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