I am designing an unregulated power supply to power a few linear voltage regulators, as shown in the image. The power supply will use a 230vac to 9vac 2 coil step down transformer. Since the power supply will supply 20kHz and 50MHz digital communication protocols, I intend on putting a safety cap at the primary coil for EMC and EMI purposes. I was wondering on whether should I put the fuse before the cap or after it.

enter image description here

  1. On one hand, putting the fuse before the safety cap will protect the primary coil and the safety cap as well, should it break short in the future(class X2). On the other hand I think I am risking on blowing the fuse in cases where the supply is powered at the peak of the 230VAC sine wave. It is a slow blow(time delay) fuse twice the rated current of the power supply.

  2. If I want to avoid blowing the fuse from inrush current, then putting it after the safety cap is the most sensible option. However, should the safety cap fail short in the future, I am risking a short circuit which will rely upon the circuit breaker of the 230VAC house distribution cabinet.

I have already seen a product on the market that has the fuse implemented after the safety cap which I guess is ok, since the main reason that cap is called "safety" is because it should not fail so easily. I am just asking for a second opinion from someone who's worked on similar stuff.

  • \$\begingroup\$ The reason it's called a safety cap isn't that it doesn't fail so easily. It's that it is designed to specifically it fails open rather than short like most caps do. \$\endgroup\$
    – DKNguyen
    May 31, 2021 at 21:45
  • \$\begingroup\$ How is that arrangement going to supply 24VDC and 10VDC at the same time from the same windings? \$\endgroup\$
    – Finbarr
    May 31, 2021 at 21:55
  • \$\begingroup\$ @DKNguyen As I said the capacitor is of class X2. Any safety capacitor of class X fails short. It is the class Y capacitors that fail open. \$\endgroup\$
    – Phill Donn
    May 31, 2021 at 23:16
  • \$\begingroup\$ @Finbarr Each coil is rated 9VAC. I use the two coils in series to get ~24vdc and each coil in half of a cycle to get 10VDC. This is basically a derivation of the center tapped +Vcc, -Vcc power supply. \$\endgroup\$
    – Phill Donn
    May 31, 2021 at 23:18

1 Answer 1


Put the fuse first .This means that the fuse protects everything .Your safety cap is not huge .It does not store enough energy to blow the fuse at peak switch on .

  • \$\begingroup\$ Well I forgot to mention the capacitance of the safety cap. Which is my bad. It is not that huge indeed, it is 0.47uF. \$\endgroup\$
    – Phill Donn
    May 31, 2021 at 23:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.