There are different types of fuses having different characteristics. Most common types are categorised as fast-acting and slow-acting fuses.
Fuse characteristics can be adapted to different loads. Common fuse types include glass-tube fuses, miniature circuit-breakers, thermal fuses.
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Key Tips for Selecting Fuses
- Prioritize Current Rating: The current rating is more crucial than the voltage rating when selecting a fuse. A fuse can operate at any voltage up to its rated level. Always choose a standard fuse size that corresponds to the current specified on the transformer.
- Follow the Standard Fuse Rule: To determine the appropriate fuse, look for the nearest standard fuse rated higher than the transformer’s rated current.
- Positioning of Secondary Fuse: The secondary fuse is installed after the transformer’s output connections on the secondary side. It provides overload protection for the transformer, and its rating must not exceed the secondary current specified.
- Choosing the Right Characteristics: The choice between slow-acting or fast-acting fuses depends on the load characteristics.
- Placement of Primary Fuse: The primary fuse is installed at the transformer’s primary connection points. This fuse acts as short-circuit protection, and while distribution transformers always have fuses on the primary side, low-power transformers for electronic circuits may only require fuses based on specific applications.
- Account for Starting Current: When a device is powered on, the starting current can significantly exceed the nominal current for a brief period. Therefore, it’s important to select a primary fuse that is as slow-acting as possible to accommodate this.
How to Calculate the Ampere Rating of the Fuse?
Suppose a transformer is rated at 230V AC primary and 12V, 1A secondary. The voltage turn ratio is given by primary voltage/secondary voltage=230V/12V=19.2A.
The primary fuse current can be calculated from current turn ratio, which is secondary rated current/primary current, that is 0.052A.
You can use a primary fuse that is 135% of this value, which is about 100mA. You can select a nearest standard size fuse with current rating not less than 100mA.
Note that you can use this fuse across the secondary side but not after the rectifier circuit. A fuse installed in an AC circuit performs differently when installed in a DC circuit.
