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- AC mains problem?
February 25, 2007, 9:32 am
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I purchased a mains powered late 70's vintage freq. counter from the
U.S. It was wired for 110v, and after first verifying that it worked,
I then rewired the trannie for 240v per manufacturer's instructions
(tap shifing). Powered up and again verifying it was functional. The
trannie simply supplies 9v AC to a board where it is then rectified
Then when in the process of moving it while powered up, I got a zap
when touching the case and another piece of earthed gear. I then
measured 120v AC from the counter chassis to an earth. The counter
only has a paired mains lead - i.e. no earth and the unit is not
The counter had a couple of disc ceramics between each mains line and
earth, and I removed these and also rewired the set in a more safe
logical manner i.e. hot AC to fuse then to power switch, then to
trannie, and the neutral direct to the trannie 0v point. As supplied
for the U.S. balanced AC system it had the switch on one side, and the
fuse in the other. With all this the chassis measured around 100v AC
to ground. Suspecting a problem with the trannie, I then mounted it on
insulated bushes but no change.
I've heard of problems with induced voltage in mains cables, but in my
experience that has been with three core cables, when one end of the
earth has been found floating.
If a three core cable is fitted it would send this 100v AC to earth -
and that may be the appropriate fix - but an explanation of the cause
of this problem would be appreciated.
Re: AC mains problem?
** Nonsense - the US, domestic 120 volt AC supply is NOT balanced wrt
Only US high energy outlets supplying 240 AC (at 60 Hz) are so balanced.
** Asked the tooth fairy for his/ her/its opinion yet ?
** So you are measuring that AC voltage with a DMM - right ?
10 or 11 Mohms load impedance - right ??
Go figure what effect 200 pF or so of stray transformer winding capacitance
has when fed from 240 volts AC into that load.
200exp-12 x 2 x pi x 50 = 16 Mohms.
Make any sense now ?
Just ground the damn thing and make it safe and legal.
Re: AC mains problem?
I would change the transformer anyway, the US is 60hz and AUS is
50hz. If the transformer isn't able to go to 50hz (and most US stuff I
have seen, doesnt, even if it is intended for export) it will just run
much hotter than it should which isnt good for it.
Earth the case properly and use an Australian 3 core cable and mains
PS: The US 120v mains is NOT balanced, Their (non 3 phase) system is
based on a centre tapped 120-0-120v pole transformer(s) and therefore
only the 240v configuration is going to be balanced (and will still
have a 120v potential to the neutral from EITHER side of the 240.).
(note: From what I have read in the past, their 3 phase supplies are
much the same system as here, but are 208 not 415. They usually are
only in industrial areas and not residential.)