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Old December 1st 16, 10:03 AM posted to uk.tech.digital-tv
Ian Jackson[_5_]
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Default I dislike ordering stuff that is not in stock

In message , d writes
On Wed, 30 Nov 2016 17:05:22 +0000
Ian Jackson wrote:
In message ,
d writes
On Wed, 30 Nov 2016 13:54:57 +0000
Ian Jackson wrote:
A more accurate explanation is that a practical transformer can be
considered as having a primary which consists of two windings in
parallel. Winding 1 carries the current which transfers its power to the
secondary winding. Winding 2 carries the current required to generate
the magnetic field which facilitates the transfer - but otherwise any
current flowing in Winding 2 is wasted.

Eh? Without current in the primary winding there is no magnetic field or
current in the secondary. The secondary is purely passive surely?


Like I said, a practical transformer can be considered as having a
primary which consists of two windings in parallel (Windings 1 and 2).
[Note that I'm not saying is that there are two physical primary
windings.] I have made no reference to the secondary.


Ok, but from my laymans point of view a real world transformer has 2 coils,
whether you call them windings or primary/secondary is immaterial.


OK. Just try to believe that a transformer has to have sufficient
inductance at the frequency in question.

So when connected across the mains supply with no load connected to the
secondary. its impedance (inductive reactance, or action as a choke - in
inductive ohms) is high enough to prevent it taking too much current.

As the reactance is proportional to frequency, the higher the frequency,
the greater the ohms. [I have a tiny transformer which is 120V in, 6V
out. However, it is out of an aircraft, where the onboard 'mains' used
to be 600Hz. If I fed it with 120V 50Hz, there would be a big bang.]

Conversely, if you lower the frequency, there are fewer ohms - and you
might find that the transformer is taking too much current for its own
good. To prevent this, the transformer has to be designed to have more
inductance (ie more turns, larger core - or both).

http://bit.ly/2fLP3xX

Bit over my head I'm afraid. I did nice tidy digital electronics (as part of a
CS course), analogue is all a bit voodoo to me

I have to admit I only have a grasp of the simplistic versions.

However, if you don't learn anything else about analogue, at least learn
Ohm's Law.
--
Ian
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