Re: strange power draw

On Sun, 15 Feb 2009 16:37:46 GMT, Johnny B Good wrote:

The message <ddWdnQleLfXZjAXUnZ2dnUVZ8oidnZ2d@xxxxxxxxxxxxxx>
from Dean <de@xxxxxxxxxxxx> contains these words:

On Sat, 14 Feb 2009 12:09:45 GMT, Johnny B Good wrote:

====snip====

The 30 watt reading seems to be more likely due to emc interference to
the digital processing within the meter, rather than the load itself
being around the required 20 watt mark for it to fall within the +/- 10
watt +/- 1 digit +/- 5% scope of accuracy for a 30 watt reading. I'd be
very concerned over a PC showing a standby figure in excess of 10 watts.

HTH

Thanks. A 10W tolerance still gives me 20-40W being pulled by the USB
freeview/aerial. Is it normal for an aerial to pull current like this or is
it due to the design of the USB stick or perhaps bad wring? The aerial lead
is connected to a splitter which in turn connects to another TV and the roof
aerial.

Ok, first things first. Is the earth connection to the PC's mains
connector good? I've seen PC cases at half mains voltage due to an open
safety earth at the (rewirable) plugtop due to the Y capacitors in the
emc filter of the psu acting as a Hi-Z potential divider. You can test
this with a neon screwdriver or a digital voltmeter (use a known to be
earthed contact for the other meter lead).

Not sure I would know where to start!

If such testing reveals an open cct earth, this does allow the
possibility of half (or even full) mains voltage from other sources (TV
sets, STB's and so on) also connected to the aerial to drive current
into the live connection you've got the wattmeter connected to. However,
I wouldn't expect starting the computer would change this condition as
seems to happen in your case.

Yes that's the strange thing. I think I'll follow Robs advice and see what
it reports with the STB/TV disconnected from the other side of the splitter.

If the PC case earth is good, any such external sources of mains
current cannot directly add to that being monitored by your wattmeter,
so it has to be an effect that causes a change in the PC's PSU load.

I don't feel competent to confirm whether the PC/PSU earth is good or not.

However, it won't be a sudden increase from 2 watts to 30 watts, more
likely a change from, let's say, 13 watts to 13.5 watts. Such wattmeters
can sometimes react out of all proportion to such tiny changes as the
one considered in this example, and your case might actually be as
simple as that, a meter reading anomaly.

If you're really concerned, and you're competent to do so, take a
current reading of the live feed to your PC with a, not so exotic,
digital multimeter (my cheap, sub ten quid Aldi special has 2, 20, 200mA
and 20A AC ranges) so you should either see the expected 15:1 change of
current (unlikely) or else get a reading somewhere in the region of 20mA
which only shows a few mA change between the 2 and 30 watt readings
you're getting of that wattmeter.

Sounds like that is the definitive way of proving whether the watt meter is
correct or not.

HTH

Thanks for the help.

.

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