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Admittedly this is quite a novice question but I just wanted to make sure before I start a potential fire as this project is destined for an enclosed space behind a gyproc wall...

I picked up a Vigortronix 230 V AC to 5 V DC PSU to power a Pi Zero W and camera. I'd like to put an in-line fuse on the live wire and after looking at the data sheet I'm unsure what size of fuse I should be using. Which figures should one use to calculate this?

Also, would it be good practise to place a capacitor on the 5 V output or unneccessary - if so, what size of cap? Lastly, would there be any other recommendations for using this type of power supply?

Thanks in advance.

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The fuse is to protect against excessive current, so you need to look at the current rating of the device.

The datasheet says the module takes 70 mA continuous at 240 VAC input for 5 W output. You could use a 1 A mains fuse. It also says the inrush current could be up to 25 A, so you need a slow-blow (a.k.a. timed or time-delay) fuse.

If the inrush current tends to blow the 1 A fuse, use a 3 A fuse.

For maximum peace of mind, find a reputable fuse manufacturer and consult their datasheets with regard to the inrush current. Note that there are fake fuses available from online auction-type websites, so you might want to track down a reputable seller.

Note that the wire used to connect it must be of at least the same current rating as the fuse so that in a fault condition the fuse will blow cleanly instead of the wire melting messily.

With regard to a capacitor (or other filtering, e.g. an inductor), check if the output specifications meet the requirements for your circuit.

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  • \$\begingroup\$ Thank you. This is my first time hearing of slow-blow fuses so I'm glad I asked. \$\endgroup\$ Feb 6, 2018 at 10:48
  • \$\begingroup\$ @user2921789 I added a note about the wire needing to be of at least the same current capacity as the fuse. 3 A mains wire isn't exactly going to be a struggle to fit in anywhere, and it is commonly available. \$\endgroup\$ Feb 6, 2018 at 10:58
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5 V at 1 A gives an output power of 5 W.

Assume it is 50% efficient (conservative) this gives an input power of 10 W.

10 W at 240 V is 10/240 A = 1/24 A any fuse around 0.1A would be enough if you can find one that small.

You should not need another capacitor across the output of that supply. In fact some supplies have a maximum allowed capacitance on the output.

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  • \$\begingroup\$ I came back to this question for a quick sanity check with a similar project and this answer is really well explained. I see that the equivalent fuse can be higher/lower whether it's placed before or after the psu. Thanks. \$\endgroup\$ Oct 30, 2020 at 15:33
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There is a inrush current of 15 to 25A. So you want a slow blowing fuse. The max steady current is 100mA so give it some margin and select one that can break your voltage.

You can easily find 250V rated slow blow fuses for various currents in a 5x20mm cylindrical package. That package size is often used as fuses for dimmer output and (old) low voltage lighting transformers and in UK plugs.

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