This was a supplementary to the aircon question.
Can anyone recommend a plug in power meter which is reasonably accurate?
Amazon offers gadgets around £10-£50 range but it is difficult to see if
you get what you pay for.
The transient current when the air-con starts will probably only last
for a fraction of a second - but if your power supply *can't* provide
that peak current, it ain't gonna start.
I'm not at all sure that you're going to find a meter which has a fast
enough response to see the transient. Certainly none of my meters - both
analog and digital - are likely to be any good.
One other instrument which I've got is a hand-held digital oscilloscope.
If I needed to do it, I'd power the air-con through a suitable shunt
resistor, and connect the scope across the shunt on a voltage range. I
would measure the steady running current using a clamp meter, and then
use that to calibrate the scope. Then I'd stop the air-con and re-start
it, and the scope would hopefully show just how big the transient is.
I've never actually done this, so others may blow holes in it - but it
seems to me that it would have a fighting chance.
On Fri, 26 Feb 2016 19:38:12 +0000, Roger Mills wrote:
Thanks - it has morphed into a more general query which is why I separated
it from the aircon thread.
It did occur to me that what was needed was not a "read the display" but
something with a more or less continuous output to a computer for later
analysis. So that peaks, troughs, spikes etc. could be graphed later.
I have no idea if there is such a device.
In more general terms I would like to find a reliable plug in (13A socket)
monitor to check everything from the dreaded MH to standby on audio-visual
kit and computer consumption.
I do have a clamp meter on the main power feed which has a remote monitor
with USB (from Eon) but I was hoping to find something for individual
Many moons ago there was a device for a Spectrum, driving a tandy plotter
via the interface 1 rs232 that had a shunt device couple by an opto isolator
that showed peak current and the waveform. It was in Electronics today
international I seem to recall. Course in those days elf and safety was
considered common sense... :-)
I think though that the shunt needs to be a buig un so it does not heat and
cause changes in resistance in the short term to be accurate.
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
Fair comment. Let's do the sums.
Let's suppose that we're looking at a device which uses 2kW in the
steady state. At 250v (to keep it simple), that's 8 amps.
You could probably get away with a 1 volt drop across the shunt. That
would require the resistance to be 1/8 ohms, and it would dissipate 8 watts.
I'm not sure that you'd easily find anything 'off the shelf' to do that,
but it would be relatively easy to create the shunt from a suitable
number of short lengths of copper wire, connected in parallel.
Many years ago, I made a series of shunts in this way to enable a cheapo
multimeter to measure automotive-type currents of up to 30A.
On Sat, 27 Feb 2016 21:42:36 +0000, John Rumm wrote:
If you're trying to measure DC amps, a visit to a local flea market may
prove fruitful. I bought a DC/AC clamp meter from "Maplin Man" for 8 quid
(afaicr I blagged him down from ten quid).
The DC amps ranges are 0 to 200 and 0 to 1200 amps with the 200A range
showing to tenths of an amp readings. However, when trying to measure the
current drawn over the 12v and 5v rails of an ATX PSU, you need to double
check the calibration since it can easily drift a few tenths of an amp
over the space of the first few minutes after pressing the "DCA Zero"
button. This drift effect can be ignored once you're dealing with tens to
hundreds of amps such as when testing lighting load and generator
charging and the obvious 'cranking amps'.
When scouting the various stalls at flea markets, you're likely to see
the more common AC clamp meter type. Tracking down a DC clamp meter is a
little more difficult. Obviously, you're looking for one with a digital
display (which isn't exclusive to AC/DC clamp meters - there's plenty of
AC only clamp meters with digital displays).
However, the stand out feature, if the labelling doesn't already make it
obvious, is the presence of a "DCA Zero" button. Without such a button,
there's no way to compensate the inevitable drift in the hall effect
sensor for DC offset to zero the meter just prior to taking a reading.
Most of these meters use a PP3 battery so it might be wise to carry a
fresh PP3 (and maybe some AA and AAA cells for the less common types that
don't use a PP3) to test any prospective purchases. If the meter has a DC
amps range calibrated in tenths of an amp, you could try the effect of
the earth's magnetic field when you fully open the jaws and turn around.
I get a half amp reading when I do this test on my own DC clamp meter.
Alternatively, a small bar magnet would be useful aid to test/confirm the
DC amps measuring feature.
HTH & HAND :-)
Accepting a voltage drop of 100mV and 1/80 R shunt would get you into
more tractable territory. Most cheap multimeters have a dedicated shunt
for 10A fsd anyway but how accurate they are on mains I don't know.
Most multimeters already have a suitable current range for 10A max.
If I were the OP I'd fit a Owl onto the main house supply lines and then
work on reducing the household base load by finding the old modem PSUs
hidden behind sideboards etc. The current clamps are not as accurate as
a dedicated 13A plug device but they let you see the entire household
consumption and seeing it will allow you to shave 10% of your
electricity bill if you want to. Finding the power hogs is the first
step. (for which you might also want one of the 13A monitor devices)
We did something similar once when wanting to assess the inrush on a
10kW transmitter. Used a clamp meter round on of the incomers. It was a
reasonably posh one that also had an external trigger output. So fed
this into a digital storage scope. Did the job quite nicely. (we had to
do multiple power on cycles to capture the worst case - which was when
switching coincided with switching near the peak of the mains waveform).
It nicely demonstrated why sometimes one or more of the three 45A MCBs
built into the amp would trip on power on sometimes - the peak inrush
was way more than the 5x allowed on a type B. They had to swap them for
either type C or D versions.
It would, but this was a control system refit on what was already a
mostly 40 year old design. So there was little appetite to make many
changes to the power or RF side of the hardware even though all the
control systems were becoming microprocessor controlled. Specifying a
change of MCB type was an easy and low cost change. (in normal
operation, these things would not normally be power cycled for extended
periods of time anyway).
Here was the beastie, a Marconi HF1141 :
The main cabinet is about 6' tall (look at the set of 3 "normal" MCBs on
the top right by the power switches to get a feel for the scale). The
lump grafted on the top was a modern fully synthesized "digital drive"
radio transceiver (all DSP based). The lump under it was basically a
400W pre-amp, some tuning circuitry (its a tunable HF amp), a 10kV step
up transformer and a 'kin big valve!
(for those familiar with the typical LC tuned resonant circuit used for
radio tuning, you can see all the bits there - what looks like a coil of
15mm water pipe is the main inductor, and all the motors and
gearboxes at the top of the bottom middle section are used to vary the
two vacuum capacitors (in white cylinders in the middle). The caps were
only a few hundred puff if memory serves, but rated for many kV
 The inductor also had a series of shorting bars that could tap it
off at shorter lengths - these were operated by a bank of large
solenoids under software control. Which all made a rather satisfying
series clanks as they were operated in quick sequence to adjust the
On Sat, 27 Feb 2016 21:33:46 +0000, John Rumm wrote:
That LC resonant circuit is most likely the 'classic' Pi "tank" circuit.
Effectively a resonant matching 'transformer' with the rather useful
characteristic of an LPF. Were the two tuning caps still labelled "Plate"
Indeed it was... with the slightly less commonly seen added wrinkle of
having both variable L as well as C components.
Quite likely - I can't remember for sure, I did not spend much time
studying the circuit diagrams for the RF side of it - I was too busy
finding bugs in the digital hardware, and rediscovering secrets of the
fine details of the control interfaces to the analogue side.
(amazingly the chap who did the original RF design still worked there
over 40 years later - but the spec he had produced for the hardware
software interface (not surprisingly) had one or two gaps or errors in it)
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