[ALUG] R31 Locks when removing adapter

nev young nev at nevilley.demon.co.uk
Mon Jan 10 08:40:42 GMT 2011


On 09/01/11 18:31, Wayne Stallwood wrote:
> On 09/01/11 09:07, nev young wrote:
>> I suspect it is caused by a (static?) discharge as I have been able to
>> measure a potential of about 150 volts between the two devices (laptop &
>> TV set) although there is no current available to do any (much) damage.
>>
>> I just fall back on the old hardware adage of "don't plug or unplug
>> under power".
>
> That isn't static, it sounds like a ground level mismatch between the
> two devices.
>
> If your TV set has an earth pin connection then I would get the set PAT
> tested or have the earth bonding in whatever mains socket you have it
> plugged into checked out.
>
> Laptops are a bit of a funny beast when it comes to Ground as they tend
> not to have a ground connection bonded to mains earth through the PSU so
> there is potential (lol) for a ground mismatch when compared against
> another item. But a 150v PD between the two would suggest to me that one
> of them is faulty or there is something very wrong with your mains wiring.
>
TVs do not have an earth connection. This is a standard for such devices 
and has been for over 40 years.

There should be no connection between earth and the output of a laptop 
power brick.

Many devices are not earthed by design. At least that was the case when 
I was an electrical engineer.  Things may have changed since I became a 
software engineer.

The PD of about 150v is very high impedance, well over 100MΩ, and is 
quite normal for such devices.  You need a *special* voltmeter to read 
it. It is not normally a problem as it leaks away eg. via moisture in 
the atmosphere, but sometimes it can bite you.

The reason for it being about 150v is it is (√2/2)*(peak mains volts).

-- 
nev





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