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August 5, 2009
Oreo Cookies

What do Capacitors, MOVs (Metal Oxide Varistor), and Resettable fuses all have in common?  The answer is that they are all constructed like an Oreo cookie...

Imagine if you will...the black cookie part of the Oreo as metal disks.  Each of the components above are essentially made with the same black cookie.  The only real difference in whether or not you have a capacitor, MOV or resettable fuse what the "white stuff" is made of.

In the case of a capacitor...the "white stuff" is a very high resistive material (like a polyester or a ceramic material).  Because of the insulator "stuffing", no current can pass through a capacitor so the capacitor acts like a temporary storage place for electrons...charging and discharging up to billions of times each second...

In the case of the MOV, the "stuffing" is an insulator type material (like the capacitor above).  The difference is that some metal oxide material is added to the ceramic material to create a semi-conductor material.  No current passes through the semi-conductor material unless a high voltage is there to help "push" the electrons through the semi-conductor "stuffing". 

MOVs are used to protect circuits from over-voltage situations (such as voltage spikes and surges).  The MOV really doesn’t do anything in the circuit during "normal" operation BUT when high voltage spike or surge occurs, the MOV "turns on" to protect the other circuit components.

So this brings us to resettable fuses...

As we learned above, the capacitors’ "stuffing" is an insulator material and the MOVs’ "stuffing" is a semi-conductive material.  The "stuffing" for a resettable fuse is a highly conductive polymer material.  This means that current flows freely from one metal plate (chocolate cookie) through the polymer material (white stuffing) to the other metal plate (chocolate cookie) during normal operation.

What is special about the resettable fuses’ "stuffing" is that this material is heat sensitive.  As the heat rises, the conductivity of the material is significantly reduced.  When an over-current occurs in a circuit (due to a short circuit or overload), heat is generated (this heat is what melts the fuse link in a non-resettable fuse).  The heat from the overcurrent quickly heats the polymer material ("stuffing") in the resettable fuse which then quickly stops conducting current.  The net result is that the current can no longer pass through the resettable fuse effectively opening the circuit and protecting the downstream circuit components.

After a short while, the resettable fuse cools downs and re-closes itself allowing the equipment to operate again(remember the stereo speaker example in yesterday’s blog).

As Paul Harvey used to say..."Now you know the rest of the story..."

Please use the following links to find more information of OptiFuse’s complete line of resettable fuses:

OptiFuse Resettable Fuses

OptiFuse Resettable Cross Reference Guide

Now I bet you’ll never eat another Oreo Cookie without thinking about Capacitors, MOVs and/or Resettable Fuses...

Thank you so much very much for all your support of OptiFuse where we believe an informed customer is an asset not a liability.

Jim Kalb

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