Q&A(FAQ) How to Select an External Fuse?
1. Selection method of external fuse
(1) For Models Equipped with an Internal Fuse
It is not necessary to install an additional external fuse. However, if the output power under the actual usage conditions is significantly lower than the rated current, etc., select an appropriate fuse according to Clause 2.
(2) For Models Not Equipped with an Internal Fuse
It is mandatory to install an external fuse. Please refer to the instruction manual of each product to determine whether an external fuse is required. An application manual is also available for selecting an appropriate external fuse (except for certain products).
2. How to select external fuse?
Here are the key points for selecting an external fuse.
<Selection Method for External Fuses>
- (1) Rated voltage : Check the maximum input voltage and select a fuse with a higher voltage rating.
- (2) Rated current : Select a fuse rated at approximately 1.5 to 2.0 times the continuous input current.
- (3) Inrush current* : Calculate the Joule heat (I²t) of the inrush current.
A time-lag (slow-blow) type fuse should be used. - (4) Abnormal condition : Ensure that the fuse will safely blow under abnormal conditions.
- (5) Safety approval : Select a fuse that meets the safety requirements for your equipment.
*The inrush current duration is approximately 5 ms (when turning on at 90° or 270°).
In the case of DC-DC converters, the capacitance of the external capacitor used for stable operation is often larger than that of the internal capacitor. As a result, the inrush current may increase due to the additional external capacitance. Line impedance may also have an influence, so please evaluate the circuit before use.
3. Troubleshooting fuse installation
The causes of fuses blowing are classified into 3 situations. According to the different causes, troubleshooting methods are also different. Please refer to table 3.
| No. | Trouble | Cause | Troubleshooting |
|---|---|---|---|
| 1 | Fuse blowing | Internal components have been damaged due to a short circuit. | Please contact us for repair. Replacing only the fuse will not restore the power supply. |
| 2 | Insufficient current capacity. | Check whether the usage conditions meet the derating requirements (current, temperature, etc.). | |
| 3 | Insufficient surge resistance. | Verify that the inrush-current I²t is within the fuse’s allowable range. |
4. Built-in Fuse Blown
When the internal fuse is blown, it is likely that not only the fuse but also other internal components have been damaged. Therefore, we recommend contacting the distributor from whom the product was purchased for repair or analysis.