0
\$\begingroup\$

I am trying to understand the spec of a Datasheet of a thermal fuse. for example the one below :

https://nl.farnell.com/panasonic-electronic-components/eyp2bn134/fuse-thermal-cut-off-2a/dp/1832031

from the datasheet: http://www.farnell.com/datasheets/2244108.pdf

I see the temperature at which it fuses. But there is also a rated current.

my initial understanding was that if the current through the fuse is higher than this rated current , the fuse would blow.So basically, my reasoning was that the thermal fuse can then also be used as electrical fuse.

However, I did an experiment with my PSU where I was passing 2.5A through a similar fuse, but it still did not fail. The fuse in the test was rated 1A.

so my question is :

what is this rated current provided in the datasheet?

\$\endgroup\$
  • 3
    \$\begingroup\$ You cannot use a thermal fuse as a reliable electrical fuse. You need an electrical fuse in series. \$\endgroup\$ – Spehro Pefhany Sep 19 '18 at 12:38
1
\$\begingroup\$

A thermal fuse is like a mechanical switch that is spring-loaded to stay in the open position. To make the fuse, they push the contacts together and fill the cavity with something like wax (this is a basic analogy, might not be 100% accurate). When the wax melts, the thermal fuse opens. Prior to opening, the resistance is VERY low compared to an over-current fuse. Running current through the thermal fuse doesn't really cause it to get very hot, so it is not a reliable way to cause it to blow.

The current rating is the largest current you are allowed to run through the fuse and still expect it to work correctly (if it gets hot). The current rating has very little to do with triggering the thermal fuse to blow. If you exceed the current rating, you may have trouble meeting safety certifications (if they are required). Also, if you exceed the current rating, the fuse may open prematurely. If you exceed it by a lot, it may fail in an undesirable way (may explode or arc-over when it opens).

Sometimes it may be OK to exceed ratings of a component if you are able to do sufficient testing to justify it. That testing would involve building samples and testing them by deliberately introducing a fault to see what happens. If that doesn't work for your schedule or budget, then it might be better to just use components within their ratings.

\$\endgroup\$
1
\$\begingroup\$

That's the current at which it will perform as rated, e.g., it's guaranteed not to open below its rated temperature as long as the current is below this value. An electrical fuse is essentially a thermal fuse with a well-established resistance and dissipation profile, and the current at which it fuses is still surprisingly uncontrolled. The resistance and dissipation profile of a thermal fuse aren't as important, so they're less controlled; if the fuse will work as specified with two or three times the rated current, that's just a bonus.

\$\endgroup\$
  • \$\begingroup\$ I think this answer is misleading in the sense that thermal fuses and over-current fuses are constructed very differently and have very different properties. The dissipation in a thermal fuse is kept as low as possible. The dissipation in an over-current fuse is engineered to generate enough heat to melt the resistive element of the fuse. They are both controlled, but the thermal fuse ideally has very low resistance. \$\endgroup\$ – mkeith Sep 20 '18 at 3:51
0
\$\begingroup\$

An electrical fuse has is often a small diameter wire where as more current runs through it, the temperature of the wire increases until it melts.

A TCO must deal with both the ambient temperature and the heat generated by the current flowing through it.

The rating is based upon the limiting of the heat generated by the current.
It takes much more current than its rated current to melt the thermal material.

If you exceed the rated amperage, the heat generated by the excess current will cause the TCO to blow at a lower temperature.

NOTE: This series of TCO is obsolete.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.