I'm doing a little fix up job on an old Electro D-612T D.C. power supply and I want to replace the existing fuse holder with a pushbutton circuit breaker. on the front of the device is printed "10 Amps Continuous Duty" and I would like to be able to use it at that current. However, printed over the fuse holder is "Use 3 Ampere Fuse" (see attached picture).

Q: Why would it require a 3A fuse if it's rated for 10A? Maybe the expectation is that you use a larger fuse only if needed not to exceed say 10A or a little over since it's rated for 10 continuous?

 

It is probably a fuse on the input, AC side of the supply.
12 volt 10 amp DC is 120 watts.
AC input 120 volts would need 1 amp for 120 watts.
A simple fast blow 3 amp fuse would be a common value on the input of such a supply.

 

A simple fast blow 3 amp fuse would be a common value on the input of such a supply.

I agree on the rating, but if the supply uses a transformer input (which it surely does) then a time delay fuse will probably be required to cope with the possible current surge at switch on.

 

Okay, I opened the case and the fuse is on the input AC side as Kermit2 suggested.

Also, AlbertHall, upon closer inspection, what I thought at first was a transformer appears to have only two leads, so I'm pretty sure it's an inductor. I guess I could put an isolation transformer in front of the variac but now I'm thinking maybe I gut this thing and use the case (it's pretty cool looking and has sentimental value to me) to build a modern isolated power supply from scratch. I'll start a new thread. Thanks for your help guys!

 

Why would you want to replace the fuse holder? Put a 3 amp fuse in it and as long as you don't overload the supply it should never need any further attention.

 

Post a picture of the inside, top and bottom.

 

The two wire thing is the smoothing choke but there is also a variable mains transformer:
http://plymouthcolony.net/starcity/radios/pages/serveq.html