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Posts: 135
Joined: Mon Apr 23, 2001 7:33 pm

Electrical System Components

Mon Jan 20, 2003 2:27 am

Hi folks have'nt been around for a while but now I am back, my lap top froze and I had to ship it outof town to have it fixed.
I got one question for the tech wizards outthere: flipping through the pages of my Learjet POH I read that DC powered gizmos are protected through a thermal fuse, while AC ones use a magnetic fuse.
While I have an idea how the thermal fuse works, it basically consists of a thin strip of bismuth or tin if I am not wrong, that melts because of the Joule effect if the current flowing through it excceds its threshold, I don't have any clue about the magnetic fuses, if you are allowed to reset them back and after how long once they have popped.
Besides if you can provide a bit of information about how a current limiter works in addition to the fact that "it is a slow burning fuse", I am quoting the POH, about how it works and what it is made up of, i would really appreciate that.
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

RE: Electrical System Components

Mon Jan 20, 2003 3:53 am

You have to be careful not to lump all "thermal" devices into one category.

The simplest form (the fuse) is pretty much how you described it.

Thermal "circuit breakers" typically use a bimetal element to release a spring loaded trip mechanism. There aren't any parts that are consumed during the trip, unlike thermal "fuses" so you can reset them. However. after several cycles, the thermal circuit breaker will wear out or after years of no use, the contacts can "weld" themselves together.

A magnetic circuit breaker uses the magnetic field formed by the current flow to overcome the field of a permanent magnet inside the device that guards the spring loaded trip mechanism. This type of device is much more precise than a thermal circuit breaker, and doesn't have the delay inherent in these devices. I don't know of any magnetic "fuse" devices that imply a one-time use device.

With respect to the Lear Jet (and many other airplanes) the passive current limiter (ANL type devices) don't actively limit current. They merely remove current when their threshold is exceeded. The use of the "current limiter" term isn't really appropriate, but they all use it! So don't expect the thing to hold the current at 100 Amps (or whatever). It will just blow.

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