What is the correct fuse rating?

[exztinct01]

I just built a power supply for my pedals. I used a transformer with 230V primary, single 12V secondary and rating is 750 mA.
Circuit includes 1 LM317 & 1 LM337 giving +-7 to +-14 volts, 2 LM7809 and 1 7909.
Question is what rating of fuse should I use? Should it be 750 mA as its the max. rating of my transformer?

Edit: I'm using the fuse for the mains

[balkanizeyou]

750mA of mains current would equate to 14A of secondary current - not good.
Quick and dirty calculation:
I assume the 750mA is RMS rating, so the peak current would be around 1A, which would equate to 50mA of mains current. Multiply that by 1.5 to cover for inefficiencies of transformer and you get 75mA of mains current. Where I live it's hard to get 75mA fuse, so I would use 100mA slow-blow one.

[merlinb]

You're unlikely to find a 100mA HRC* (ceramic) fuse. You MUST use an HRC fuse for mains. The smallest HRC you can conveniently buy is often 500mA (you don't want to be ordering special fuse values every time one blows, right?).

You should therefore be using a convenient 500mA value HRC fuse on the primary, and an LRC (glass) fuse on the secondary. Glass fuses are readily available with smaller values, but a 1A or 2A glass fuse would be about right for a 750mA transformer.

*HRC = high rupture capacity

[exztinct01]

I can afford some of these http://ph.rs-online.com/web/c/fuses-sockets-circuit-breakers/fuses/cartridge-fuses/#esid=4294960249&applied-dimensions=4294650317,4294828536

Is a quick blow fuse okay? The slow blow here is a bit expensive for me

[merlinb]

I can afford some of these http://ph.rs-online.com/web/c/fuses-sockets-circuit-breakers/fuses/cartridge-fuses/#esid=4294960249&applied-dimensions=4294650317,4294828536
Is a quick blow fuse okay? The slow blow here is a bit expensive for me

http://ph.rs-online.com/web/p/cartridge-fuses/0563352/
http://ph.rs-online.com/web/p/cartridge-fuses/7833286/

[exztinct01]

I can afford some of these http://ph.rs-online.com/web/c/fuses-sockets-circuit-breakers/fuses/cartridge-fuses/#esid=4294960249&applied-dimensions=4294650317,4294828536
Is a quick blow fuse okay? The slow blow here is a bit expensive for me

http://ph.rs-online.com/web/p/cartridge-fuses/0563352/
http://ph.rs-online.com/web/p/cartridge-fuses/7833286/

oh, that first one's definitely affordable
thanks

[merlinb]

One other thing I should have mentioned:
Measure the DC resistance of the transformer primary winding. Divide your primary voltage (230V) by the resistance. The primary fuse rating must be LESS than this figure, otherwise it may never blow during a fault.

[amptramp]

You can get small fuse current ratings easily enough - most DMM's have a 200 mA or 315 mA fuse to protect the low-range ammeter function and they are available in most places.  Just make sure you have the right voltage rating.

[Transmogrifox]

You can get small fuse current ratings easily enough - most DMM's have a 200 mA or 315 mA fuse....

You're unlikely to find a 100mA HRC* (ceramic) fuse. You MUST use an HRC fuse for mains. The smallest HRC you can conveniently buy is often 500mA ...

*HRC = high rupture capacity

The output rating of your transformer gives a hint how small your fuse can be, but the purpose of the fuse is to reduce the risk of fire in case of a transformer fault (output short circuit, transformer leads short, etc).

You want to make sure the fuse blows relatively quickly before stuff lights up.  Thus this advice:

One other thing I should have mentioned:
Measure the DC resistance of the transformer primary winding. Divide your primary voltage (230V) by the resistance. The primary fuse rating must be LESS than this figure, otherwise it may never blow during a fault.

One way to be sure you have a reasonably sized fuse is to momentarily short the transformer output when power is applied.  Make sure this blows the fuse within about or 2 seconds.  You waste a fuse on the test but you give yourself more confidence that you won't start a fire if a short happens in uncontrolled circumstances.

Fast blow fuses are great for safety purposes but they can be a hassle if your circuit has much inrush current (say you have a lot of capacitors hooked up to the output), or brief harmless faults (accidentally touch the output leads for 1/2 second).  The I²t rating on the fuse will tell you how much time it will take a fuse to blow at a certain amount of current.

[PRR]

Recapping important points:

> slow blow here is a bit expensive for me

More expensive than burning the house down? Or more likely: the line cord sets the carpet or drapes on fire, leading to big fire?

Fuses are not expensive, they are savings.

> You MUST use an HRC fuse for mains.

Why? Because a dead-short on household wires "could" suck nearly a MEGA-WATT of power for an instant.(*) Yes, the main and circuit breakers will quickly open. But meantime your little fuse must "handle" some very large energy. Without exploding or bursting into fire! Line-Rated fuses MUST be used. (Not car or meter fuses which never face such large energy.) Merlin says HRC for UK uses; and he knows. In the US, "3AG" fuses rated 250VAC are customary line-cord fuses.

> 750 mA as its the max. rating of my transformer?

750mA at the _12_V_ side. Transformer is a lever. 12V@0.750A is same-as 120V@0.075mA or 240V@0.038mA.

So use a 0.050A fuse? No. Such small transformers have losses beyond what you get out. They also have a turn-on surge. When you work it out, you can NOT "protect the transformer", because normal currents are not-much less than some abnormal currents, and because you can't buy a line-rated fuse that small.

In a similar case, I had trouble with a 0.25A fuse, "had" to go 0.5A for reliability.

In small gear (a few Watts) the line-fuse protects the Power Cord. You don't want red-white hot copper igniting your papers, carpet, curtains. Small US line-cord is good for 10A though we'd never fuse it that high. UK cords can run smaller, like 2A safe? You want to fuse well below this level.

For larger, 100 Watt, loads, Merlin's check of DCR is an excellent point. (The full calculation is more complex, thus rarely done.) In practical terms, you do not fuse much-much (like 10X) larger than the expected load. Example: I had a 60VA transformer with 6 Ohms DCR on 120V connection. 120V/6r= 20 Amps. A fuse larger than 20A would not blow when transformer was shorted. But in fact it was a nominal 0.5A @ 120V load. 1 or 2 Amp fuse should be fine. (To resist user abuse, I fitted a 2A breaker.)

But for this very-small 9-Watt transformer, you may not be able to fuse this small for other reasons.
_________________________________
(*) Assume a house is supplied for 2% voltage drop at 230V and 63 Amp in the line from the street transformer. Nominal load 14,500 Watts. 2% regulation means a dead-short will induce 49 times that current when voltage drops to zero (dead short). This is 710,000 Watts of power. Most of that will be lost in the power wires, but a huge amount of short-term power is available in your fuse.

In another window I am reading breaker/fuse discussions from 1897(!). It is clear that both theorists and practical men were astonished at the many ways Things Go Wrong in dead-shorts. We've learned some in the century+ since then. We now have reasonably safe cord and fuse systems even for very small loads.