Raspberry Pi Fuse
The main fuse is located at the top right hand corner of the board near the SD Memory card slot. It is the green coloured SMD (Surface Mounted Device) component.
This is a Polymeric PTC (Positive Temperature Coefficient) fuse which increases it's 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 it's resistance when a current larger than 0.75 A flows through it and will become fully open at 1.1 A.
These fuses are designed to reset back to normal operating mode within a finite time period. 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.
As an Electronic Engineer, back in the prehistoric days, I remember using a freezing spray to cool such a fuse down, however it rarely had any effect, as the fuse is made of a material which changes its chemical structure, and reverts back to its original structure in its own time.
Whilst Polyfuses are fairly reliable, they can sometimes be damaged permanently if the overload was large enough and lasted for a long enough duration. In such a case the fuse will exhibit a small resistance which may cause a slight voltage drop to the Raspberry Pi. And in some cases this condition can impact upon the proper operation of the Raspberry Pi board.
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 the 5 V 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 is also directly fed to the Raspberry PI USB ports to power the keyboard and mouse that may be connected to it. Hence a slight drop in voltage can have an impact on those devices as well, such as not being recognised at boot time.
If the fuse is permanently damaged and exhibits a small voltage drop, it may be necessary to replace it with the proper correct replacement. It is highly unprofessional (not clever) to bypass a fuse especially since 1. These fuses are readily and cheaply available, and 2. It's a safety device that protects the power source.
There are also 'cowboy' solutions such as soldering a small resistor across the fuse to compensate for the increase in resistance - two resistors in parallel form a lower effective resistance. Again a very unprofessional solution and highly not recommended.
SMD Fuses are available from many online electronics shops. They can be easily purchased and fitted to the Raspberry Pi board. It's a simple question of desoldering the blown fuse and replacing with a new one. When soldering a new fuse use the lowest possible temperature of the soldering iron, and the shortest duration of heat applied, since excessive heat can damage the fuse.
On the Raspberry Pi Board the fuse is marked T075. Schematic number F3. According to the European fuse markings T stands for träge which is German for slowly reacting characteristics. 075 denotes 0.75 A, which is the rating of the fuse.
A simple digital multimeter is all that is needed for diagnostic purposes. Some of the most affordable digital meters can be bought on eBay for as little as £3.00 and ideal for students.
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Set the digital multimeter to check for DC voltage. With the negative (black) probe of the digital multimeter connected to the ground use the positive (red) probe to check the voltage at either ends of the fuse as shown above. You should see 5 V at both ends of the fuse.
All voltages are measured 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.
The designers have also provided Test Points TP1 and TP2 on the PCB for measuring voltages. If you look at the article Raspberry PI Power Supply Minimum Voltage you will see a diagram of where the test points are located. TP2 is ground.
If there is 5 V at only one end of the fuse but not the other then consider the fuse is open and therefore disconnect the power supply and remove the overload condition to the board, and leave it for some time to see if the fuse resets itself.
If there is 5 V at one end but a significantly lower voltage such as 3 to 4.5 V at the other end, then leave the board un-powered for 24 hours. If it's still the same the next day then consider changing 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 ends of the fuse consider checking the power supply adapter. Is there 5 V coming out of the adapter?
Author: Peter J. Vis