250V DC would
be dangerous

Leo Simpson’s idea of powering DC-compatible household appliances from 250VDC sparks a memory of an article I saw some years back. I am not sure if it was in SILICON CHIP or in Electronics Australia. The subject was the change-over from DC to AC in a particular Sydney suburb.

The writer told a story of the days of DC. It seems that he turned on a light and it blew. Normally that would be the end of things but because the supply was DC, the arc struck by the breaking filament did not extinguish. Instead, it travelled up the light globe and was half-way up the flex to the ceiling before the author had enough presence of mind to turn off the switch.

The point of the story was that DC can maintain a spark under extreme conditions. This is obviously quite dangerous. Another problem is that an electric shock from DC is far more dangerous than a shock from the same voltage using AC. This is because AC tends to throw the person away, while DC tends to paralyse the muscles, making it difficult for the victim to escape the shock.

The higher the voltage, the greater is the paralysing effect and hence the greater the danger. In short, I think the idea needs a rethink.

Jonathon Waller (via email).

DC has drawbacks
for TVs and monitors

I read the Publisher’s Letter about DC mains in the October 1999 issue with great interest. However, may I bring up a few points about existing appliances and DC mains? The first is that you can’t use most switches on 240V DC as the distance between the contacts is insufficient to extinguish the arc formed when the switch is opened. Appliances and house wiring for DC mains generally use heavy duty tumbler switches.

Second, colour monitors and TVs are unsuitable for DC as the degaussing coils would actually cause magnetisation of the CRT’s metalwork on power up; quite the opposite of what you want! You would have to feed the degaussing coils off the switchmode transformer & take into account the higher frequency and asymmetrical waveform that would have a DC component.

Third, switchmode power supplies and compact fluorescent lamps have a DC supply of typically 320V. When you specify a 250V DC source supplying these things, it would be equivalent to a 180VAC supply. This would be at the limit of regulation for a lot of power supplies. Compact fluorescent lamps don’t get enough drive to their switching transistors which overheat and die with this lower supply (I’ve experimented with this). This is why you shouldn’t use a 240V peak-peak square wave inverter for the electronic kind of CFL.

Normal fluorescent tubes can be used with a resistor or incandescent lamp instead of a choke but this reduces the efficiency.

It is interesting to note that the widespread DC mains in the UK were subsequently killed off by the National Grid in the 1960s. Likewise in Australia, DC mains were used in some country towns, the overnight power being supplied by batteries charged during the day.

Sydney’s CBD lost its DC mains in the mid 1980s but by this time it was only being used for lift motors. I hope these points are of some interest.

John Hunter (via email).

AC switches not
suitable for DC

The idea of supplying 240V DC to certain domestic loads was floated in your latest editorial. It might look attractive at first but carries some risk. There is no problem with the logic concerning the equivalence of AC & DC for heaters, incandescent lights and switchmode power supplies used in consumer goods.

However, the ratings on appliance ON/OFF switches and domestic light and power point switches are all qualified by the words AC ONLY for very good reason. Some arcing always occurs between switch contacts when any load current is interrupted. Inductive loads create a severe "back emf" and are particularly hard on switch contacts but arcing still occurs even with purely resistive loads.

With DC, the only passive way to extinguish such an arc is to provide generous contact clearance – maybe several centimetres. (We are not likely to install automatic compressed air blast arc suppression on every household and appliance switch!) The switch contacts must also be robust enough to tolerate repeated arc attack.

Since AC voltage passes through zero 100 times/second, an AC supply offers an inherently reliable way of extinguishing contact arcing with small contact clearance – typically a minimum of around 1mm. This allows switch designs to be compact, safe and cheap.

I recall an anecdote related several years ago by the late Neville Williams in Electronics Australia. He vividly described what happened when a pendant light globe, supplied with DC power at the time, failed spectacularly in a factory where he worked in the ’30s. The globe went "pop" and an arc occurred between the two filament supply leads. This arc burnt into the lamp base, up through the socket and then just kept going up the twisted pair rubber and fabric cable into the ceiling rose. Fortunately, the power was cut in time to prevent a major building fire.

Noel Erbs, Trafalgar, Vic.

DC makes sense
in remote areas

Yes, DC power in the home DOES make sense. Your editorial in the October 1999 issue, in my opinion, is very sensible. I live about 400km west of Rockhampton. For many years we generated our own 240VAC, then solar panels became available, not quite as cheap as now but affordable. We went into the cost structure very thoroughly. Batteries were the biggest cost but we were lucky in obtaining a couple of sets of ex-Telecom 500A.h 2V cells. These are harder to find now but a 350A.h unit is on the market.

So our power unit was trickled charged at about 5A by two 42W Solarex panels. These are now nearing 30 years old but the batteries have been replaced.

All lights in the home are fluoresents modified to run from 12VDC. My amateur station (VK4KAL) is operated from batteries, direct where possible or by DC/DC converters. A 1500W inverter is on standby, although rarely used. The 486 DX2/66 computer I am using is also running from 12V DC.

We are now connected to the grid but only for the deep freezer and washing machine (not an automatic – these are water wasters in our dry area). The welder has to be AC-operated although the frame of our home was welded using battery power – 36VDC (we live in white ant country!).

We use gas for cooking and our hot water is solar-heated. Where possible, every gadget we use is 12VDC operated. We don’t have blackouts, although recently the system in our area was out for three days due to 3km of mains being blown down in a freak storm; but our lights were on.

For what it is worth, invest in a smallish Solarex panel and a 12V deep cycle battery. I run as a "emergency" an 18W panel charging a deep cycle marine battery to "fire up" an FT 747GX which draws 20A on transmit. This also lights my "shack" if ever needs be. A couple of amateurs living in Brisbane have similar small setups running 12V fluoros because of blackouts.

Don’t use inverters unless absolutely necessary. They waste power while idling. I throw DC plugpacks in the rubbish bin. Why shouldn’t I, with 3kW of DC power at my disposal? We have the best of both worlds.

A. Loveday, Rubyvale, Qld.

DC concept
is worthwhile

I have just read your "Publisher’s Letter" for October 1999. I must say that you have touched on a point that I have been trying to make for quite some time, much to the amusement of my friends and colleagues. I believe that there is little to be gained by having domestic dwellings on the 240VAC mains. Apart from heating, all appliances could be powered by low voltage DC, be it 12, 24 or 32 volts. Heating, and by this I mean cooking, space heating and hot water systems, can be more efficiently be handled by gas.

There are quite a few domestic appliances (eg, TVs, videos, etc) that are dual voltage and cooling is not a problem with 12VDC fans, made for large trucks, on the market. Refrigeration could also be run from 12VDC because of the relatively new 12V compressors available and of course, the large 12V absorption refrigerators have been around for a while. Air conditioning can again be via 12V compressor or evaporative, as can all manner of power tools. Even audio amplifiers can be low voltage powered; those massive "thumper" systems in cars are testimony to that.

You did mention some of these in your article and my knee-jerk reaction was "now someone will listen to me!" I could go on and on about this subject but I had better stop now and say thank you for a timely article.

The amusement mentioned above? I am a qualified electrician! Maybe a traitor to my trade?

J. Smith, Middleton, SA.