A calculation problem:

How can I calculate the value of a fuse in a transformer circuit?

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Electronics every day!

 

Basically by measuring the maximum current your circuit takes, then applying a reasonably sized margin on top. You also have to consider the power on surge, which is considerable for toroidal transformers!.

Mostly the mains fuse is protecting against failure in the transformer itself (shorted turns), or the rectifiers going S/C (although you should really have a fuse between the transformer and rectifier as well). So you might also try measuring (VERY briefly) the primary current with a short across the secondary winding. If it's a toroidal transformer you can even simulate a shorted turn by placing a loop of wire through the toroid.

You might even consider simply trying different fuses?, start small and work upwards until it doesn't blow any more? - although the first size you find that survives will probably be too small as well, and only have a limited life.

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You've also got different types of fuses to consider. quick blow, time lag, and there are even choices to be made regarding the material of the fuse. Should it be a glass fuse, or sand filled etc.

Not that I'm trying to complicate matters of course

Brian

 

What is the rating of the transformer?
What is the current rating of the electrical load?
If the transformer is rated at 1amp then that is the maximum current it should draw.

If its a step-down transformer and you know the ratio of the windings such as 10:1

Then the input current should be 1/10th of the output current.

The input fused current should be limited to 1/10 of designed maximum.

So if the Maximum load is 1amp then the fuse should be rated at 1/10amp if connected on the input side. That would protect the transformer and the circuit load.

If you put the fuse on the output side then use a fast-blow fuse of 9/10ths of an amp to be safe. and would also protect the transformer.

Just remember to look at the input voltage and use a rated voltage fuse that's at or exceeds that voltage.

Don't use a 32volt 1/10th amp fuse on a 117volt side. You would probably get arcing and ruin the transformer, etc.

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I'm sure others will be more explicit but at least this is an easy concept to absorb.

I know there is more involved than this, but at least its a simple rule of thumb and no hard math need.

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