MCU 20A 450VDC fuse investigation

[kiev]

i was hoping to obtain several for comparison, e.g. 20,30,40A rated, in the ZXISO style, but only have an OEM PEC 20 and a Littlefuse 30 at this point due to lack of stock in the US distributors.

On the PEC drawing there is a call-out for item 7, a small dot or blob of a tin alloy (solder?), which can be seen below. It is also seen on the Littlefuse version and in the vapor trail of the blown fuse, so this must be some secret sauce for making fuses.

Maybe it adds some thermal mass to extend or delay the breaking current, or it adds a concentrated mass point to control where the fusing will start (like a score line on a piece of glass to control where it will break when stressed), or ...?

Top view, L to R: 30A, 0.075" wide reference, new 20A, blown 20A



Side view, same order as above except reference is 0.036" width



rotation to get good side view of 30A fuse element

[WReed82]

Kenny,

How did you get those very cool photos?!

One guess on the alloy blob is something that would heat up, release smoke, and
discolor the inside of the fuse so it is obvious the fuse is blown.
Alternately, if it has a relatively low melt point, it could tell a tech that the
fuse has been near its thermal limits, but didn't actually exceed it.

Thanks and good health, Weogo

[coulomb]

On the PEC drawing there is a call-out for item 7, a small dot or blob of a tin alloy (solder?), which can be seen below. ... so this must be some secret sauce for making fuses.

From the end of this page:

"Another slow blow fuse is the "M" type (named for the inventor). This has a small blob of dissimilar metal "soldered" onto the fuse wire. As the "blob" heats up and melts it diffuses (blends or alloys) with the main wire to change composition and melting temperature of the wire, at which time the wire melts and breaks the circuit. This action provides a time delay function at slightly excess current, while the main wire will blow instantly at significantly higher excess current."



But that's strange, since this fuse seems to blow quickly compared to other fuses that are in series with it.

[kiev]

That's a good finding on that reference and the history of the thermal blob.

i've been looking at the Laef OBC and it appears that they use a 420VDC 30A SHV27 fuse on the Positive side output. This is classed as an inrush-withstanding type of fuse. The 2012 FSM wiring diagram for the VBC shows it as a 25A fuse? This same type is used in the Mits OBC output, but is only rated 20A.

Another note on the LOBC, there are no snubber caps across the POA and NOA terminals such as is found in the mits, even though they have the same pinout of the waffle plate.