View Full Version : Laptop power pack

24-11-2001, 08:27 AM
I cant get a connection to the internet at home when using my laptop with the power pack, as I can hear interference on the line caused by the power pack. On battery I get a perfect connection as well our normal computer gets a connection so its not the phone line. Im guessing we have a power problem has anyone had this problem and solved it? The power pack works at work so it has to be power at home.

24-11-2001, 10:44 AM
You're not wrong Bruce!

Those switch mode power packs can kick up a lot of noise. Assuming that the power supply is not actually faulty, the principal problem is probably induction into the phone lines or phone itself, more so than noise carried down the power cable. The fact that you can hear the interference on the line is a very helpful diagnostic tool.

Firstly, put as much distance between the power supply and your phone system as possible, using the phone noise as a guide as to which direction is best. You might find that your power supply is sitting right alongside a phone cable hidden in the wall for example and inducing noise directly into it.

If the are any coiled up loops in either the power supply leads or your phone line, try unbundling them as they can act as transformers and magnify the effect.

Actively trying to increase the noise by moving the power supply and/or its cables nearer to various components of your system will also help narrow down the primary source.

Alternatively, sometimes if helps to tightly coil any slack in your phone line as this acts as a choke and inhibits the noise. This may sound a direct contradiction of my advice to un-bundle but it's horses for courses!

The power supply should not be injecting noise direct into the power as this would contravene the relevant EMC regulations but if you find suspect that it is, fitting ferrite chokes on the power line may help.

Try some of these ideas then post your results and we'll take it from there.

Sheikh Billy @8-[)

24-11-2001, 02:01 PM
the other thing is to check the voltage at the powerpoint.

had a couple of cases where the power company had set the voltage wrong on the transformer. it caused a few werid faults and one dead t.v.

24-11-2001, 02:49 PM
Sorry tweak e`

Not often I'd presume to contradict a super-geek like yourself, but laptop power supplies are almost always multi-voltage, and can cope with wild variations in power input. I have two that can handle 100v to 240v which would be the norm. If the Powerco transformer setting was wrong the laptop would just ignore it.

Alternatively, if the voltage at the PP itself was faulty due to poor connections there would be bigger problems getting the laptop to go at all!

On the other hand, most TVs have big problems with voltage variations unless they are multi-supply like the laptops, and the first thing to pop is the power supply.

I'll stick with radiated or induced switching artefacts as the most probable cause. These are anathema to digital communications.


Billy 8-{)

24-11-2001, 03:07 PM
RFI causes a lot of lost hair. You've probably got a ferrite choke between the PSU and the laptop (a plastic cased lump on the cord), so you need to stop the excreta gettingback into your house wiring where it can re-radiate in the phone lines.

I was about to suggest looping the power cord through a large ferrite toroid, but you won't want to cut off the power plugs for that. You can get clamp-on ferrite blocks for this -- try Jaycar, or DSE. Try various places on the power cord -- I would go for as close as possible to the unit.

You could try a few turns of the phoneline into the modem through a ferrite or iron powder toroid.

Experiment -- there's no one simple and easy answer to this.

If the PSU does not run hot, you could try wrapping it in aluminium foil (earthed if possible).

24-11-2001, 11:24 PM

EMC from an SMPS will either be conducted via the cables or magnetically induced via switching fields (H fields) from the transformer. Very little will be radiated as E fields so aluminium foil is not much use. There would be some eddy current losses in the foil but apart from that the H field will go straight through & earthing makes no difference to H field attenuation.

I think Bruce is better advised to carry out a few simple tests as per my earlier post rather than to poke around in the dark fitting chokes that may not be needed and may not help.

In reality there should be very little coupling between the mains supply to his laptop and the telephone wiring to his phones & modem. I spend a lot of time tracing interference problems like this and the answers are often quite simple, hence my advice to take a systematic approach to the problem.

The possibility of a faulty PSU cannot be ruled out but that will be identified if physical relationships between PSU, power lead and phone wiring to the modem do not show any clues.

It is hard to diagnose effectively when you can't apply test equipment to the problem but digital signals do not like switching impulses so it seems likely that this is the cause of the problem.

It may also be that there is nothing wrong with his PSU or telephone system but the phone wiring may be run in the same bundle as the power wiring (not supposed to be but it happens) and if that is the case then some serious choking might help. The chokes will have to be the right grade too as the RF variety have little effect at low frequencies.

Post progress Bruce and further advice will be forthcoming.

Billy 8-{)