Re: measuring output of adapter
- From: adrian@xxxxxxxxxxxxxxxxxxxxxxxxxxxx (Adrian Tuddenham)
- Date: Thu, 30 Jul 2009 11:52:24 +0100
Tim Hodgson <thnews@xxxxxxxxxxxxxxxxxxxxxx> wrote:
Jaimie Vandenbergh <jaimie@xxxxxxxxxxxxxxxxxxxxx> wrote:
On Thu, 30 Jul 2009 10:21:44 +0100, thnews@xxxxxxxxxxxxxxxxxxxxxx (Tim
Hodgson) wrote:
Ok, excuse my appalling ignorance, but:
I want to check whether an ultra-cheap mains adapter for a Powerbook is
outputting the wattage it claims. Can I do this with a multimeter or
will I blow myself up? (Ok, those aren't mutually exclusive outcomes...)
Unlikely, but you might pop the adapter if you short-circuit pin+ring
and it doesn't have designed-in protection against that sort of thing.
Presumably you just apply the probes to the sleeve and tip of the plug?
Nope, that'll only get you voltage. Or ampage-through-the-multimeter,
if you set amps, which is no use to anyone.
Particularly as trying to measure amps in that way will short-circuit
the power supply and may burn out the meter.
Ah. That's where the appalling ignorance comes in. I'd assumed I could
just measure volts then amps, then P = VI. But you mean that amps
without the laptop connected will be a meaningless figure?
What about measuring ohms, then V^2 / R = P ?
No, not that way either.
The only thing you can measure directly with the meter is the off-load
volts. If it is a cheap adaptor, that could be anything up to 50%
higher than the on-load volts, even if the adaptor can handle the power
drawn.
If the adaptor is a bit better than total rubbish and does attempt to
stabilise the voltage, the meter will only read the average voltage. A
stabilised adaptor stores charge at the peak of each half-cycle of the
mains so as to tide it over until the next peak comes along (100th of a
second later). If there isn't a big enough storage capacitor for the
load, the voltage will droop below the stabilising level 100 times a
second and give really bad ripple, an averaging voltmeter won't
necessarily tell you for certain if that is happening.
Some adapters can handle the load without ripple, but there isn't much
room to get rid of the heat from the stabiliser chip. If the chip
overheats, some types will shut down safely and re-start after they have
cooled (which is a bit of a nuisance), others will burn out.
Excessive ripple and drooping output voltage will not upset a
well-designed piece of equipment (like a Powerbook ...I hope) which may
already have built-in stabilisers delivering a lower voltage level
internally - or it may have a switched-mode power supply which adjusts
itself to whatever voltage is available.
If you use the adaptor and it overheats or doesn't work the Powerbook
properly, take it back and get your money refunded because it doesn't do
what it was sold to do.
--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
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