Wiring question: house current to transformer

[BR60103]

I have some surplus small transformers (from a ceiling 12V halogen (?) light) that I want to use to power a throttle and other items.
I looked a Peter Thorne's book where he shows transformer wiring. His diagram shows the wiring in as having a fuse on one side and a switch on the other. Now, if I use a polarized plug, which side should get the switch and which the fuse?
What size fuse would I use on the 120V side if the output is rated 40W? -- Can you buy 1/3 amp fuses?

[nachoman]

I have some surplus small transformers (from a ceiling 12V halogen (?) light) that I want to use to power a throttle and other items.
I looked a Peter Thorne's book where he shows transformer wiring. His diagram shows the wiring in as having a fuse on one side and a switch on the other. Now, if I use a polarized plug, which side should get the switch and which the fuse?
What size fuse would I use on the 120V side if the output is rated 40W? -- Can you buy 1/3 amp fuses?

The 120 volt side is already protected by your house breaker. The lower voltage side is not protected against a short circuit if the fuse is on the high voltage side.

[rrinker]

Fuse belongs on the hot side. As does the switch. With a polarized plug and assumign the house wiring is done correctly, there really shouldn;t be any voltage between ground and neutral (but don't try this out by touching it!). That's one of the things the "wiring fault" light on a surge protector or UPS detects. Or better yet get one of those little testers witht he three LEDs and plug it in, it will tell you if the wiring from the outlet back to the box is right or if someone flipped the hot and neutral or the ground is bad.
Don;t rely on the house breaker, fuse the input to the transformer. A fault could develop in the primary that draws 10 amps - it won;t trip the 15 amp breaker but that little transformer will have 1200 watts cooking through it - plenty to set things on fire.

--Randy