210902-0851 EDT
TheGingerElectrican:
An MDL is slow blow fuse.
Transformer inrush current --- see my curves at
http://beta-a2.com/EE-photos.html
Plots P! thru P4 are of a tungsten filament bulb.
Plot P4 is of an Ohmite power resistor.
Plots P6 thru P8 are the primary current of an unloaded transformer.
The residual flux state of a transformer from its last turn off, and the point in the applied AC voltage at next turn on, and loading will determine peak inrush current at next turn on.
This worst case turn on transient will determine how you size a primary fuse. You want to use a slow-blow fuse, and size it to protect your secondary load, and not trip on in rush current. Look at fuse curves for MDL and your worst case input in rush current.
75 VA and 24 V is 3.1 A at 24 V. If we assume 120 V for the transformer input, then primary input current at full load is about 0.63 A. But there is also some additional magnetizing current. Assume steady state input is about 1 A. Assume peak in rush is 10 times peak steady state, then peak 1/2 cycle current will be around 14 A. Look at MDL curves for 20 milliseconds. What MDL fuse might handle this? Is that suitable for your steady state load current? If not consider adding a series negative coefficient thermistor in the input circuit.
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