i am trying to repair a laser driver circuit.Apparently, it was connected to a lab power supply (the driver is rated 4-5,5V, 8A) and someone turned the knob, so it got way higher voltage.
I identified a 3,3v Linear Regulator (smd-part in the center of first pic) that looks a bit burned on one Pin, but have a hard time to identify the small (diode-like) part below it. It is marked like this (the ST being the ST logo):
ST EBBGC623It has no polarity marking and seems to be in series with a capacitor, which i find a bit weird. Might be some kind of protection diode? Can anyone help me to identify it?
I hope the linear regulator failed due to overheating and did not destroy a lot more of the circuit.
The capacitor seems to be a KEMET Tantalum capacitor. It is marked
4776K333From what i have found that means 6V, 333 is the date of manufacture, and 477 the "picofarad code".
Does that mean 47 * 10^7 pF = 470uF? That seems quite large. Or is it 4,7 * 10^7 = 47uF?Also, i always thought you should oversize the voltage rating of these tantal capacitors considerably, so 6V seems to be a bit tight with 4-5.5V input. Or am i misinterpreting the markings somehow? The 4 other caps seem to be 10V, and so maybe the 6V one is actually the output capacitor of the regulator, so with it seeing 3,3V a 6V cap makes a bit more sense.
Pictures:
Better image:
And the Back side:
The regulator and burned diode is exactly on the opposite side of the big inductor, so it is hard to follow the traces, and most are actually in the middle layer of the PCB.However, after consulting the Datasheets it seems like the regulator would be very resilient to overvoltage and has internal overheating protection. VMax is stated as 26V and i am quite sure that was not exceeded. So i suspect only the cap and the diode thing blew. Does anyone know the expected failure mode of these Tantal Caps? It does not even look that burned, or at least only the surface. It did apparently get hot enough to melt the solder a bit since it is no longer flat, maybe that heat was coming from the "diode-part" though and the cap survived.They are really expensive to replace sadly.
It is a bit hard to be sure since it is a 3-layer board, but i suspect the diode might be actually in series to the input Voltage and might be some protection diode that shorted due to the overvoltage, and burned up until the polyfuse shut down