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I am trying to design a linear power supply to learn how AC is stepped down and converted into DC. However, I am having trouble figuring out how to choose a fuse value for use on the primary side of the transformer. I have done some research and learned that for inductance loads, such as transformers and motors, you need a slow blow fuse because of onrush current. However, I have read in some places that the fuse should be valued at the amperage of the primary side. In other places I have read that the fuse should be valued 1.5x or 2x the primary side? Which is it?

For my particular instance, I am using a step down transformer which takes 115V on the primary side and provides 12.6VCT at a maximum of 2A on the secondary side. (I will be using this on US mains, which I assumed would be okay, so I expect the secondary voltage to be a tad bit higher.) This gives me 25.2 VA, which means that the primary side should draw about 25.2VA / 115V = 219 mA of current, plus some for inefficiency.

Because I am planning on drawing only 1.5A out of this supply in order to preserve my voltage regulators, I thought that a 200mA slow blow fuse should be adequate for my use. I also plan on placing a 2A fast blow fuse on the secondary side as well. Do these values seem correct?

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  • \$\begingroup\$ It depends. Show your schematic. Simulate maximum inrush current and calculate the I^2*t under the graph. \$\endgroup\$ – winny May 9 '18 at 22:30
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A fuse should be large enough to handle not only the rated current but also surges. A fuse should be small enough to guarantee that it opens under fault conditions. A fuse is not accurate enough to be a current limiter.

So what fault conditions do you want to catch? A fuse isn't suitable if you define a fault as 'secondary drawing 1.5x rated current'. A reasonable fault to catch would be 'secondary gets short circuited'. One option is measure the transformer draw under short circuit conditions. Another is to compute, from the winding resistances, what it should be. This puts an upper limit on your fuse value (see below).

For the minimum value, start with a time lag fuse slightly larger than your rated current. Switch on 100 times. If it fails at switch on, replace with the next size up (small fuses tend to be available in an E5 series, so 10, 16, 25, 40, 63). If you need to rinse and repeat this too many times, make sure you're using time lag, and not fast blow.

Not only do transformers have an inrush, so do rectifier-capacitor filters. You can use a fast blow after the main reservoir capacitors, where it will be most useful, not before.

(I did once use a bench instrument from a well-known manufacturer (no names, no pack-drill) that employed a crowbar on the DC power supply, that was prone to nuisance tripping. Unfortunately, when triggered, it didn't draw quite enough current to take out the main fuse, so the transformer sat there and cooked, destroying the instrument by coating its insides with condensed transformer insulation)

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