DMMT5401 Component U14 – Dead Pi B+ (V1.2)

In this final part of the article to repair a dead Raspberry Pi B+ (V1.2), we focus our attention on the component U14 responsible for producing a current sensing mirror.

U14 is DMMT5401, which consists of a pair of matched PNP bipolar transistors. These are small signal transistors with a maximum collector rating (Ic) of -200 mA and they form the basis of a bipolar junction transistor (bjt) current sensing mirror. A current mirror circuit usually generates a current at the output, which is directly proportional to the current at the input. In this two-transistor circuit, one transistor generates a constant reference current established by the input, and the second transistor follows this reference to maintain its constant output current. This has the advantage of producing a constant current source, and additionally tracks the input currents. As you can see, this part of the circuit also controls the gate biasing of the MOSFET.

The precise matching of the transistors in the U14 package is extremely critical, as this is the only way a current sensing mirror can work. Damage to either one or both transistors would result in your Raspberry Pi appearing deceased. Unfortunately, fully shorted or partially damaged transistors in the U14 package may also interfere with the operation of the MOSFET; therefore, you may have to remove the MOSFET first in order to determine whether it is at fault, and then continue with measuring voltages on the U14 pins.

With the MOSFET removed, you can test it first, and if it appears fine, then try measuring voltages on the pins of the U14 package. There should be +5 V at pin 6. Since the PCB trace joins pin 1 and pin 2, you may have to remove the U14 package and check each transistor individually. If you have never checked a transistor before, then one simple method is to use your digital meter as they usually have a transistor check function, however, you may need to solder some temporary extension leads to the pins.

Logic and Information

As with everything in life, you have to collect the best information you can, before making a logical deduction. I hope that you will already have the logic, including precision instruments to collect the information, and the skill to carry out the repairs.

I hope that somewhere in this article you found the information you needed to repair your Pi B+ and put an end to your Pi woes. If it saved you some money then consider donating a little something to the USHMM.

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