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PUBLISHER’S LETTER
www.siliconchip.com.au
Publisher & Editor-in-Chief
Leo Simpson, B.Bus., FAICD
Production Manager
Greg Swain, B.Sc.(Hons.)
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ISSN 1030-2662
High voltage warnings err
on the side of caution
This month one of our correspondents in the
Mailbag pages, Otto Hoolhorst, takes us to task
for being unduly cautious in including a warning
about monitoring mains voltages. The particular
warning was on page 38 of the July 2004 issue
and pertained to 240VAC voltage and current
waveforms in the Energy Meter project.
Mr Hoolhorst makes the point that many people would have the equipment and the know-how
to monitor these waveforms and for these people
at least, such measurements would be routine.
That is entirely realistic and yes, the warning really does only apply to electronics novices. We do tend to err on the side of caution when we consider
the possibility of novices attempting to make high voltage measurements.
If a novice did have a scope and a high voltage probe, it would certainly
be possible to make the voltage measurements, with the scope earth return
connected to earth of the measured circuit. It should not be connected to
Neutral, unless it has already been confirmed that Neutral is at earth potential. Why? Because it is possible that the power point wiring or the power
cord itself could have Active and Neutral transposed. That rather muddies
the water doesn’t it?
So while Mr Hoolhorst correctly notes that all voltages in the Energy Meter
circuit are referenced to Neutral, it is not necessarily safe to put a scope earth
return on the Neutral line, because Active & Neutral might be transposed.
That possibility would be taken care of if you are using differential probe
connections but again that implies fairly high knowledge levels.
I agree that warning notices possibly do deter hobbyists from learning how
such measurements can be made but if we were to include all the information
on such methods in each project article where high voltages are involved,
it would be a major undertaking. And then of course, we would risk boring
all those readers who actually know what is involved. So we have taken the
safe way, hoping to discourage novices from getting into trouble.
Even experienced people can get into trouble making measurements
on today’s cramped PC boards and a moment’s inattention can lead to accidents. I have to admit to a little accident of my own this month when
checking waveforms in the CFL Driver project featured on page 76 of this
issue. I was checking the buffer output switching waveform at pin 14 of the
switchmode controller IC when the probe slipped ever so slightly, shorting
pin 15 (+12V) to pin 14.
There was a slight crack and all the smoke suddenly escaped from one
of the Mosfets as it blew apart. It wasn’t a good moment for my self esteem.
Maybe I should have left the measurements to someone else on the staff . . .
Incidentally, we’ve also got a warning on that project, due to its high voltage DC output of 350V. Such a warning needs to be included, otherwise it
would be too easy to be lulled into the idea that, because it’s such a small
and innocuous PC board running from 12V, its output can’t be lethal. Well
think again: in the wrong circumstances, 350V can be lethal! So while we
have presented it as a PC board, it needs to be safely installed in a case so
that accidental contact with high voltage cannot happen.
Are we being too cautious? We don’t think so. What do you think?
Leo Simpson
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