Power meters

[ppppenguin]

Lots of inexpensive plug-in power meters available, such as:

http://www.amazon.co.uk/Power-Energy-Mon...B00E4H0L40

http://www.screwfix.com/p/energenie-ener...cket/3477h

Are these any good? Can anyone recommend anything better?

[hamid_1]

About 5 years ago I bought a couple of these: http://www.maplin.co.uk/p/15a-plug-in-en...itor-l61aq

I didn't actually buy them from Maplin. There's a guy at a local car boot sale who sells customer returned Maplin products very cheaply - I got the meters for £1.50 each. They had minor faults but I fixed them by opening and cleaning the switches / PCB contacts.

The meter displays mains voltage, current, frequency, power factor, VA and 'real' watts. It also keeps a cumulative total of KWh used, which is reset when you unplug it. It only reads down to 1 watt, but I still found it useful for working out the standby power consumption of things like set-top boxes. It also enabled me to work out how much electricity it takes to make a loaf of bread in a bread machine, which is rated at 800 watts but operates intermittently.

I found it convenient to plug my variac into the meter, then use the meter to measure the current drawn by the variac and its load as the output voltage is turned up - this can alert you to possible faults.

A point to note: this Maplin meter is unfused, and claims to be able to measure up to 15 amps, but clearly it will be unsafe to draw that amount of current for any length of time. The BS1363 socket is only rated at 13A. I did test it briefly with an electric fire (2000W) and it worked without any problems.


I haven't used the meters you mentioned on Amazon and Screwfix, but they look like updated versions of the ones I've got. They have the facility to calculate cost if you program in the price per unit, however the reviews on Amazon suggest that's fiddly to set up. Other reviews said the instructions were poorly translated and some thought their meter was faulty. Maybe worth buying one from a shop that you can easily return it to?

[Mark Hennessy]

I have a relatively old Brennenstuhl PM230 - I think it came from Maplin, but possibly CPC. Example photo here:

http://www.ebay.co.uk/itm/Brennenstuhl-P...1712204369

It's OK. I've not checked the accuracy against a DMM, but it's fine for indication. In terms of functionality, is sounds a lot like Hamid's - it does all the same measurements, and will keep a running total for energy cost display. Only "fail": plug in a device, and the lead obscures the buttons and display

I get the impression that these plug-in types are much of a muchness; they work well enough for basics. If you need more accuracy, then you need to step up to something like the Voltech PM100:

http://www.govliquidation.com/auction/vi...Id=7350504

I'd love one of these, but I believe that they've been out of production for a while, and you rarely see them come up s/h.

As a half-way house, my MDU with monitoring outputs works quite well. The metering isn't quite in the Fluke class, but the accurately scaled outputs on the BNCs can be hooked up to external meters when more accurate results are needed (saves messing about with mains wiring, etc). And of course, you can measure phase with a 'scope and calculate PF if the load current isn't too complex. Looking at the current waveform is incredibly useful if you do a lot of PSU work...

[ppppenguin]

Mark, that's a nice bit of engineering you did on that MDU. Did you consider the KISS solution for current resolution: Winding 10 turns through the CT. Looks like they would just about fit.

[Mark Hennessy]

Thanks

Yes, I considered it briefly. But the hole in the CT is only about 6.5mm, so it would have been a tight fit - perhaps only 2 or 3 turns possible with the chunky cable I was using. I needed the op-amp anyway, so it was no hardship to provide the gain...