selecting fuse value

[V_King]

Hello,

trying to figure out what value fuse I should use for the circuit protection from the mains supply side.

I've got a power supply unit, which supplies 12V 150mA to the circuitry. The unit does not provide any mains input requirements and I can not find any data sheet for it.

Also, I found that it is recommended to have a fuse selected with constant load of 75%, and in other reseources they recommend to use the x2 rating fuse for the constant load.

Any input is much appreciated.

Cheers

[Psi]

Does your device have a large inrush current. (like an audio power amp with large 10,000uF caps)
If you have large inrush you'll want to use a slowblow fuse.

The figures  75% -> 100% (1.3x -> 2x) are the typical ranges.

If you have little to no inrush then 1.3x should work fine.

[V_King]

Thanks for the answer. I have two high value caps at the power input, so slow blow (same as time delay?) fuse it is. On the other hand, is there an easy to implement way to prevent the surge at the beginning of the cap charge and use fast blow fuses?

I am more puzzled with selecting the current rating for the fuse.

Power consumption at 12VDC and 150mA, is 12*0.15=1.8W

So at 240VAC RMS (340V true) I would get only 1.8 / 340 = 5.3mA (not taking phase into account)

Even if the power supply efficiency is 50%, the current would be 11mA.

I looked in the familiar output switch mode power supplies in farnell's catalogue and they show current input at around 50-100mA for 240VAC inputs. Would 100-150mA fuse would be the value to choose?

[PuterGeek]

The fuse rating depends on the fuse type, opening speed and the load. If you don't have input ratings you should at least measure the steady state current.

If you look at the data sheet for a fuse you will find how long an overload takes to blow the fuse. For a typical 5x20mm medium acting fuse, a 400% overload will blow in 40 to 2000 ms while a 210% load will take a maximum of 10 minutes. A low current fast acting fuse takes 3 to 300 ms @ 400% and 30 minutes max. @ 210%.

These values are from Littlefuse but should be typical.