Calculating fuse value for transformer

[jrc4558]

Is the fuse value equal to the secondary VA primary voltage divided by the primary voltage of transformer?
(Basically - is this: http://www.itemspecialty.com/audio/calcfuse.html true?)

As in 6VA trabsformer would need (with extra 30% for loss and turn-on surge) a 0.065A fuse?

[RGW]

Basically if you know the VA and the primary and secondary voltages, it is pretty easy.

Ideally Primary VA=Secondary VA, assuming no core losses. In reality there will probably be at least some core loss but for the sake these calculations assume 100% efficiency.

Let's say a transformer is 60VA with an input voltage of 120V and an output voltage of 12V. The turns ratio is 10:1, this relates directly to the voltage. The primary to secondary current ratio is the reciprocal of the actual turns ratio, in this case 1:10.

60VA/120V gives us a max primary current of .5 amps.
60VA/12V gives us a max secondary current of 5 amps

If there are no primary or secondary taps, only a two wire primary and a two wire secondary; you could fuse either the primary at .5 amps or the secondary at 5 amps still protected the transformer from burning up. In neither situation would the 60VA rating be exceeded. 

I wouldn't cut it this close, maybe derate things a bit and keep the VA at about 80% of its actual rating. In this case .4 amps on the primary or 4 amps on the secondary.

But you can fuse only one side or the other and protect the transformer in the case where there are no taps creating multiple ratios.

hope this helps,
Robbie

[R.G.]

Selecting a fuse value is a tricky task.

The rating stamped on a fuse is not the current it opens at. Rather, its the current that it's guaranteed NOT to open at. A fuse will carry its rated current essentially forever. Fuses carrying 110% of rated current may take a couple of hours to blow, depending on the construction. Normal blow fuses will clear in seconds with 200% of rated current.

Current correlates poorly to fuse blowing. What correlates well to fuse opening is the energy delivered to the fuse, calculated as the square of the current times the time, or I-squared-t. A glance at a fuse maker's selection guide will make this clear.

Most of the time, fuses are used in the AC line to prevent fires, not to save transformers. Selecting them to save transformers is much more work. I put a fuse in every single secondary winding of the Workhorse amp transformers, and then spent a week calculating values and testing whether the transformer winding would blow the fuse without damaging the transformer. It was a busy week.

The simplest thing to do is to fuse to just carry your maximum load, not the transformer rating.

[gez]

The rating stamped on a fuse is not the current it opens at. Rather, its the current that it's guaranteed NOT to open at. A fuse will carry its rated current essentially forever. Fuses carrying 110% of rated current may take a couple of hours to blow, depending on the construction. Normal blow fuses will clear in seconds with 200% of rated current.

Well, I learnt something new today.  Thanks for that RG! 

[QSQCaito]

Nice info RH thanks a lot.

The simplest thing to do is to fuse to just carry your maximum load, not the transformer rating.

What would happen if your maximum load is near the transformer rating??


Thanks, bye bye!


DAC

PS Sorry for hijack :/

[R.G.]

What would happen if your maximum load is near the transformer rating??

Then you're fused for both your load and the transformer rating, Mother Nature is happy, the fuse is happy, you're happy, and all's right with the world. 

You will also run into the problem that fuses are not available in all possible ratings. You can find a 1A, a 1.5A, and a 2A, but what if you want a 1.732A fuse? You're out of luck.

If the exactly size of your current limiting is critical to your design, you're going to have to use a solid state current limiter and figure out how to make that work for you. There are many examples of prior art.

[QSQCaito]

Awesome, thanks a lot RG

[jrc4558]

Thanks everyone!!! I learned a lot from this!