Here is further info on the EFT DC blocking capacitor use by Schaffner
generators. It seems there can be 2 capacitors (in series) which is
probably why the confusion started in the first place! Hope this clears it
up!
Tony (Colorado)
To the EMC Discussion group, freom greg Senko
Perhaps my last reply was to brief and should have covered more details. I
hope this clears up the remaining questions about the generator:
IEC 1000-4-4 Section 6.1.1 Characteristics and performance of the fast
transient/burst generator specifies a "DC blocking capacitor inside the
generator" of 10nF. This capacitor is internal to the generator and
therefore any connection to the generator whether it is an internal mains
CDN, an external CDN, the capacitive clamp or a direct injection test.
Schaffner uses the internal 10nF blocking caps in series with the generator
output.
Section 6.2 Coupling/decoupling network for a AC/DC mains supply port
describes the CDN for AC and DC mains ports and the circuit diagram is
shown
in Figure 4. The text and diagram show a 33nf coupling capacitor in series
with each mains connection.
When these two functional blocks are connected together, the CDN is
connected to the EFT generator, we then have a 10nF cap in series with the
33nF inside the generator. The effective capacitance is 7.6nF which is not
that different from 10nF. Since the 10nF dominates the effective
capacitance, the tolerance of the 33nF cap is not very important. For
example, a 20% high coupling cap will increase the 7.6nF capacitance to
7.98nF, a change of just 5%.
Greg Senko
Sales Manager - EMC Test Equipment
Schaffner EMC
Tel: (603) 642-4694
Fax: (603) 642-7975
Cell: (978) 764-7358
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