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AC/DC


As well as the famous rock band, and a euphemism for "bisexual", AC/DC is also an electrical term which means "Alternating Current / Direct Current", meaning that an electric machine will work with Alternating Current or Direct Current. The expression "AC/DC" used to be more commonly seen in the specification of electrical devices, especially high powered machines, but is now less usual. Partly this is because AC is much more fashionable for mains electricity supplies and has become more standard, but also manufacturers are much more cautious about claiming a device is truly AC/DC because of various modern safety rules and standards.

However, a great many electrical appliances are AC/DC, although they don't admit it!

Before I go on to tell you about my experiments to find out which domestic electrical appliances are AC/DC, I'll explain the background, because not everyone knows what AC and DC are, so here goes:

DC: Direct Current

With batteries and other sources of DC, the voltage is about the same all of the time. A 12 volt car battery has a voltage of 12 volts between its positive and negative terminals now, and will still be 12 volts next second, and the second after that. This unchanging voltage is like a constant head of steam or potential height of water, and provides current which is continuous. Hard disc drives are powered by 12 volts DC and 5 volts DC. For most low voltage applications, DC is the favoured choice. With machines that require a low DC supply, such as the equipment inside computers, the electricity is changed from AC mains to DC at low voltage, by a power supply unit (PSU).

AC: Alternating Current

With mains supplies such as those supplied by most electricity companies, the electricity is AC. Many different standards apply, but it's usually 240 volts AC as in the UK or 110 volts AC as in the USA. The two terminals (usually "live" and "neutral") don't always have the stated voltage across them all of the time, but the ratings are based on an equivalent DC supply. The UK electricity supply standard is a 240 volt AC electricity supply, the actual voltage varies from about minus 340 volts to plus 340 volts in a continuous wave 50 times per second. The US supply is similarly alternating, but goes from plus 155 volts to minus 155 volts at a frequency of 60 cycles per second. The voltage continuously changes in a style which is a sinusoid, a smooth waveform which has characteristics like a swinging pendulum, a motion related to rotation.

If you're wondering why electricity companies have chosen to use Alternating Current which flips to and fro many times per second, rather than keep it a simple flat voltage in the DC style, you'll be pleased to hear there are good reasons: AC is much easier to transport across the countryside as it can easily be stepped up to a higher voltage with a transformer, relatively efficiently sent along power lines, and stepped down by another transformer at the customer's end. The same is not true of DC. Also, DC is more difficult to control and requires much larger heavy duty switches.

Alternators are small electric generators in cars. Curiously these are AC, because they are easier to build than DC dynamos. The electricity is then converted to DC to charge up the car battery, which is obviously DC.

The experiment

I don't recommend trying this, but if you are confident enough, daring enough, and/or silly enough, it's up to you! I'll tell you what I did. It was many years ago, and I had acquired a DC generator from a 2nd World War military wireless transmitter. This was 275 volts DC and the house electrics were 240 volts AC. Here's what I did:

1. I switched off all of the appliances in the house, including any standby devices, the fridge, the doorbell, etc, so there was nothing connected at all.

2. I switched off the main switch on the fusebox. This doesn't make the circuit dead, but simply isolates the house electrics from the supply. All the house power outlets remain connected together, but are not powered.

3. I plugged the DC generator output into one of the plug socket outlets in one of the bedrooms. At this point all of the house electricity was DC but nothing was switched on.

4. I went round the house switching things on experimentally, one at a time. I had a good idea in advance which domestic appliances were likely to work and which were not. Also, quite important to the safety of this otherwise very dangerous experiment, the DC electricity generator was very small and would have its voltage dragged down by any overload, rather than blowing itself up or destroying the appliances plugged in.

The results were quite interesting. At least half of all of the equipment was AC/DC by this test.

* Vacuum cleaners are the best example, as the motors tend to be DC brush motors which work on AC as well. Any device which consisted of a motor with a split ring commutator worked. Electric drills, a Kenwood Chef kitchen mixer, hairdryer, hedgetrimmer, etc.

* Anything with an induction motor did not work. Induction motors require an AC input, so it was not surprising that the very old washing machine with a quarter horsepower 1425RPM motor did not work on DC!

* Anything with a transformer as the first part did not work. The doorbell, radio sets, the tv, hi-fi ... none of these worked on DC. Transformers rely on the fact the current alternates to generate voltage in the secondary to power the device.

* Fluorescent lamps DID work in this experiment, but it was later revealed that it was not perfect DC and had small amounts of AC in, a fact which saved the fluorescent light tubes from bursting. A later experiment with a 240 volt battery completely destroyed a fluorescent lamp tube.

* Light switches of the old bakelite style were perfectly ok as they had a strong spring mechanism similar to the old "knife and fork" switches of old. Some of them were even specified as AC/DC. Modern neat square switches fared less well, and the one on the fluorescent lamp in the kitchen would not turn off, as the switch was arcing across. This is a typical DC problem, that switches need to be sturdier to prevent arcing.

* Neither the fridge nor the freezer worked on DC. Both had standard hermetically sealed compressor motors, which are AC only.

* All of the lightbulbs worked, whether in chandeliers, table lamps, wall lights, or other lightbulb fittings. This was a long time ago, well before modern economy lightbulbs, which are a miniaturised version of the fluorescent lamp idea but in a lightbulb sized unit designed to fit a lamp socket. Filament lightbulbs have no problem with AC or DC as the device consists of an element which is heated up in an airless bulb. It has been speculated that filament lamps do slightly better on DC, as the filament is not being vibrated by the reaction to magnetic fields. If you'd like to do some scientific experiments to see if lightbulbs live longer on DC than they do on AC, make sure you use a fully rectified SMOOTHED supply, as my experiments with just a bridge rectifier weren't promising.

* In the 1980s, a BBC Computer worked regardless of whether the mains was AC or DC, provided it was of the switch-mode power supply type. The manufacturers would probably not have stated anything about DC use, but experiments reveal success with a 240 volt battery. No other home computers of that era, at least none of those I tested, worked on the DC supply. That's because they all had mains transformers. Also, although the BBC Computer worked and proved to be AC/DC, its Microvitec Cub RGB monitor was AC only in tests. Incidentally, "AC only" on a label doesn't truly mean the device won't work on DC. It usually just means that the manufacturers don't recommend you use it on DC. But it's best to be sensible about this; if you can see it's got a transformer PSU, you know it's not going to work on DC.

* Modern computers? None around at the time to test, as it was a long time ago, but the question of whether a modern computer will work on DC mains depends on what type of power supply it has. If it's a switchmode, then it's likely to be more like the BBC Computer than the ZX Spectrum, in its success with DC.

* All of the gas appliances in the house worked even though the mains was no longer AC. But don't laugh too soon! If you've got a gas central heating boiler it is likely to have an induction motor (AC-only) as the circulator pump, in which case, if you have no AC mains, your gas central heating will stop working! See Gas appliances in electricity outage

* Electric fires with bars, bathroom heaters, and other resistance based electric heating appliances were compatible with DC, although the generator I was using was not powerful enough to get them very warm. On coal effect electric fires, the red lightbulbs worked, and also the rotating smoke-effect wheels worked, as these the mini turbines powered by convection of warm air rising off the lightbulbs.

* The cooker elements, same as for electric fires. But the cooker clock did not work, as it was an induction device. Also note that fan heaters require the motor to work, so I would guess that most of them are AC only.

* The kettle would have worked if the generator had been bigger, but we can only hazard a guess on whether Russell Hobbs made switches that would have been able to turn it off with three kilowatts of DC.

* In an age before microwave ovens, there was no microwave to test, but I can tell you now, it wouldn't have worked. It involves a transformer to get the 1200 volts required to power the magnetron.


Well that's about it then. Testing AC domestic appliances on an experimental DC mains system. About half of them worked, and half didn't. Best success: Table lamp. Next best: vacuum cleaner. Most disappointing: doorbell and any other low power devices which required a transformer. Also fridges, as it would have been nice if these had been able to be used on a makeshift DC system in case of long power cut.

Also, to make it clear, we are talking about substituting AC 240 volts by using a similarly high DC supply. This is not the same as using a car battery, which is 12 volts DC. (There is no such thing as an AC Battery even though electrical engineers may have jokes about such a thing!)

Other useful information: Domestic electric appliances of the types mentioned on this page are available from various competitive electrical shops, although I don't think you'll get anywhere asking the shops if the appliances will work on DC!

Or, if you'd like to buy an AC/DC t-shirt or other band promotional merchandise, there are some notable places at the Fashion category, including AC/DC variants from a shop whose name is Shot Dead In The Head

On the question of which is more dangerous AC or DC?, there's another page about that.

There's also a company, ACD-See which is a digital photo management software company. I just thought I'd drop that in.