Trembling laptop

I've just purchased a used laptop - a Lenovo IdeaPad. It works perfectly, except for one rather worrying thing - when I gently brush my hand or finger on the metal panel surrounding the keyboard I feel a very slight trembling. If I unplug the charging unit the trembling stops. It's clearly a mains frequency trembling. The charging unit is marked INPUT: 100V-240V~1.5A 5-60Hz OUTPUT: 20V = 3.25A. The '=' sign is actual a horizontal line above a horizontal dashed line.

Does this signify a serious fault or am I a little paranoid?

Hello Gary,

I suspect an earth leakage fault. Test between the panel surround for the keypad and a known good earth.

Regards Brian

Edited By Brian Wood on 15/07/2020 08:08:40

Hi Gary,

Replace the charger unit and test again !

Does the fan change speed when you disconnect the power supply? Could it be a resonance?

The laptop and charger will be double insulated, so there will be NO earth continuity between the laptop and mains earth. This is confirmed by the symbol on the charger label "box within a box".

Like Clive I suspect a different fan speed, determined by the profile when the unit is on charge. I don't think it is anything to worry about.

This is a common phenomenon. It is likely that the metalwork is at a slight AC voltage to "earth" as represented by your body. Though double insulated there will be some capacitive coupling to the laptop "earth". If you have a multimeter try measuring the AC voltage between the keyboard surround and some known earthed metalwork - you may find a few volts.

If only we had a Basic Electrics as applied to Electronics Topic!

I'm not convinced this is a fault at all - I've noticed such tingles on other mains powered electronics before and decided it's innocent. Here's my case for the defence, let's see what the prosecution and jury make of it.

First, underlined in RED, a tingle from a metal framed mains powered appliance like a lathe needs urgent attention. The machine is earthed for safety and a tingle means something is leaking (which could be dangerous) AND the lathe isn't earthed properly. It's a fault.

There are legitimate reasons for some earth current to be present, such as suppressor capacitors, which causes the tingle. The safety issue is a live wire accidentally coming into contact with unearthed metalwork and passintg lots of lethal amps through the unhappy owner. It can happen for many reasons - poor maintenance, chaffed or perished insulation, loose connections, corrosion, heat damaged or damp motor etc. Old equipment is more likely to be dangerous than new. It's vital to earth mains powered workshop equipment properly.

Electronics are different. Chances are a laptop shouldn't be earthed. Gary's black box is a mains to DC power supply. It contains a bunch of electronics and a small High Frequency transformer. There's no direct connection between the AC inputs and DC outputs, Gary's unit is 20Vdc at 3.25A.  Designed properly - and Lenovo are a reliable make - the power supply will keep mains well away from the output and the laptop. DC minus isn't earthed. Safety is provided by the double insulated power supply, not by a mains earth. A laptop is nothing electrically like a lathe.

Why do DC powered electronics tingle? I suspect it's through transformer and other stray capacitance. Although the transformer's primary and secondary aren't in contact, they're close enough to form a small accidental capacitor. A tiny current can flow through the phantom capacitor, which is detectable but not hazardous.

An electronic tingle could be a legitimate concern because not all DC power supplies are well designed. But it's quite difficult to be electrocuted by a badly designed switch mode power supply.  They're more likely to catch fire or fail completely than zap the owner.

Dave

Edited By SillyOldDuffer on 15/07/2020 09:41:12

The '=' sign you refer to is not an equal sign, but indicates DC current.

Dek.

This tingle is caused by leakage current from the power adaptor. The leakage current is often caused by interference suppession components, as has been mentioned by others, in particualy a "Y" capacitor across the isolation barrier. This current should be less than 210 microamps (0.2 mA 0.0002 A). and not normally detctable.

My concern is that this is a used device so you don't know it's history and the AC adaptor may have been replace at some time, possibly by a "clone" that might not meet current safety standards. From the photo it doesn't look the 4 years old that date on the unit implies and used latopts often come with "new" power supplies. Ideally you dhould have it checked.

If you have a meter that reads AC milliamps you can do a quick check yourself. Place the laptop on an insulating surface (plastic cutting board is good), switch it on with mains power and connect the meter (set to AC milliamps) between a known earthed object (e.g head of the screw holding down the mains socket) and an exposed metallic pat of the laptop. The OUTER metallic part of a USB socket is good, but don't put the probe into the socket itself. Use proper test leads and don't touch any metal parts when doing the test. If you get a reading of more than 0.2mA stop using the charger and get professional advice.

Robert G8RPI.

20V should be barely detectable, unless you have an unusually low skin resistance (usually it is about 140K Ohms )

Maybe the tingle is because the voltage is not sufficiently smoothed?

In the Electronics and Vibration Lab, we used used to check ourselves, weekly, with the AVO 7. If we felt unwell, the reading was lower.

But remember, "it's the volts that jolts, it's the mils that kills"

Current can be more deadly than voltage. I have had lots of shocks from Kettering type ignition systems (Coil and contact breaker points ) but would not like the same from modern solid state systems. Indeed some years ago, SAAB specifically warned of the dangers from their ignition systems.

Howard

Since Lappy DC PSUs have a ring and tip plug connecting to the laptop, where is the perceived "Earth" connection?? Is the positive or negative connection dual purpose?

Regards Ian.

Trembling or tingling? I keep all my old laptops, and the on the older ones the battery does not hold a charge, and they are effectively mains laptops. Could it be the fan or hard drive stop when the power is turned off? I certainly have a noisy fan on one of my old laptops.

Hi Garry, Dek is correct, a full line above an equal length dashed line always denotes a DC output as opposed to a sine wave for an AC output.

Regards Nick.

Gary. Mindful that you wrote trembling rather than tingling I have an alternative suggestion...

It is quite often the cooling fan is either clogged up with filth or having lost a blade. Unplugging the powersupply will nearly always reduce fan speed (because it changes power plan), reducing the said trembling.

Fans full of gunge also are one reason that laptops run slower.. The temp sensors reduce speed when the fan cannot keep up, especially true on laptops with faster graphics cards which need more airflow.

Easy thing to check (and fix)

Cheers,
Mark

Och new fangled electronics, 'we're doomed I say; doomed.

George.

I have found that some people can feel these currents others can't. You are one of the (un)lucky ones who can. The picture looks like it is an original PSU so design-wise should be fine, but four years of unknown abuse could cause a little extra leakage. The number of times I have pulled a blistering hot PSU out from the sofa cushions, darn kids!

If you have the funds, for peace of mind pick up a good quality replacement PSU, Looks like you can pick one up for about £20. Avoid the generic bargain bucket PSUs, they are built down to a price and often are just the safe side of lethal. But any 20V 3.25A (probably 3A too) will work as long as the plug polarity and type is correct. The little diagram after the 3.25A shows the + is the pin and the - is the ring.

Oh incidentally as I found out after finding a server I was working on gave me the trembling feeling. The equipment can be perfectly earthed, it can be the building that floating.  

Adrian

Edited By AdrianR on 15/07/2020 12:42:02

My previous laptop did this for years - nothing to worry about, The newone is all plastic so doesn't do it. I tused also to be a problem with CRT TVs that were plastic so had no earth wire. If tthe aerial cable wasn't earthed then the owner might buy a metal cased VCR which became 'live ' through the scart cable but it was just an indued votage so no current to do actual harm.

The above is incorrect and misinformed to put it nicely. You cannot possibly know if there is a problem with the OP's laptop / powersupply or not. Just because you were not electrocuted does not meant that your old laptop was OK either.

Don't give safety advice when you are not competent in the area.

Robert G8RPI

Thanks for all the useful replies. I couldn't try a different charger but, encouraged by the optimism of your replies I spoke to a local PC repair shop who agreed with all your responses, but nevertheless suggested calling Lenovo, which I did. I'm now encouraged to accept that the trembling is very common and nothing to worry about.

Thanks again.

The following is not always true (e.g head of the screw holding down the mains socket)
Untrue where the socket is insulated and fitted on a plastic box, not all sockets have earth straps extended to encompass the fixing points.
These days of stud walling more plastic boxes are used so always confirm a reliable earth path is available.

Emgee