[Amps] Direct rectification of AC mains to derive the amp VDD supply

[peter chadwick]

In some countries, connecting the neutral to an outside earth could be extremely dangerous, leading to fires or even electrocution. Where a Protective Multiple Earth system is used, a ruptured neutral could pass many amps down any earth lead, or lead the coax outer to float up to full mains voltage. This is why it is not permitted to bring any conductor within the unipotential zone unless it is bonded to the system earth, and  earth leads need to be able to carry large fault currents.

Where PME is not used and the neutral is earthed somewhere else, you can still get quite a large potential between neutral and earth. When I lived on a housing estate in Swindon where the whole estate was fed from a substation where the neutral was earthed, on a Sunday morning with washing machines, electric cookers, immersion heaters etc all blasting away, I could get 25 volts at 500mA - free - between a decent ground and the mains neutral. No RCD (GFI in the US) there - if there was one fitted, earthing the neutral to a decent external earth would be very safe - you would have no electricity supply because the RCD would trip!! At this QTH, at the end of the line feeding other houses and a half mile of wire from here to the transformer, the neutral is usually around 1 volt above earth potential.

So any system earthing  the neutral is not a good idea and may even be illegal in some places.

If one used TLT instead of conventional transformers, the blocking capacitors would need to be Class X, and for 160m, at least 10nF. But Class X capacitors are not designed for handling large amounts of RF current (ca. 3 amps at 500 watts) , and are not as low inductance as one would want for 10 metre operation. Cascaded RF transformers is, to my mind, asking for trouble. From memory, an isolating transformer to meet the regulations needs to withstand 2kV, although I could be wrong on that.

As Leigh says, the other regulatory requirements offer a problem too. Although in Europe at least, kits for amateur radio and home brew equipment are exempt from the requirements of the Radio and Telecommunications Terminal Equipment and EMC Directives, it is questionable as to whether such exemption applies to other directives. Interestingly, kits for non-amateur purposes, such as model train controllers, aren't mentioned anywhere......

73

Peter G3RZP