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.

[kiev]

Moving back over here from some discussion about failure mode of the OBC (snubber caps or fuse first?) on the troubleshooting and repair thread page 47,

i ran a simulation with a nominal 10 Amps flowing thru a 40R load until a fuse blew, then opened the switch to turn off the "Boost" and let the energy ring out from the coils and caps. The scope time-history displays the peak voltage at the 1nF snubber cap when the fuse blows on the left side, and the latest time step value on the right.





[EDIT: added more circuit details and repeated the sim, peak voltage across snubber is a bit higher]

[DBMandrake]

2.4kV ?

So the original 2kV caps would probably suffer a failure while my 6kV caps would survive in this scenario ?

[kiev]

Here are the I-t fuse curves for the MCU and OBC 20A devices. The MCU version is physically larger.

[kiev]

2.4kV ?

So the original 2kV caps would probably suffer a failure while my 6kV caps would survive in this scenario ?

If they could survive a 354 kV impulse--the peak of the spike.

i haven't measured the miev coil inductances but the laef coils were about 200 mH. Any simulation, of course is GIGO, so this is just trying to illustrate how if a fuse lets go, then it generates a spike that could take out the snubbers.

The scope chart has the peak on the left side; at the right side is the data for the last time step when it was stopped. Sorry for any confusion.

[Don]

I'd be up for preemptively replacing the fuse(s) if we can come up with a group buy on a suitable fuse. My fuses are probably less stressed than most as our cars have 30K and 35K on them and have almost never been charged 'full tilt' on L2 as I've set my EVSE's to a max of 12 amps and probably 50% of the time or more, I use L1 at either 8 or 12 amps - I have 4 EVSE's plugged in all the time, so choosing how fast I need to charge is quick and easy . . . . just plug in with the applicable one. One EVSE is set to 16 amps L2, but I never use that one on either of the iMiEV's

I do kinda like the idea of mounting a pair of clips on the tabs removed from the old fuse so a clip in replacement can be used - I'd imagine there is a much greater assortment of clip in fuses than sourcing only those with tabs on them, right?

Don

[kiev]

i ran the Paul Falstad (Physics site) sim with a switch on the snubber so i could short it out or go open circuit. i let the fuse get to 50% melted before hitting the switch, then opened the Boost switch and stopped the scope.

Opening the Cap: i also ran this case a long sim time (1.1 sec at 20usec steps) until the fuse cooled back down to 50% melt



Shorting the Cap: also ran the fuse cool down sim for this case (391msec at 20usec)



Accidentally double clicked and bounced the switch during a Shorting run which produced some interesting results: