The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: "Phil Allison"
References: <email@example.com> <firstname.lastname@example.org> <8e+6RgBNico9EwL3@jmwa.demon.co.uk>
Subject: Re: Doubling wallwart power rating
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
X-MIMEOLE: Produced By Microsoft MimeOLE V5.50.4522.1200
Date: Tue, 8 Oct 2002 20:49:07 +1000
NNTP-Posting-Date: Tue, 08 Oct 2002 20:37:18 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"John Woodgate" wrote in message
> >> How did you measure it, exactly?
> [snip interesting account of meter design]
** Like to see the circuit diagram ? I could send a jpeg.
> >Did you disconnect the rectifier, so as
> >> to measure the *transformer's* no-load current?
> > ** Oh come on, to remove filter capacitor leakage current!! Bit
> Well, while you might know that the cap wasn't faulty, it does happen.
** I can view the primary current wavefrom - pure magnetising
> > Most wall warts I see are similar - when loaded at the rated
> >current the magetising current peaks drop down to about 60 % of the off
> >value and charging peaks appear at slighlty less amplitude. The rms value
> >the same +/- 10%.
> With 12 V at 500 mA d.c. output, the secondary voltage must be about
> 13.5 V and the current must be about 800 mA, assuming a bridge
> rectifier. That's a secondary side power of 10.8 W. Only if the
> transformer were 100% efficient would that translate to 45 mA (your
> measured off-load current) at the primary side. In fact, the copper loss
> must be about (not exactly) 1.4 W and there is some iron loss, likely to
> be less than the copper loss unless the transformer uses semi-scrapless
> laminations. Say 2 W total losses. Primary current 53 mA, plus
> magnetizing current in quadrature. That was 45 mA offload. Suppose it's
> 30 mA on load, due to the primary IR drop reducing the voltage available
> for generating induction. That's 61 mA total.
** The figures were only from memory John - just to give you the
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.
> >> Remember our discussion about mains leads? When you gave me ALL the
> >> facts about your tests, we agreed.
> > ** Not quite the way I remember it.
> We initially disagreed because I did calculations based on the wire
> having the normal resistivity of copper. When you disclosed that in fact
> the wire had a much higher resistivity, we agreed. I passed your warning
> on to the British Standards committee responsible for 60065 and 60950,
> with advice to members to increase goods inwards checks on mains leads.
> I wonder if any others did that in their countries.
** 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!)
> >> >
> >> > You have to do tests to know - theory will not tell you.
> >> Ah, well, that depends on whether you know how to apply the theory, of
> >> course.
> > ** Believing that is your problem John. Therory will not tell you
> >some maker in a factory decided to do.
> Applying the theory when evaluating products *includes* knowing how
> other designers apply it. No, the theory won't tell you what design
> practices are *used*, but it will tell you how to interpret the results
> of measurements.
** That is true.
> I do tests; it's what I do to while away the long hours between e-mail
> discussions with you. (;-)
** I do them when designing, repairing, de-bugging and re-engineering
Some stuff is beyond re-engineering .. . . ;-)
> 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.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup