What a difference a meter makes...

[DamonHD]

Hi,

I had been measuring power consumption of devices with my PM230 plug-in power meter.  I know that it's not very good at the low end, eg for devices with a consumption under (say) 30W.

I have been lent an alternative plug-in meter and compared the two:

http://www.earth.org.uk/note-on-plug-in-power-meters-for-the-UK-REVIEW.html

Basically, the 2000MU seems to measure smaller consumptions with more precision and seems also to handle power-factor better.  In any case, it seems to resport at least somewhat lower consumption figures for almost everything, but also lets me see smaller changes in device consumption, eg ramping up and down processing load on my server and laptop.

In particular it seems that my server/laptop mains power supply is 90%+ efficient and thus when I take the laptop off-grid the residual consumption is only 2W, and on-grid only 20W is drawn when I'm conserving power in my code.  (It could easily be as high as 60W or 70W.)

Rgds

Damon

[Opera House]

This is a common problem at the low end.  I have an older larger watt meter with an internal toroid transformer to sample current.  This has a terminal strip connector on back so I added a couple extra turns through the toroid and reduce the reading by a multiplier.  The extra turns can be a lot smaller when space through the hole is limited.

[rossw]

Many years ago, I made this very simple current-amplifier.

I took a conventional power board (just an extension lead would work, the board just made it more practical for my use), stripped the outer insulation only (very carefully so I didn't nick the inner insulation) for about 8 inches or so. I then wound the "active" 5 turns clockwise, and the "neutral" 5 turns anticlockwise and stacked them one on top of the other. With some electrical insulating tape, I wrapped them all together sort-of neatly to make this nice tidy loop. The Earth wire is wrapped around the base of the coil to keep it out of the way. NONE of the conductors were touched nor the insulation cut, no electrical work need be done at all.

The finished unit is convenient and neat enough, safe, simple to use. Easy to plug in the computer, monitor, printer or TV, DVD, Amp etc.

In use, simply clip the clamp-meter through the loops and read the current. This amplifier produces a 10* reading - so 0.1A is actually 0.01A.

Great for measuring current draw of small devices, or standby current of things.

Hope someone else gets some use out of it.

[DamonHD]

Very neat, and basically what the clip-on-at-the-consumer-box devices do, I think.

But, I'm guessing that it's going to be hard/impossible for them to correct for power factor, ie you'll get a VA estimate (from a A measurement) rather than true W, which seems to make for up to a factor of 2 error in a couple of devices I know of.

Rgds

Damon

[feral air]

Is there a simple/cheap way to divide by 10? I've got several cheap meters that only measure up to 2amps...

take it easy

[joestue]

Replace the resistor with a shunt.

[feral air]

I kind of figured it would involve a shunt but..huh? I know it's bad form to ask for hand-holding but it would be much appreciated...

I've got the prong-type meters. Can I add a shunt without replacing anything in the box? Any links to shunts or suggestions on what to look for? take it easy

[joestue]

Oh God.  Find an ebook on electronics, add some asbestos, a water rheostat, some mercury and some creative thinking.

Do you want an elegant or a quick & dirty solution?

You can use some nichrome wire, copper.. anything.

For nichrome wire about 20 or so strands about 2 inches long should get you in the ball park. Each of those values is +/- 100%, so experiment, just use enough material to keep the temperature down & stable.

If you make one and it works well, build 20. Tremendously easier than building 20 different ones over 10 years, something I never learned.

Achieving high accuracy from sh*ty equipment is in some ways a lost art, find an old electrical engineering book from the 1910's and you will find a good fifty different methods to calibrate and compensate for every piece of test equipment.

[feral air]

Yeah, I should've googled it first but I thought someone would have a bookmark handy or want to be helpful...guess not.

You could've just linked to Make Your Own Shunts and been done with it. Thanks anyway.

take it easy