Here is the next installation of the OptiFuse Fuse Selection Guide. We’re now through most of the more "technical" points but there are still several other criteria to consider when selecting the proper fuse...
Just to review...below are 15 information items that are needed before selecting the proper fuse for an application. Each week I’ll attempt to explain 2 or 3 of these items in detail. After completing the series, I’ll post the entire selection guide on the OptiFuse website:
In order to select the proper protective device you need to consider the following factors:
- What is the normal operating current of the circuit?
- What is the operating voltage?
- Is the circuit AC or DC?
- What is the operating ambient temperature?
- What is the available short-circuit current?
- What is the maximum allowable I²t?
- Are there in-rush currents available?
- Is the protective device being used for short-circuit protection, over-load protection, or both?
- What are the physical size limitations?
- Is the PCB surface mount or thru-hole?
- Does the fuse need to be "field-replaceable"?
- Is resettability an issue?
- What safety agency approvals are needed?
- How will I mount the device?
- What are the cost considerations?
Is the protective device being used for short-circuit protection, over-load protection, or both?
Ok...same old story here...an over-current can be a powerful and quick short-circuit or it can be a long-term overload. To review the differences in detail I suggest that you click here.
If the device is to be used as short-circuit protection, the fuse or circuit breaker must interrupt the fault quickly (generally less than 4 milliseconds) in order to give the maximum protection to equipment and personnel.
If the fuse or circuit breaker is intended for over-load protection only, then it can be much slower in reacting to the over-current - seconds or even minutes as compared to milliseconds...
All fuses offer some form of both short-circuit protection as well as over-load protection whereas many circuit breakers however are over-load protection ONLY and have no capabilities to protect against dangerous short-circuits.
What are the physical size limitations?
Many times the fuse or circuit breaker needs to be mounted into a place with physical size limitations.
It is this reason that fuse and circuit breaker manufacturers have created a wide selection of components with varying physical sizes. Typically however, there are a trade-offs that the engineer must consider.
Generally speaking, the smaller the fuse, the less current and/or capabilities that the fuse or circuit breaker may have. For example, a subminiature fuse maybe limited to 15A where as the larger 1/4" x 1 1/4" glass tube fuse can accommodate up to 40A.
Additionally, although the fuse can be smaller, the corresponding fuse holder maybe substantially bigger adding to the consideration.
Is the PCB surface mount or thru-hole?
This sounds like a "no brainer" but these days there are several different options for both surface mount fuses and thru-hole fuses.
With surface mounted fuses, there are multiple sizes available from a 6125 (6.1 x 2.5 mm) package all the way down to a 0603 (0.6 x 0.3 mm) package.
The thru-hole options are even greater with axial-leaded options available on all of our standard glass and ceramic fuses as well as a variety of leaded subminiature and micro fuses.
These same options are available in resettable fuses as well.
Additionally, the fuse mounting can also play a part in the designers’ decision especially if the fuse needs to be field-replaceable.
Look for part VI next Wednesday as we continue this technical series. As a reminder, the entire OptiFuse Fuse Selection Guide will be available in a downloadable .pdf format on the OptiFuse Website at the completion of this series (we don’t want to let people cheat and read ahead of the class).
Thank you once again for your continued support of OptiFuse where we know that education is a process not an event...