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From: John Woodgate
Subject: Re: Doubling wallwart power rating
Date: Tue, 8 Oct 2002 14:24:33 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Tue, 8 Oct 2002 15:19:23 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Phil Allison
wrote (in ) about 'Doubling
wallwart power rating', on Tue, 8 Oct 2002:
> ** Like to see the circuit diagram ? I could send a jpeg.
Yes, please. Or a .gif. Something as compressed as possible that is
still readable. The follow-up address of my newsgroup posts is real.
> ** I can view the primary current wavefrom - pure magnetising
No iron loss? Hysterical! (;-)
> ** The figures were only from memory John - just to give you the
Ah, memory. That's what gets you (one) to quote a 35 K rise as a 15 K
rise above 20 C ambient. (;-)
> But I have some exact ones for you now:
> Dick Smith Electronics cat. no. M 9525 3 -6- 9 volt DC plug pack @
>200 mA nominal.
> Primary 45 mA rms off load - secondary 13.3 volts DC
> Primary 43 mA rms with rated load of 200 mA @ 9.1 volts DC
> Primary 42 mA rms with overlaod of 375 mA @ 8.2 volts DC
> Primary ohms = 930 at room temp ( 25 C)
> I get similar results with an 4 VA AC plug pack too.
I don't think I'll buy any of those. Would you please measure the no-
load current at 240 V +10%?
> ** The Western Australian "Energy Safety" people have now seen the
>actual lead and were horrified. They have commenced a major investigation
>taking over 100 sample IEC leads from various PC dealers across Perth for
>tests. They issued warnings and gave safety law info to the same dealers.
> I could not get the people in NSW to do a f......g thing.
> A colleague in Perth did the job for me. ( A$1 post each way - it
>is 2000 miles!)
Pertheverance pays! (Sorry, irresistible!)
>> If you design a transformer so that the core is deep into saturation at
>> nominal mains voltage (which is what is implied by the input current
>> being the same on no-load as on full load), it WILL overheat under
>> safety testing at high mains voltage (+6% or +10% depending on the
>> standard; one even requires +15%!).
> ** Well, plug paks tend to get 24 hour, 365 day usage and often with
>low or no loads.
> I don't see overheating problems very often at all - except
>where a clear overload has been applied.
Well, you will surely agree that the test I propose above is relevant?
If the core is deep into saturation, no-load is the worst case for
overheating with high input voltage, because no-load gives the highest
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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