Powering AC adapters
on DC Mains

WARNING: The following article is for information only - operating any equipment as described below must be done with a sure knowledge that you know what you are doing and with full permission from any required parties.

"DC Mains" is a lot more common than is realised. The reason is simple, it offers the easiest means of backup supply in the form of rechargeable batteries. Electrical sub-stations for example will usually have 110 or 220VDC power for all operating equipment such as breakers. Although sub-stations are not "public" areas the general public is a lot more exposed to DC mains than is realised.

Boarding a train recently attention was grabbed by an annoying flickering neon light on a standard 13A outlet beside a seat. The note above it, however, was interesting: "Not for public use - 220volts DC". This was used for the cleaning staff who would be armed with vacuum cleaners. These are usually fitted with brush motors being usable on both AC or DC and are not polarity sensitive. The same goes for most power tools including drilling machines - the only exception being the speed control will not work on DC.

The power supplies of most modern equipment are now Switch Mode types especially portable equipment including laptops. Apart from the usual filters after the connector the first component encountered by the power is a bridge rectifier whose purpose it is to convert the AC into DC. Following the rectifier is the normal smoothing capacitor.

If you were to investigate the label of e.g. your laptop AC adapter you would notice the voltage range to be quite extensive such as "110-240VAC" and usually a frequency band e.g. "50-60Hz". The ranges found on industrial equipment will be a little more extensive, usually "85 - 265VAC , 47 - 440Hz". Labels like this are almost a dead give-away the unit is a Switch Mode Power Supply (SMPS).

The first range of figures is to assure the general user of the equipment he may travel between the USA and Europe without having to bother with the various voltage and frequency standards. The second shows the actual limits of the equipment, the 440Hz indicating it is suitable for use in aircraft maintenance as the standard frequency of aircraft power is 400Hz.

The reason for the lower frequency limit being imposed is owing to the smoothing capacitor not being able to hold enough charge between cycles. The lower the frequency the higher the ripple, the higher the ripple the hotter the cap will run. The upper limit is due to the above mentioned input filters starting to filter out the mains frequency and, as they would now be absorbing energy, would run warm.

Powering a SMPS from DC eliminates both the above problems. The capacitor is no longer required to hold a charge between cycles as the DC will maintain a constant input voltage and the filters would merely prevent any unwanted spurious generated by the SMPS from reaching the incoming supply.

The only concerns when operating a SMPS from DC is the incoming voltage range that ensures correct operation. The lower DC limit is the lower AC limit multiplied by 1.414 e.g. 85VAC would mean 120VDC lower limit. The upper limit of 240VAC equates to 340VDC, 265VAC being 375VDC, levels not usually found.

I would not suggest you rush out to try this but should you ever be in a real jam at least you'll be able to keep the mobile phone charged, emails working, and generally ensuring you have your techno-fix. Before using any equipment on DC please do ensure the power supply is an SMPS, there are some very small transformers and using them would result in the usual smoke on which all electronics run being released thus killing your equipment.



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14.08.00