# Is this the correct fuse to use on the primary side of a transformer?

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?

• It depends. Show your schematic. Simulate maximum inrush current and calculate the I^2*t under the graph. May 9, 2018 at 22:30