Duncan,
I will take a stab at this since if I really mess up and get flamed by
various
and sundry members of the group I at least won't know about it until I get back
from a four day weekend.
Based on my understanding of the requirements there is good news and bad
news.
The good news is you should not have to meet the tolerance spec; at least not as
far as having to use a precision capacitor RATED at 1% tolerance. You need a
capacitor with the proper voltage rating that's actual value is between 0.2222uF
and 0.2178uF. Theoretically you can measure the value of a group of 20% 0.22uF
capacitors and find one that is within 1% and use it.
The word "theoretically" above brings me to the bad news. The distribution
of
values around the mean tends to be skewed, and it is unlikely you will find a
20%
capacitor that is actually 1% because the manufacturer normally preselects and
sorts a batch, removing the 1%, then the 5%, then the 10%, and finally the 20%
(or
whatever tolerances they offer). You will get two peaks in the distribution near
the positive and negative bounds.
Luckily there is some more good news (or least not terribly bad) if you can
live with a four capacitor network. Go back to that bin of 20% caps and start
measuring actual values again. This time you are looking for a pair of
capacitors
whose values add to about 0.44uF. When you find one pair look for a second.
connect
each pair in parallel (0.44uF), and then connect the two parallel pairs in
series
(0.22uF). I wouldn't want to design that for a production run product, but a
little
mix and match for test equipment shouldn't be too bad if you can live with four
capacitors.
I hope this helps,
Paul McCoy
[email protected] wrote:
> Group,
> I am trying to construct a simple earth leakage test circuit as in Annex D
> of EN60950. I am finding it impossible to find a capacitor for Cs (0.22uF) as
> I
> cannot find one that is the required value, tolerence and voltage rating.
>
> Am I missing somthing here? Even if I use a network of capacitors I would have
> to use quite a few of them to get the right value! It would appear that the 1%
> tolerence that is requested is a bit unrealistic given the value and required
> voltage rating.
>
> Has anybody else found this and if so how was it resolved? Are there any
> manufacturers of earth leakage testers who may be able to hint on how they
> manage to overcome this?
> Any suggestions would be greatly recieved...
> thanks,
> Duncan.
>
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