The main fuse is located near the SD memory card socket. It is a green coloured surface-mount device (SMD). The fuse is marked “T075”, schematic number F3. “T” indicates slow acting characteristics, and 075 denotes 0.75 A.
This is a Polymeric PTC (Positive Temperature Coefficient) fuse, which increases its resistance in over-current situations. In this case the Raspberry Pi (Board B) fuse is rated at 0.75 A, and will begin to increase its resistance when a current larger than 0.75 A flows through it, becoming fully open at 1.1 A.
These fuses are self-resetting and usually return to normal operating parameters within a finite time. The time to reset back to normal operating mode can take anywhere between minutes to hours and in some cases days. It depends upon the level of overload current that passed through it and the duration.
Damaged PTC Fuse
Whilst a Polymeric PTC fuse is reliable, it is possible to damage it permanently with a large overload current of long duration. In such a case, the fuse will exhibit a permanent resistance, which may cause a slight voltage drop. Moreover, in some cases this condition can affect the proper operation of the Raspberry Pi board and devices connected to its USB socket.
The Raspberry Pi is very sensitive to its input voltage. Ideally, it requires 5.1 V, as the board contains various DC-DC converter circuits and voltage regulators that convert this to lower CMOS voltage levels. Hence, a voltage drop of just 0.5 V can have an impact on the proper operation of the circuitry. In addition, the 5 V at the input directly feeds the USB ports to power the keyboard and mouse connected to it. Hence, a slight drop in voltage can have an impact on those devices as well, preventing them from initialising at boot time.
Cowboy Methods to Repair a Fuse
There are also “cowboy” solutions such as soldering a small resistor across the fuse to compensate for the increase in resistance, because resistors in parallel form a lower effective resistance. I received this advice from Gordon who is a British hobbyist; unfortunately, these types of solutions are very unprofessional and highly not recommended. It is highly unprofessional (not clever) to bypass a fuse especially since 1. These fuses are readily and cheaply available, and 2. This is a critical safety device.
In the prehistoric days, my friend Rufus… used a freezing spray to cool such a fuse, however, it rarely had any effect, as the fuse is made of a material, which changes its chemical structure, and reverts to its original structure in its own time.
As can be seen from the fuse marking “T075”, this fuse will start to exhibit a resistance when the current passing through it is 0.75 A. If you have power hungry circuits taking power from the same point, it could pose a problem; therefore, you have to be mindful of this.
Using a Correct Replacement
Replace the damaged fuse with the correct replacement. You will have to learn about SMD fuse markings, how to select them, and correct symbols. Compatible replacement fuses are available from many online electronics shops.
Repair is a simple matter of removing the damaged fuse and replacing it with a new one. When soldering a new fuse, use the lowest temperature setting of the soldering iron, and apply the shortest duration of heat, since excessive heat can damage the fuse.
A simple digital meter for diagnostic purposes will go a long way to figure out what has gone wrong. Some of the most affordable digital meters are on eBay and start from as little as three pounds. If you are looking to buy your first digital meter that will last you well into your adult career then Fluke make some of the best quality brand meters.
All voltage measurements are with respect to ground, which is at zero potential. On the Raspberry Pi there are many such points. The official ground rail is pin 6 on the GPIO header socket.
Set the digital meter to check for DC voltage. With the negative (black) probe connected to the voltage ground, use the positive (red) probe to measure the voltages.
Diagnosing F3 PTC Fuse
As you can see, the fuse has a pair of soldered terminals. Using the red probe of your meter, you need to measure the voltages at those terminals.
If you measure 5 V at only one terminal of the fuse but nothing at the other terminal, then consider the fuse to be open. There will usually be a short circuit on the board, Therefore, disconnect the power supply and remove the overload condition. You will need to give the fuse some time to reset itself.
If there is 5 V at one terminal but a significantly lower voltage such as 3 V or 4.5 V at the other terminal, then leave the board un-powered for 24 hours. If the condition remains the same the next day, then it might be a good idea to replace the fuse. However, note that you will void your guarantee if you solder on the board. If your board is under guarantee, you should consider getting support from the seller.
If there is no 5 V present at either terminals of the fuse, then consider checking the power supply adapter, and the Micro USB B socket. Is there 5 V coming out of the adapter and entering the board?
This Article Continues...Raspberry Pi Dead
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