# Fusible Resistor Calculation [on hold]

How do we calculate the value of fusible resistor used in the input side of a capacitive power supply. The supply produces an output voltage of 5V DC. I want to add the fusible resistor (RT1) to protect the diode (1N4007) in the circuit from surge current . The data sheet of 1N4007 specifies that the diode can withstand a peak forward surge current of 30A for a period of 8.3 ms . How do I calculate the value of fusible resistor from this data.

## put on hold as too broad by Marcus Müller, Sparky256, Bimpelrekkie, StainlessSteelRat, John BirckheadFeb 19 at 20:55

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• Without being way more specific, this is far too broad to be answered here, I'd say. Can you add more specific requirements? – Marcus Müller Feb 10 at 13:58
• we have zero knowledge of what circuit you protect, but at least we now know against what! That's a great improvement, so please edit your question to include this info, along with a schematic of the circuit – we really have no idea what you're talking about, otherwise. – Marcus Müller Feb 10 at 15:14
• Edited the question and included schematic – CS Thakur Feb 11 at 7:36
• It is a known principle of electronics that a transistor (or diode) will always blow to protect its fuse. You design fuses to protect against a fire or electrocution hazard and additional damage, the fuse should be dimensioned to your expected operation range, not the maximum range of a specific component. – Edgar Brown Feb 13 at 16:41
• Maximum current of the diode is meaningless. You need to specify your requirements. Specifically, current required by load and type of load. Follow the instructions at: Everything for capacitive power supplies from a single source . – StainlessSteelRat Feb 18 at 16:21