Problem Occured Due to Printer Crash

[Jaiwanti]

hello

I am having a serious problem with my usb connectivity.
Few days back my scanner got felled of the table and landed
on its backside. It is not working, so I thought I must have damaged
it, even though there are volts present on all the USB connector pins.

I also tryied another USB device, the power LED would show for a
second and then go out.

I then thought that i have blown a protective fuse/fusible resistor in the
USB 5V supply most likely. Probably gone high resistance and could
charge a capacitor on the mobo enough just to get that show on the
power LED.

I'm good with soldering and electronics generally but I don't know how
to locate the part. Does anyone have any clues ?

Please help me regarding this issue

[Bon-Z]

A polymeric positive temperature coefficient device (PPTC, commonly known as a resettable fuse) is a passive electronic component used to protect against overcurrent faults in electronic circuits. They are actually non-linear thermistors, however, and cycle back to a conductive state after the current is removed, acting more like circuit breakers, allowing the circuit to function again without opening the chassis or replacing anything. These devices are often used in computer power supplies, largely due to the PC 97 standard (which recommends a sealed PC that the user never has to open), and in aerospace/nuclear applications where replacement is difficult.

These devices are sold by different companies under various trademarks, including PolySwitch (Tyco Electronics), Everfuse (Polytronics), Polyfuse (Littelfuse) and Multifuse (Bourns).

A PPTC device has a current rating. When the current flowing through the device, (which has a small resistance in the on state) exceeds the current limit, the PPTC device warms up above a threshold temperature and the electrical resistance of the PPTC device suddenly increases several orders of magnitude to a "tripped" state where the resistance will typically be hundreds or thousands of ohms, greatly reducing the current. The rated trip current can be anywhere from 20 mA to 100 A.

A polymeric PTC device comprises a non-conductive crystalline organic polymer matrix that is loaded with carbon black particles to make it conductive. While cool, the polymer is in a crystalline state, with the carbon forced into the regions between crystals, forming many conductive chains. Since it is conductive (the "initial resistance"), it will pass a given amount of current, called the "hold current". If too much current is passed through the device, the "trip current", the device will begin to heat. As the device heats the polymer will expand, change from a crystalline state into an amorphous state. The expansion separates the carbon particles and breaks the conductive pathways, causing the resistance of the device to increase. This will cause the device to heat faster and expand more, further raising the resistance. This increase in resistance is sufficient to substantially reduce the current in the circuit. A small amount of current will still flow through the device and is sufficient to maintain the temperature of the device and keep it at the high resistance level ("latching" functionality).

When the power and fault are removed, the PPTC device will cool. As the device cools, it contracts to its original shape and returns to a low resistance level where it can hold the current as specified for the device. This cooling usually takes a few seconds, though a tripped device will retain a slightly higher resistance for hours, slowly approaching the initial resistance value.

Since a PPTC device has an inherently higher resistance than a metallic fuse or circuit breaker at ambient temperature, it may be difficult or impossible to use in circuits that cannot tolerate significant reductions in operating voltage, forcing the engineer to choose the latter in a design.

[Gonzo]

look for a Polyfuse.

They're typically green in color, with a "dimple" on
either end of them. I see three in a picture of the KT7A,
up near the I/O connectors. They can be used to protect
PS/2 mouse, PS/2 keyboard, and USB devices

[Pugsly]

Don't reach for the soldering iron just yet.
Use a multimeter, and check to see whether
+5V is available on both sides of the fuse or
not. Both before and after plugging in a USB
item.

A Polyfuse resets itself, after the overload
is removed.

[Regina]

Littelfuse PTC (resettable fuse) technology utilizes a unique Lead-Free polymer-based, Positive Temperature Coefficient (PTC) material to protect electrical circuits from overcurrent conditions. During normal operation, numerous carbon paths within the polymer allow the device to conduct electricity. As current through the PTC (resettable fuse) reaches its rated threshold, the polymer material begins to heat causing the polymer to expand. The resulting expansion breaks the carbon chains to reduce the current through the circuit to a small leakage current. The increased resistance in the resettable fuse works to protect circuitry by limiting potentially damaging current. When the fault (overcurrent) is removed from the circuit, the PTC (resettable fuse) will reset itself thereby allowing current to flow through the circuit again