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From: "Phil Allison"
References: <email@example.com> <firstname.lastname@example.org> <8e+6RgBNico9EwL3@jmwa.demon.co.uk>
Subject: Re: Doubling wallwart power rating
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Date: Tue, 8 Oct 2002 08:35:24 +1000
NNTP-Posting-Date: Tue, 08 Oct 2002 08:23:40 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"John Woodgate" wrote in message
> I read in sci.electronics.design that Phil Allison
> wrote (in ) about 'Doubling
> wallwart power rating', on Tue, 8 Oct 2002:
> > ** John is speaking off the top of his head again. How many wall
> >warts has he measured I wonder - less than me for sure.
> Since you don't say how many you have measured, your statement is not
> worth much. I've measured about 100.
** Similar number here - most have the same characteristic.
> > The last one I saw was rated at 12 volts and 500mA DC. It
> >the same primary rms current with both full load and no load.
> How did you measure it, exactly?
** Using a instrument called a Current Monitor. My own design
published in Electronics Australia magazine a few years back. It uses a LEM
Hall effect transducer (LTA 50P) followed by a AD536 true rms to DC
converter IC and a 3.5 digit led DPM. There is range switching for 2.000 and
20.00 amps done by selecting a 1 or 10 turn wind on the LEM. It also has a
BNC output for waveform monitoring on a CRO. The LEM has a 100kHz bandwidth
and the AD536 exceeded that at most levels. Accuracy and linearity is better
There is an internal power supply for the electronics which
remains connected to the AC line while the metered circuit is kept seperate
so a Variac can be used. A 6.3 amp fuse protects the 2 amp range and a 16
amp triac is wired with gate to MT2 prevents interruptions to the load
current when the range is selected by a 16 amp SPDT relay.
You did say exactly !!
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
Did you use a true
> r.m.s. meter, because the magnetizing current is very peaky? If that
> transformer drew around 1 A (12 V x 500 mA x 1.6) on no-load, there was
> something seriously wrong with it.
** I think you meant 10VA not 1 amp. Yep, the off load current was
about 45 mA. The primary was about 350 ohms which gives a copper loss of
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 load
value and charging peaks appear at slighlty less amplitude. The rms value is
the same +/- 10%.
> 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.
> > 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
** Believing that is your problem John. Therory will not tell you what
some maker in a factory decided to do.
........ .. love, Phil
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