[sdiy] Leaking line filter (was: Grounding & computers)

[Ken Stone]

At the company where I used to work MOVs (metal oxide varistors) were also
frequently installed in these locations to help supress spikes. The practice
was discontinued after safety switches were introduced.

Ken

>Magnus Danielson wrote:
>>
>> Jim Patchell wrote:
>>
>> Hi Folks,
>>
>> >     Yikes!
>> >
>> >     One posiblility is there is a line filter that the mains goes
>through.
>> > Each side has a cap that goes to the chassis....this makes a voltage
>divider,
>> > and will give you 110v on the chassis...you really need that earth
>ground to
>> > prevent this.
>> >
>> >     Anyway, this is my guess.
>>
>> I'd go even further... I'd simply state that this is the most common
>> case of getting a hot chassi. The reason why is however interesting.
>>
>> The caps are chosen to not produce too much leakage... for 117 V, but
>> when you put the same filter in a 230 V system (which is what Europe
>> have these days) you do sense the leakage.
>>
>> There is a simple enougth way to test this. If you have the box
>> unpowered and still sense the current, then it is the filter since it
>> is allways hot, regardless of the power-switch.
>>
>> Despite this, I allways recommend the usage of these filters. They do
>> the rigth thing for most situations, so there is not real reason to
>> avoid it.
>>
>> Also, one should if possible run with earthed contacts, there are
>> other benefits with doing that.
>>
>> Cheers,
>> Magnus
>>
>
>I was suspecting the line filter of the computer too. Now why are those caps
>in the line filter directly connected to the earth ground plug? Isn't it a
>little odd to leak current to the safety earth when this has nothing to do
>with safety measurements? If those caps are at their max limit of leaking
>it's very possible (especially in areas where you have 230V) that they set
>off the earth safety switch. And if people then stop grounding their
>computer to keep it from triggering the safety earth switch, they create a
>very unsafe situation in their home with 110-115V on their computer case.
>All because of these 2 caps in the line filter. Why?
>
>Cheers,
>Martijn
>
>
_______________________________________________________________________
Ken Stone   sasami at hotkey.net.au  
Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/>
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