How to choose External Fuse?

1. Selection method of external fuse

(1) In Case of Fuse Equipped Type

It is not necessary to install a fuse outside of the power supply. However, in case that the output power of usage condition is too small against rated current etc., choose appropriate fuse according to Clause 2.

(2) In Case of Non Fuse Equipped Type

It is mandatory to install a fuse outside of the power supply. Please see instruction manual of each product to check the necessity of external fuse. Applications manual would be available for selection of external fuse (Except a part of products).

2. How to select external fuse?

Here is the point to select external fuse.

< Selection method of external fuse >
(1) Rated voltage : Check the maximum input voltage, and choose higher rating fuse.
(2) Rated current : Choose approx. 1.5 ~ 2.0 times from continuous input current.
(3) Inrush current* : Calculate Joule heat (I^2t) of inrush current.
                                  Time-Lag type (Slo-Blo) should be chosen.
(4) Abnormal condition : Evaluate that the fuse blown out safely.
(5) Safety approval : Choose appropriate fuse for your equipment requirement.

* Inrush current term would be approx. 5ms (At 90deg/270deg turn on).
In case of DC-DC converter, the capacitance of external capacitor which is used for stable operation would be higher than internal capacitor. Inrush current would be larger by external capacitance. And there may be influence of line impedance as well, so please evaluate before use.

3. Troubleshooting fuse installation

The causes of fuse blowing are almost classified into 3 types. According to the different types of causes, troubleshooting methods are also different. Please refer to table 3 .

Table 3 The causes of fuse blowing and troubleshooting methods
No. Trouble Cause Troubleshooting
1 Fuse blowing Internal components broke down by short circuit. Contact us for repair. Because only changing the fuse can not recover the power supply.
2 Current capacity is not large enough. Confirm whether the usage condition against derating (current, temperature) is appropriate or not.
3 Surge resistance is not enough. Confirm whether the inrush current I²t is within the range of fuse resistance.

4. Blown out of built-in fuse

In case of internal fuse blown out, internal components not only fuse but also others shall be broken down for some certain reasons with most cases. So we recommend to ask purchased distributor for repair/analysis.

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