How to pick a proper fuse?

[redgear]

The PSU has a nominal input current of 0.15A and has a peak inrush current of 40A. No info about how long the inrush current lasts is given.

The ballast has a nominal current rating of 1.5A with a max peak inrush current of 21A lasting for 185 microseconds.

After reading the Little Fuse's guide, I got some info on how to account for these inrush currents while selecting a fuse. I do not have an idea about the waveform of the inrush current so went with the waveform that will give me the maximum i2t value from chart 1 in the guide.

For 100,000 cycles I get a i2t value of 0.185. So a 2A fuse with a i2t value greater than 0.185 should be used as F1, am I right? How do I calculate i2t for F2 where I don't the time period of inrush current? The F3 is to protect the micro and 7 seg display? Will a fast blow 1.5A fuse be a right choice? Cos I don't have both inrush current or timeperiod to calculate the i2t.

Also, I need a fuse as soon as the mains enter, will a 3A fuse with the greater i2t value(F1) be a right choice?

Thanks

[wizard69]

I don't have a good answer for you because most of my experience has been industrial.   Branch circuit protection there is about meeting NFPA/NEC code.   Device protection is attempted but to be honest protecting some devices with fuses is a bit like swimming upstream.    Generally the device fails thus blowing a fuse.   If this is the case it really doesn't pay to set the fuses to close.   In the end you may need some history with the device to get an optimal fuse size.

[Raj]

correct me if I'm wrong. But you'll need a slow burn fuse with a little more than 0.15A rating and fast burn fuse with little more than 40A rating

[thm_w]

How do I calculate i2t for F2 where I don't the time period of inrush current? The F3 is to protect the micro and 7 seg display? Will a fast blow 1.5A fuse be a right choice? Cos I don't have both inrush current or timeperiod to calculate the i2t.

Can you measure it?

That IRM-10-5 module has a maximum output current of 2A and overload protection already, so I don't know if I'd even bother with a fuse on the output.
But you could throw a 1 or 1.5A 0603 fast blow on there sure, if you want.. Assuming the maximum current consumption of the circuit is <0.5A.

[chris_leyson]

I think the best approach would be to measure the power supply inrush current under worst case conditions, maximum VAC and switch on at the peak. You would get a better estimate of i2t for the power supply.