I'm looking at this design

enter image description here

For a split supply. I get that the thermistors are there to limit the inrush current, but I don't understand the purpose of C9 and how its value was chosen.

  • 3
    \$\begingroup\$ That's an X rated cap for some noise reduction \$\endgroup\$ Commented Jun 25, 2019 at 0:40
  • \$\begingroup\$ @SunnyskyguyEE75 Would you mind to elaborate please? Is it an important component? What kind of noise is it suppressing? I have not seen it often. \$\endgroup\$
    – darksky
    Commented Jun 25, 2019 at 0:43
  • \$\begingroup\$ WHen output Ripple is 10% CUrrent pulses are also 10% duty cycle and thus 10x bigger amplitude and much faster dV/dt so more noise on line. TH can be ICL's \$\endgroup\$ Commented Jun 25, 2019 at 0:47

2 Answers 2


In addition to Autistic's correct answer, the value of C9 (X rated) is a compromise between consuming excessive AC current at 60 Hz while suppressing the noise voltage spikes by about 3 dB to 6 dB at the transformer primary if power is cut off when the 60 Hz sine wave is at or close to peak current. C9 does NOT behave like a MOV or TransZorb, but an active noise filter. When the power is cut off as mentioned above there is a burst of noise, the back EMF from the transformer. This can send substantial RF noise into local air space as well as increase arcing of the switch contacts.

C9 consumes about 150 µA of current at 60 Hz (18 mW at 120 VAC), not enough to heat up anything. If C9 were large enough to suppress most of the noise burst (say 0.33 uF), it would consume 15 milliamperes of current and 1.8 watts of reactive power. The design engineer was being careful about the power budget vs. its ability to absorb a useful amount of RF noise, likely based on oscilloscope readings of the primary voltage when the switch is turned off.

Also if C9 had a high value then it could cause a brief current spike when the power is turned ON. Keeping C9 at a low value avoids this problem as well as the extra space and cost of a large X rated capacitor.

  • 2
    \$\begingroup\$ Thank you both. I can only select one answer. This makes a lot of sense. \$\endgroup\$
    – darksky
    Commented Jun 25, 2019 at 14:32
  • \$\begingroup\$ @darksky Thankyou for accepting... \$\endgroup\$
    – user105652
    Commented Aug 27, 2019 at 3:35
  • \$\begingroup\$ I'm pretty sure I had already accepted this answer. I must have accidentally tapped to unselect on my phone. \$\endgroup\$
    – darksky
    Commented Aug 27, 2019 at 3:44

C9 snubs the transformer primary inductance. This damps the inductive spike that occurs when the power switch is turned off when current is flowing. This capacitor is often present in audio systems.

This can save speakers from a turnoff plop sound that can be annoying or even destructive.

  • \$\begingroup\$ I see. This design actually does come from an audio power supply application note. Any idea how to choose the value? \$\endgroup\$
    – darksky
    Commented Jun 25, 2019 at 0:55

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