Hi,
I'm trying to build my own version of a Switchmote PSU v2. I've learned a lot from Felix's work but need to add some flexibility in my own design.
First prototype fails because of the picofuse is blowing up at first power-up. I've calculated the maximum current and used a bit higher value of the fuse. Even choosing a much bigger value, same result.
I think the switch-on current is much higher then just the maximum values stated on the components (I use the one on the AC-DC converter block TMLM-04105 from Tracopower which is stated as being: 65mA /230V typ.). I've tried fuse values of 125mA and 250mA to no avail....
How do I calculate a working fuse value?
Many thanks for your help, Remco.
Are you sure there's no short somewhere in your design?
Have you tested your AC-DC modules by themselves without any fuse, do they power up and hold a load that is within the specs?
There should not be any inrush current, but maybe the modules you're using are spiking for a split second to charge their internal caps, long enough to blow the fuse.
I don't think there's a way to calculate, it's really a value that is slightly above the max current you'd ever expect your project to draw. Something like 500mA or 1A is a good value. If something fails catastrophically then much higher currents are possible, which will blow the fuse and should cut off the large current and any potential fire hazards.
Thanks Felix.
With the fuse footprint shorted the setup works fine and is consuming about 100mA full load.
I also thought of the small switch on current spike. I will follow your sugestion and trial and error with higher values. The fuses are 'fast' so even a short spike could blow the fuse indeed.