# How to chose resettable fuse for 24 V DC to 5V DC converter?

I want to use a 24V to 5V DC converter, the PYBJ10-Q24-S5 (https://www.mouser.fr/datasheet/2/670/pybj10-om-1548233.pdf).

Page 7 of the datasheet, they specified the EMC Recommended Circuit which includes the fuse I'm trying to choose :

If I understood correctly the datasheet, the converter accepts a maximum input current of 508mA and has a current overload protection of at least 110% :

So if I'm correct, in the worst case scenario it should not exceed 558mA.

I was planning to use a resettable fuse. If I understood correctly, the holding current is the maximum current such as the fuse won't trip.

The tripping current is the minimum current such as the fuse will trip.

In conclusion, I have to choose a resettable fuse with a holding current = 508mA and a tripping current = 558mA.

Problem is : in this configuration, I can't find exactly the right fuse.

Knowing all this, how do I choose my fuse from all the existing models?

For example, I found some fuses with currents of 550mA versus 1A but I'm afraid that 1amp is much too high for my converter.

I guess I'm missing something.

• What will a fuse bring to the party i.e. what is your rationale for a fuse? If you think it will protect the converter, I believe you will be disappointed. – Andy aka Sep 25 '19 at 15:28
• I assumed that this was necessary to protect the converter since the manufacturer himself specifies that a fuse is recommended. Why can't this fuse protect the converter? And if he really can't, why does the converter manufacturer offer us a recommended circuit containing a fuse? – A.Girafe Sep 25 '19 at 16:16

## 1 Answer

Knowing all this, how do I choose my fuse from all the existing models?

Typically fuses are for short circuit currents, and good for removing the circuit from operation if the current goes well above it's intended operation.

If I were sizing a fuse for a circuit, I usually use at least 50% of the rated current as a starting point, and go to the next size fuse. So in your case I would use a 1A fuse. The fuse size should be high enough to not blow in normal operation (which it will if rated close to the current max).

A better way would be a current monitor with a shutoff if you really must have current protection. A better way is to use a DC to DC circuit that has built in current limits that self limits the current.

• But if I use a 1A fuse, won't the dc converter blow long before the fuse? – A.Girafe Sep 25 '19 at 16:19