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**kikkoman** · 07-05-2012, 05:44 AM

Re: Help ID a sot-23 device

I'd say since they label diodes as D and ICs as U, you probably have got a transistor there. could be a FET too but usually you see BJTs for gen. purposes.
If you can tell if it's on the high or low side (any of its pins @ ground or at Vcc?)
this is the basic idea: http://ikalogic.cluster006.ovh.net/w...ads/swtch1.jpg

hope it helps.

**theagent** · 07-06-2012, 01:34 PM

Re: Help ID a sot-23 device

Thanks for the reply. I'm not too experienced... can you tell me how I would test that? From the base to ground I get 23ohms, from base to vcc 0.9ohms, and from vcc to ground I get 23ohms as well. Setting my multimeter to the diode setting I get 0.001V base to ground, and 0.020V base to vcc, and 0.020V with vcc to ground. I get this result regardless of the orientation of the probes (pos to base or neg to base). And it beeps continually, as if it's shorted (and it probably is).

**budm** · 07-06-2012, 02:05 PM

Re: Help ID a sot-23 device

That transistor can be 2SA1XXX (Japanese SEMICONDUCTOR P/N system), which will be PNP type. On your pictures of the transistor, the left pin will be base, the middle pin (stick up to the top) is Collector, the pin to the right is Emitter.
Can you take the DC voltage reading with black probe of the meter to the Ground (right terminal of CA9 as shown in your picture), the red probe to each pin of the transistor, report what you read on each pin, unit must be powered up. By the way, your transistor is shorted out, it may be uesed as a switch to supply the votage to the circuits, when the TANTALUM failed (Tantalum CAP usually fail in shorted mode, when it failed it shorted out) shorted, so it take out the transistor with it isne the transistor will try to feed the voltage into shorted out circuits.

**theagent** · 07-06-2012, 03:13 PM

Re: Help ID a sot-23 device

Ok, just soldered it back on and took the readings...
Looks like they're fairly similar at each pin.
~3.580V on the base, ~3.650V emitter, and ~3.555V collector. Thanks.

**budm** · 07-06-2012, 06:10 PM

Re: Help ID a sot-23 device

I mean to test the voltage with it removed, since it was shorted out and you had already remove it to take the picture, I just need to find out the voltage at each pads without the transistor.

**theagent** · 07-06-2012, 06:45 PM

Re: Help ID a sot-23 device

Oops... ok, desoldered it and tested the pads.
I have ~4.056V for the Base (left), ~4.253 for the emitter (right), and 0V for the Collector (middle/top). Of course the unit wouldn't power on without the transistor in place, but the batteries were inserted, so there was voltage to it.

**budm** · 07-06-2012, 07:04 PM

Re: Help ID a sot-23 device

OK, it is an PNP transistor since B+ is fed into the Emitter, so you just have to get PNP transistor with Vce of about 50V, Ic of about 500mA (or 300) SOT-23 footprint. Do you have any scrap board with SMD components? They just use this transistor as a switch, since it has shorted Emitter to Collector then you can not turn it off.
One more thing to verify is that the Collector is connected to that Tantalum cap or not, it should be if I am correct.

**theagent** · 07-06-2012, 07:10 PM

Re: Help ID a sot-23 device

Ok, great! Thank you for the help, I really appreciate it. I do have scrap boards. I will have to go through them and see if I can find one that will work. I will report back Thanks again.

**budm** · 07-06-2012, 07:11 PM

Re: Help ID a sot-23 device

Please read my edit post on the last statement.

**theagent** · 07-06-2012, 07:19 PM

Re: Help ID a sot-23 device

Seems to be connected yes. The left side of EC3 is connected to the collector with 640ohm between them. No connection to any pins of the transistor from the right side of the cap.

**budm** · 07-06-2012, 07:28 PM

Re: Help ID a sot-23 device

Hmm, I expect it to be direct connection from the Collector to the cap since it is supplying the voltage to the cap, if the 640 Ohm is in SERIES with the cap leg and the Collector, that does not make sense.

**theagent** · 07-06-2012, 07:42 PM

Re: Help ID a sot-23 device

It's actually behaving very oddly. Setting the meter to ohms, it is now jumping up to about 20Mohms, then quickly falling to about 500kOhms then 0L, then jumps right back up in the megaohms, and repeats. That's with the probes on the right side of the cap, and on the collector.

Here's the breakdown.

Meter set to Ohms.
Probe on right side of EC3
Probe on Collector
Jumps to 20-23Mohms, then falls quickly to 500Kohms, then 0L and repeats

Probe on right side of EC3
Probe on base
1.36Mohms, then slowly falls

Probe on right side of EC3
Probe on emitter
rises until it reaches 6.23Kohms... then stops.

So, looks like the cap is charging, then draining.

Probe on LEFT side of EC3
Probe on collector
1.452Kohm

Probe on LEFT side of EC3
Probe on base
Climbs from ~500ish Kohms

Probe on LEFT side of EC3
Probe on emitter
Climbs until it reaches 6.153kOhms then stops.

**budm** · 07-06-2012, 08:43 PM

Re: Help ID a sot-23 device

OK, I look at the picture, the left side of the EC3 is connected to the Ground plain, the right side of EC3 is connected to U1 (Regulator) middle pin, so EC3 is not fed by the transistor.
The Collector looks like it is connected to the Anode of D8, and also to the top pin of U8.
So I wonder now if U1 is OK. You may want to check the left pin and the middle pin (as oriented as shown in the picture) to see what voltage you are getting from left pin to ground and middle pin to ground.

**theagent** · 07-06-2012, 09:14 PM

Re: Help ID a sot-23 device

Thank you for taking the time. I'm getting a steady 0.103V from all 4 pins of U1. Do you still think Q3 is a PNP transistor? I believe I found one I can try... it's 45V 500mA bipolar... do you believe that would work?

**theagent** · 07-06-2012, 09:32 PM

Re: Help ID a sot-23 device

Well, I went ahead and tried the PNP transistor. It didn't work. No lights, nada. The only other time the wheel lit up was when I used the SI2301 mosfet, but like I said, only for a millisecond.

**budm** · 07-06-2012, 10:20 PM

Re: Help ID a sot-23 device

so the transistor has shorted between Emitter and Collector, so when you put it in, the unit comes on but cannot turn off, which make sense, but when you put in the PNP, it does not turn on, so it may mean that the transistor is not being bias on. So you need to put red probe on the Emitter and the black probe on the Base, it should show 0.6V if it is being turn on.

**theagent** · 07-06-2012, 10:35 PM

Re: Help ID a sot-23 device

With the red probe on Emitter and black on Base, it's showing 0V unfortunately.

**budm** · 07-06-2012, 10:47 PM

Re: Help ID a sot-23 device

Ok, it is being told to turn on.
I look real close at the picture again, I see the two pads labeled with 'BATTER' and one of pad's trace leads directly to Emitter of Q3, it is also connected to the bottom leg of R1 but I cannot tell if the top leg is connected to, C1 is also connected to that trace. I can also see right side of R2 is connected to the Base. Am I corrected? It sure looks like this Q3 is a switched transistor.
I wish the picture is better focused and brighter.

**theagent** · 07-07-2012, 04:59 PM

Re: Help ID a sot-23 device

I traced out what I thought was important... would this help? https://www.dropbox.com/s/535l3gv1b9...oardtraces.jpg

It's the battery that's connected... the line that says +5v is for the power adapter, which isn't connected.

I also ran the board through Adobe Lightroom and brightened it up quite a bit. https://www.dropbox.com/s/6qqhxomi93...oardBRIGHT.jpg

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Help ID a sot-23 device

Help ID a sot-23 device

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