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Alasdair's EMF Musings - View Comments

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Shocking laptops

There have been recent postings on internet bulletin boards about high voltages being meaured on some laptop computers (particularly Dell ones) when connected to their mains power supplies (PSUs).

This is an issue that we have been concerned about for years. It arises from most (but not all) laptop PSUs having only a two-wire connection to the mains supply. They are what is called "double insulated" and do not require an safety earth connection. Unfortunately this also means that the low-voltage supply to the laptop then "floats" to typically half the mains supply voltage (i.e. about 115 volts AC in the UK. There is not enough current to electrocute you, but it does expose you to high levels of electric fields of several hundreds of volts per metre at your hands (and lap). I believe that this is not good for your health and well-being.

We have measured our new Dell (an Inspiron 1501 with a PA-12 PSU) and, sure enough, the video socket screws on it measured 111 volts AC to Earth. We then tried our other two (both Novatech of differnt vintages) and they were 85 and 0.002 volts AC. The low voltage one had a proper 3-pin (not double-insulated) power supply.

There is no need or excuse for these power-supplies to be double insulated - indeed as the connectors on the laptop are connected to the PSU output, I wonder whether this is, in fact, legally double-insulated at all. All desk PCs have proper Earthed power supplies, with all their exposed metal parts being properly Earthed. So, if you are buying a new laptop, we suggest that you choose one with a 3-pin mains connector on the PSU. If not, then we suggest you always use a safety Earthing Lead when you use your laptop with its mains PSU.

To make and earthing Lead, connect a wire (best to use an 'Earth coloured' wire - green with yellow tracer) from the Earth pin of a mains plug to a crocodile clip (or similar) that you attach to one of the screen fixings next to the external video ouput connector (the 15-way miniature D connector). This will reduce this stray voltage to zero.

EMFIELDS can provide such a lead if you want a quick and easy solution. Email info@emfields.org for details and costs. It will be added to the website soon.


Posted at: 09/02/2007 13:03:01 :: 2 Comments


Comment By nick :: Posted at: 28/05/2007 06:44:01


As an electronic engineer I think your reasoning behind this may be slightly wrong. the transformer in the mains plug pack should electrically isolate the laptop from mains, so even though it will show 110 volts ac, no current will flow.

Also for safety I would reccomend earthing the laptop thorugh a 1 Meg Ohm resistor to earth, and not directly.

Cheers, from New Zealand.

Comment By Andy Philpot :: Posted at: 18/06/2007 10:44:16

I'm also not sure about your reasoning and would suggest it's not a good idea to suggest the general public make up an earth lead going to a three pin plug! I would agree with Nick that if it was done then it should go through a high value resistor but I would suggest a lead that clipped to an earth point, say a copper pipe, rather than having a mains plug on the end would be far safer.

I must disagree with Nick about the transformer in the power supply. Most PC's and laptops have switched mode power supplys, that is how they get them so small and able to cover such a wide mains voltage (often 100 - 240 Volts AC). These do NOT rely on isolating transformers and therefore are not electrically isolated from the mains.


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