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From: "Phil Allison"
References: <firstname.lastname@example.org> <email@example.com> <8e+6RgBNico9EwL3@jmwa.demon.co.uk>
Subject: Re: Doubling wallwart power rating
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Date: Tue, 8 Oct 2002 20:49:07 +1000
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"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.
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