[ARC5] Receiver AC Power Supplies

[Brian]

Actually, core magnetisation, and hence saturation, is dependent on the 
Voltage swing peak, not the current. This is a major problem with 
transformers designed for the European (220 V) and Japanese (100 V) markets. 
Unscrupulous marketers would sell equipment with these transformers in them 
to Australia, New Zealand, South Africa where the mains is nominally 240 V 
and to the US where the mains is 115 V. The extra 10 to 15% Voltage was 
sufficient to take the transformers into saturation. The recent so-called 
'harmonisation' of the mains in Australia to 230 V is a crap-out - that's 
the lowest Voltage and the swing just after meal time can often go to 265 V 
before lazy tap-changers start working.

Another problem for core magnetisation is the operating frequency. 
Transformers designed for 60 Hz mains will soon overheat when fed with 50 
Hz, unless the input is dropped to 50 / 60 of the nominal Voltage; here, the 
volume of the core is important. It takes a brave manufacturer to design and 
offer goods for all world frequencies, without changing transformers, eg, 
Collins, HP, IBM, Racal, Tektronix.

73 de Brian, VK2GCE.

On Saturday, December 13, 2014 2:14 PM , Roy said:

On Dec 12, 2014, at 9:58 PM, Brian <brianclarke01 at optusnet.com.au> wrote:

> ...Be very, very, very careful of marketing 'claims'. A transformer rated 
> at 25 V and 2 A, may not be designed to deliver both at once. The actual 
> deliverable VA depends on the design specifications, not the marketing PR.

Indeed,

I remember a report about the Radio Shack small filament transformer (maybe 
the 12.6 volts at 1.2 amps one).  Reportedly, the core in this device went 
into saturation at a load current much less than the rating . At full 
current, the output wave form was seriously clipped at both upper and lower 
peaks.  And, under these conditions, the thing became so hot as to burn the 
fingers if they were held on the transformer.

<snip>
Roy Morgan