Sorry this is a little late, but it is Schaffner's official response to the
recent question from Antonio Cinquino Re: EFT DC blocking capacitor on Dec
5th.
{Posted by Tony O'Hara 12/11/2000, LARCTEC Marketing, Colorado]
Re: EFT DC blocking capacitor question from Antonio Cinquino
To the EMC Dicussion group,
I believe that using a 33nF cap in series with the generator as indicated
by Schaffner is the correct way to apply EFT pulses for mains and power
ports. Every diagram in the IEC 1000-4-4 standard shows 33nF capacitors
which are external to the EFT generator. The relevant paragraphs from the
EFT standard are listed below. In addition, it is important to note that
according to Section 7.3.2, if the capacitive clamp cannot be used for I/O
and data ports, discrete 100pf caps are used instead of the 33nF caps.
IEC 1000-4-4 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.
Section 7.2.2 Methods of coupling the test voltage to the EUT, Power supply
ports and Figures 8 and 10 show the 33nF cap in series with the generator
as well. This section covers conditions where direct injection test must
be applied.
Section 7.3.1 Test on power supply ports and on protective earth terminals
states that "If AC/DC blocking capacitors are necessary their capacitance
shall be 33nF." This section also refers to Figure 10 where the caps are
shown in series with the generator.
And finally, "to clear the air", all Schaffner burst generators incorporate
DC bocking capacitors with the mains and direct outputs.
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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