Unknown SMD component - help needed with identification

jpanhalt

Joined Jan 18, 2008
8,335
Welcome to AAC, Kacy.

I am assuming it is one of those electronic microscopes. Three-terminal devices can be a lot of things. If there is no crystal in the device, it may be a ceramic resonator to set the frequency for the MCU, but those often are X codes on the PCB. Since it is a U code, I am thinking an integrated circuit (1st choice) or transistor (often Q code, but could be U).

If integrated circuit, I would lean toward a voltage regulator for the entire board. I am assuming the board doesn't work. Can you identify U1 and U2? Are those two adjacent capacitors connected to the "now gone" pins? Which ones?
 

Thread Starter

Kacy Night

Joined May 13, 2019
12
It looks like it is connected with C20 and then giving out 3 signals or not as the lead skips it anyway in The middle
 

jpanhalt

Joined Jan 18, 2008
8,335
Here's a typical smd voltage regulator:

upload_2019-5-13_17-53-9.png

They do not all have a tab connected to pin 2 (common, aka ground, negative). The pins are numbered left-to-right as 1,2,3. Pins 1 and 3 will very often have a capacitor from the pin to ground. The pin itself is input voltage or output, regulated voltage. Do you have an ohmmeter to test the pads for what they connect to?

The problem is, if the VR blew, what caused it? Did you plug the device in with reversed voltage? Was there a short somewhere?
 

Thread Starter

Kacy Night

Joined May 13, 2019
12
The device fell and your answer seems very plausible. I cant check it right now with a ohm meter, will do that tomorow. More info coming in tomorow. As the battery in ohm meter died. Thank you for trying to help me, I really appreaciate it.
 

jpanhalt

Joined Jan 18, 2008
8,335
That's quite helpful. While the pinout I mentioned above is quite common for voltage regulators, an alternative where pin 1 = GND, pin 2 = regulated out, and pin 3 = unregulated input is also common. TI and Linear Technologies (now part of Analog Devices) used that pinout , e.g, LM1084 and LT 1584.

If it is a voltage regulator like I am discussing, pin 3 should go the the unregulated supply. We still need to consider outer devices too, like mosfets and other transistors. A power mosfet could easily self destruct with a short. I am assuming a surface mounted device with no tab and all 3 pins on the same side.

Have you cleaned up the PCB from whence it came? Is that the correct pin arrangement?
 

jpanhalt

Joined Jan 18, 2008
8,335
Now that is interesting. USB voltage is not well regulated, so that is consistent with it being a VR. Also, if your PCB is cleaned, then it appears pin 2 was also connected to a small heat sink tab (the silver stuff at the bottom)

If you can make some approximate measurements, it may be a SOT-89 package, like this:
upload_2019-5-14_12-34-47.png

That is a Microchip voltage regulator ( I am definitely not suggesting that is it, only that the configuration might match). Here's a link to the datasheet with dimensions for that package: http://ww1.microchip.com/downloads/en/DeviceDoc/MCP1700-Low-Quiescent-Current-LDO-20001826E.pdf
 

Thread Starter

Kacy Night

Joined May 13, 2019
12
The dimenssions are about the same as the SOT-89. Ive read the datasheet for it and one of the usage for it was usb cameras. So it would be about right. More ideas appreaciated but I think that SOT-89 is the answer. I will check now with what the third pin is connected. If i find something interesting i will post here.
 

Thread Starter

Kacy Night

Joined May 13, 2019
12
Okay so I connected the third pin with 3.3v from external source... and it worked but I f-ed up one of the soldering pads and a jumper wire was needed, anyway now i need only the component you written. Issue fixed. Thank you for help.
 

jpanhalt

Joined Jan 18, 2008
8,335
You beat me to it. Since the input was about 5 V, it seems logical that the output would be less. 3.3V is common, but even lower voltages are becoming more common. I was going to suggest checking the MCU part number and find its operating voltage. But, you have already given it the "smoke test" and it worked. There are several parts that have that pinout and package. IMHO, Microchip's are as good as any.

Now, on to your PCB. Can you provide a clear picture of the front and back there the pads are? My major concern is that with a multi-layer board, pulling a pad may affect more than just what you see on the surface. The good news is that package size is not particularly small by today's standards, and the board may be more easily fixed than you fear.

Before making matters worse, what type of soldering iron do you have? Is it temperature controlled? Small tip? Can you get and use leaded solder rather than lead-free solder?
 

Thread Starter

Kacy Night

Joined May 13, 2019
12
Im not home right now but the pad was ground and I solderd some wire to fit to the place where the old pad was. I was lucky enough that the pad was for ground and not for the 3.3v. I did the smoke test while holding wires some smoke came but it still works so again im very lucky. I got image after the smoke came so nothing importend burnt, the think that happend was i shorted pad 3 with 2 and bridged 5v into the controller, but still works or shorted 2 with 1 but then i tested the voltage on pin 2 and it was still 5v so no clue with that one, as long as it works ... I soldered the wire to the ground pin of usb seems to work. I use leaded solder but right now I use some questionable soldering station with tip as big as a finger. It has some temp control but ... the error margin might be 50c+/-. Im planning to buy new soldering station. I will show you pics of the jumper wire when i get home.
 

jpanhalt

Joined Jan 18, 2008
8,335
Hi Kacy,

I forget to mention that in addition to a smaller soldering iron tip (or maybe my fingers are just too fat), you need solder wick. That is a flux coated copper braid and sucks up excess solder from the pads. There are several brands, and some brands seem to work better than others. I use Chemtronics (https://www.digikey.com/products/en/soldering-desoldering-rework-products/desoldering-braid-wick-pumps/265?k=solder+wick&k=&pkeyword=solder+wick&sv=0&v=75&sf=0&FV=ffe00109&quantity=&ColumnSort=0&page=1&pageSize=25 )

I use #80-2-5 for almost everything. If you have a large area, it will still work. If you go much wider (say 4X) , you require more heat to get both the braid and pad to soldering temp. That can be a disadvantage. Narrower, such as 80-1-5 uses a lot of braid. The final digit is the amount in the package in yards/meters.

As for your soldering iron, for SMD, most people will recommend a temperature controlled iron with a tip that is either conical or chisel shaped. Each type has its advantages. I use a short conical tip (cone about 0.3" long) for almost everything. Years before I got my current soldering set, I used a 25W Weller fixed temperature iron with a 1/8" wide chisel tip. I still use it after >>40 years. It would also work for this device, but I would not recommend buying a fixed temperature iron like that if you have a choice.
 
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