/Preamble]

Ma> So here's the question:
Ma> I need a 9v 6vA AC output (input 240V UK mains). So I've purchased a
Ma> transformer that meets exactly that spec. What arrived is kind of
Ma> this, but not quite... At the top of the transfomer are two termoinals
Ma> marked 230-240V 50/60Hz. So far so good. However, at the bottom, I have
Ma> 4 terminals in a horizintal line in 2 pairs. Each pair has 9v 3vA
Ma> written above it. Now before I set my house on fire wiring this up
Ma> incorrectly (the input will be fused :) ), can someone enlighten
Ma> this newbie as to how I get the correct output from this transformer?
Ma> (Combine the two outputs? Send it back and ask for the _correct_
Ma> transformer this time?)
Ma> Sorry for the noddy question!

Ma> TIA
Ma> Mark

.... Which sparks some mnemonic circuitry.
 
"Sam Goldwasser" <sam@saul.cis.upenn.edu> wrote in message
news:6wwuc7wi3y.fsf@saul.cis.upenn.edu...
"Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> writes:

You can safely operate the 2 secondaries in parallel to achieve the
required VA rating. There is no danger in this because the transformer
will automatically split the power to each winding. The only precaution
is to match their phases.

Careful. This assumes they are identical with the same number of turns.
Any difference will show up as wasted power flowing throught he two
windings determined by the voltage difference divided by the winding
resistance.
Click to expand...
To all that replied to my message: I followed all the instructions noting
the comment about testing the phase first and was happy to see 0V between
the non-connected terminals. Phew! I've since connected the independent
secondaries in parallel and got the output I'm after. Strange that there is
no dot or * to mark the phases on the seconary terminals, but c'est la vie;
at least I know how to test it in future (noting the comment about what if
the number of turns is different).

Thanks to all!
Mark


--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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Click to expand...
 
"Sam Goldwasser" bravely wrote to "All" (17 Sep 03 11:27:13)
--- on the heady topic of "Re: Transformer newbie question"

SG> From: Sam Goldwasser <sam@saul.cis.upenn.edu>

SG> "Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> writes:
You can safely operate the 2 secondaries in parallel to achieve the
required VA rating. There is no danger in this because the transformer
will automatically split the power to each winding. The only precaution
is to match their phases.
Click to expand...
SG> Careful. This assumes they are identical with the same number of
SG> turns. Any difference will show up as wasted power flowing throught he
SG> two windings determined by the voltage difference divided by the
SG> winding resistance.

That's what one would assume at first thought but in fact no heavy
current will flow between the windings as one might otherwise imagine.
The flux is shared by the two windings. The core's flux couples both
windings with the result it naturally adjusts for the small difference
between them to some middle value.

Imagine the reverse situation in which we apply the same emf to both and
these induce the sum of their flux in the core to energize the primary.
The winding with the less turns will simply contribute less but the
total flux is the sum of each.

.... Is reactance then illusory? No, it just appears that way...
 
"Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> writes:

"Sam Goldwasser" bravely wrote to "All" (17 Sep 03 11:27:13)
--- on the heady topic of "Re: Transformer newbie question"

SG> From: Sam Goldwasser <sam@saul.cis.upenn.edu

SG> "Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> writes:

You can safely operate the 2 secondaries in parallel to achieve the
required VA rating. There is no danger in this because the transformer
will automatically split the power to each winding. The only precaution
is to match their phases.

SG> Careful. This assumes they are identical with the same number of
SG> turns. Any difference will show up as wasted power flowing throught he
SG> two windings determined by the voltage difference divided by the
SG> winding resistance.

That's what one would assume at first thought but in fact no heavy
current will flow between the windings as one might otherwise imagine.
The flux is shared by the two windings. The core's flux couples both
windings with the result it naturally adjusts for the small difference
between them to some middle value.
Click to expand...
So are you saying that if a pair of windings of slighlty different
voltage are connected out-of-phase to obtain the difference voltage,
there won't be significant current flow if the outputs are shorted
together?

If I take a 5 VAC and 6.3 VAC winding and connect them together such
that the result is 1.3 VAC, then short these together, I will get a
large current. I measured it for a Stancor PM-8423 - 3.5 A. I would
assume that if there was 1/10th the differenc - 5 V and 5.13 V windings,
it would be about 0.35 A - still significant power wasted in IR heating.

Or do I misunderstand what you are saying?

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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"Sam Goldwasser" bravely wrote to "All" (17 Sep 03 20:08:09)
--- on the heady topic of "Re: Transformer newbie question"

SG> So are you saying that if a pair of windings of slighlty different
SG> voltage are connected out-of-phase to obtain the difference voltage,
SG> there won't be significant current flow if the outputs are shorted
SG> together?

Connecting them out of phase will let out the smoke... phase matters!
Heck, short out a normal winding and one will get smoke since each
terminal is the opposite phase to the other. What are you asking?

Normally, if one winding is say 5.9v and the other is 6.1v, are parallel
connected in phase there won't be a massive current flow. For example if
the windings have an impedence of 0.01 ohms each they will not conduct
10 amps of current internally, nor will there be heating in the core.

If you don't believe it then there should be a hell of a lot of fires
being set all over the world because it is done quite commonly.


.... Over a hundred billion electrons were used in crafting this tagline.
 
"Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> writes:

"Sam Goldwasser" bravely wrote to "All" (17 Sep 03 20:08:09)
--- on the heady topic of "Re: Transformer newbie question"

SG> So are you saying that if a pair of windings of slighlty different
SG> voltage are connected out-of-phase to obtain the difference voltage,
SG> there won't be significant current flow if the outputs are shorted
SG> together?

Connecting them out of phase will let out the smoke... phase matters!
Heck, short out a normal winding and one will get smoke since each
terminal is the opposite phase to the other. What are you asking?
Click to expand...
Read what I said - the voltage difference at the output is 1.3 VAC,
they are out of phase in series. The two terminals for the output
would be connected together to parallel them. If I put an ammeter
between these I get a large current even though they are almost the
same voltage - as expected. I measured 3.5 A and that was with
the series resistance of the meter probe wires as well.

Normally, if one winding is say 5.9v and the other is 6.1v, are parallel
connected in phase there won't be a massive current flow. For example if
the windings have an impedence of 0.01 ohms each they will not conduct
10 amps of current internally, nor will there be heating in the core.
Click to expand...
Have you done the experiment? There may not be a huge current but
there will be I would guess a few hundred mA. We may be arguing
about not that much but it's still not insignificant.

If you don't believe it then there should be a hell of a lot of fires
being set all over the world because it is done quite commonly.
Click to expand...
No it isn't. Most of the time, the parallel connections are made
using windings that are very nearly identical in number of turns.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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"Sam Goldwasser" bravely wrote to "All" (18 Sep 03 08:11:25)
--- on the heady topic of "Re: Transformer newbie question"

SG> From: Sam Goldwasser <sam@saul.cis.upenn.edu>

SG> Read what I said - the voltage difference at the output is 1.3 VAC,
SG> they are out of phase in series. The two terminals for the output
SG> would be connected together to parallel them. If I put an ammeter
SG> between these I get a large current even though they are almost the
SG> same voltage - as expected. I measured 3.5 A and that was with
SG> the series resistance of the meter probe wires as well.

If you don't believe it then there should be a hell of a lot of fires
being set all over the world because it is done quite commonly.
Click to expand...
SG> No it isn't. Most of the time, the parallel connections are made
SG> using windings that are very nearly identical in number of turns.

Okay, for now I'll take your word for it until I do my own homework.
No sense arguing without being sure about the quantities...

Asimov
******

.... Is reactance then illusory? No, it just appears that way...
 
"Sam Goldwasser" bravely wrote to "All" (18 Sep 03 08:11:25)
--- on the heady topic of "Re: Transformer newbie question"

SG> From: Sam Goldwasser <sam@saul.cis.upenn.edu>

SG> Read what I said - the voltage difference at the output is 1.3 VAC,
SG> they are out of phase in series. The two terminals for the output
SG> would be connected together to parallel them. If I put an ammeter
SG> between these I get a large current even though they are almost the
SG> same voltage - as expected. I measured 3.5 A and that was with
SG> the series resistance of the meter probe wires as well.

If you don't believe it then there should be a hell of a lot of fires
being set all over the world because it is done quite commonly.
Click to expand...
SG> No it isn't. Most of the time, the parallel connections are made
SG> using windings that are very nearly identical in number of turns.

Just did a quick test and you were right. I used a small filament
transformer with separate 5 volt and 6 volt secondaries. I connected
these so there was a 1 volt difference (in phase). When I connected the
2 remaining terminals to a 1 ohm 2 watt resistor the voltage dropped to
0.4 volts. Obviously power was being drawn and I must now repeat the
experiment a little more thoroughly for my peace of mind. I'm not sure
the voltage drop was so large because the winding impedance affected the
circuit or there actually was a counter emf reduction as I had said.

Sorry if you blew your meter's fuse.

Asimov
******

.... Is reactance then illusory? No, it just appears that way...
 
"Asimov" <Asimov@-removethis-bbs.juxtaposition.dynip.com> writes:

"Sam Goldwasser" bravely wrote to "All" (18 Sep 03 08:11:25)
--- on the heady topic of "Re: Transformer newbie question"

SG> From: Sam Goldwasser <sam@saul.cis.upenn.edu

SG> Read what I said - the voltage difference at the output is 1.3 VAC,
SG> they are out of phase in series. The two terminals for the output
SG> would be connected together to parallel them. If I put an ammeter
SG> between these I get a large current even though they are almost the
SG> same voltage - as expected. I measured 3.5 A and that was with
SG> the series resistance of the meter probe wires as well.

If you don't believe it then there should be a hell of a lot of fires
being set all over the world because it is done quite commonly.

SG> No it isn't. Most of the time, the parallel connections are made
SG> using windings that are very nearly identical in number of turns.

Just did a quick test and you were right. I used a small filament
transformer with separate 5 volt and 6 volt secondaries. I connected
these so there was a 1 volt difference (in phase). When I connected the
2 remaining terminals to a 1 ohm 2 watt resistor the voltage dropped to
0.4 volts. Obviously power was being drawn and I must now repeat the
experiment a little more thoroughly for my peace of mind. I'm not sure
the voltage drop was so large because the winding impedance affected the
circuit or there actually was a counter emf reduction as I had said.

Sorry if you blew your meter's fuse.
Click to expand...
Test performed on a Variac with meter on 10 A range. :)

Thanks.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

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