[Boatanchors] (Another) question about clamp-on ammeters

[Phil]

Hi All,

Awhile back I posted a question about using clamp-on ammeters, if using 
multiple turns through the meter was a linear function.  I.E. If 4 turns 
through the sense probe clamp gave 4 times the sensitivity.

Awhile back I DID buy a relatively inexpensive unit, the Harbor 'Fright' 
Cen-Tech 95683.  Think I paid a bit less than $20 with a coupon.

I DIDN'T want to put a lot into one as it will only receive occasional 
use, and those not critical measurements (I DO have several DVMs, from 
high quality down to 'throw-away' meters as well as VTVMs).  So the last 
thing I needed to do was sink a lot into another one ;-)

When I replaced the Astron RS-20M 20Amp PS with the Astron RS-50A I 
wanted to measure and compare the "idle" (no load) current of the supply 
while turned on but not in use (basically I would say mostly transformer 
losses).

To this end I put one of my freebie HF outlet strips on the bench, 
opened it up and cut the hot wire to the power switch.  I then spliced a 
12 inch wire between the power cord and the switch, and routed this 
'loop' out the side of the outlet strip.  This will now be what I use 
when I want to measure AC line current on something. It's long enough to 
allow multiple turns through the ammeter when need be.

1.  First I measured the RS-20M, it measured between .4 to .65 Amp 
primary with no load.

2.  Then I measured the RS-50A.  It measured between about 1.2 to 1.75 
amps with no load.  Makes sense, the supply is rated at about 2.5 times 
the current, and it looks like the transformer losses are in a similar 
ballpark.

HOWEVER: *******************************

What I don't understand is why the large variation in what I read, 
DEPENDING on where the wire was located in the pick-up 'jaws' of the 
meter. If I centered the wire (the hole is about 2.25 inches) it read 
the least, IF I located the wire loop near the edges, especially the 
left edge it read the maximum.  I'm pretty sure that is due the better 
coupling to the (presumed) ferrite in the clamp proper, and that makes 
sense.

*******************************************

DUH, I just went back and read the 'skimpy' manual again (for the 
umpteenth time) and it did answer the question I was going to ask.  It 
said to locate the wire in the CENTER of the jaws for best accuracy.

At that point I nearly scrapped this email before sending, but thought 
it might emphasize how important it is to read the manual over several 
times before asking stoopid questions. Especially when said manual is in 
"Chinglish" (though this one is better than most that come with Chi-Com 
products).

-- 

73 From "The Beaconeer's Lair"
Phil, KO6BB
http://www.qsl.net/ko6bb/  (Web Page)

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