where to place fuse in power supply?

[crawler486]

Heres another wuick question to u guys.
I just built the 9V Ultra Claen Power Supply fro GeneralGuitarGadgets.
My question is where woul I place a fuse and how many ampere is required?

Many Thanks!!

[Ge_Whiz]

For thoroughness, I would include two - one before the transformer primary, and one just before the supply is split to the various effects.

The current rating of the LM317T is (I think?) 1 amp, so a 1A quick blow will provide protection at the output. If you are only going to supply very-low-consumption analogue pedals, something like 100 - 200mA would be more appropriate. The current pulled from the mains is miniscule, but there will be a surge at plug-in, so 500mA slow-blow should be adequate.

[niftydog]

The current rating of the LM317T is (I think?) 1 amp, so a 1A quick blow will provide protection at the output. If you are only going to supply very-low-consumption analogue pedals, something like 100 - 200mA would be more appropriate.

That's fine for the DC side of the circuit, but if you put a 1 amp fuse on the primary side of a 1ADC rated supply, you have a way over-rated fuse!

I would suggest you load the power supply up as much as you intend to in normal use; Then get a meter that can read AC current, place that in place of the fuse to get an idea of the normal operating range of the supply, then, put in a slow blow fuse that's rated 5-10% above the normal current.

[Ge_Whiz]

Dear Nifty

Please read remainder of posting, with particular reference to 'start-up surge'... :?

[primalphunk]

For thoroughness, I would include two - one before the transformer primary, and one just before the supply is split to the various effects.

The current rating of the LM317T is (I think?) 1 amp, so a 1A quick blow will provide protection at the output. If you are only going to supply very-low-consumption analogue pedals, something like 100 - 200mA would be more appropriate. The current pulled from the mains is miniscule, but there will be a surge at plug-in, so 500mA slow-blow should be adequate.

I personally don't know what the heck I'm talking about so take anything I say with a grain of salt...even better with a salt lick.  

I too am building the same power supply but with a 600ma transformer.  So I asked a couple of my students that have a better background in this kind of thing about useful values for the two fuses.(Both have marked my recent zeal for building groovy effects with mild amusement.)  One guy has built amps as a hobby for some time while doing more serious stuff at a high tech/engineering type of gig and the other works at TI.  Each of them concurred that fast blow for both fuses would not be a bad idea.  On the primary side both agreed that 250v .25amp would be fine and tried to take into consideration the surge factor.  For the DC side they also thought that a 1amp would be cool.  One of them kept telling me to get a 25v 1amp fuse for the DC side.  But it's kinda hard to find that value.  I found some 24volt but I'm not interested in messing with surface mount stuff.  It seems like I've read elsewhere on this site that going with a higher voltage 1amp shouldn't be a problem.  

Anyway...I'm willing to learn if any of this a bad idea.

[niftydog]

a slow blow fuse is designed to deal with start up surge...

You don't want to over-rate your fuses just to deal with start up surge!  If it's a big problem, stick in a small, high wattage surge limiting resistor!