Help ID a sot-23 device

[budm]

Re: Help ID a sot-23 device

Something does not look right on the redlines is that U8 and U1 output pin (center pin) are tied together, which does not look right unless they are not voltage regulator IC's. You may want to find out what they are. And since the EC3 is burnt up so bad, I would think U1 AND U8 will be shorted out.
On the Q3 transistor, the Collector output is connected to the TANTALUM and the CERAMIC cap (they are connected in parallel), where else does it go to, it has to power something.
The junction of R1 and R2 must be going some where that will control the Q3 to turn it on and off.

[theagent]

Re: Help ID a sot-23 device

The resistors tied to Q3 go to one of the IC chips covered in epoxy.

U8 is a DC-DC step-up converter
and
U1 is a voltage regulator. http://www.datasheetarchive.com/6206...datasheet.html. The datasheet corroborates the fact the the center pins are connected.

I believe they're both ok.

[budm]

Re: Help ID a sot-23 device

So that junction of the resistors that go to the IC, that output of the IC has to go low to turn on the PNP Transistor. That means the IC is running off the batteries at low current so it not drain the batteries and it sends out the signal to turn on Q3 to supply the power to the rest of the circuits when the soft switch (power ON/OFF) is activated.
This is what I think what happen:
The EC3 cap shorted and then burnt up, it is fed by the output of the step-up regulator.
Q3 burnt up due to over current draw due to shorted load, EC3 shorted. I ma surprised that the step-up regulator did not get damaged.

[theagent]

Re: Help ID a sot-23 device

That makes sense. But the PNP transistor didn't work. Is it possible that it's a PFet? I read this on a different forum:

A transistor is switched on and off by a small current flow into (NPN) or out of (PNP) the base lead. The base lead behaves like a diode while this current flows and the voltage will never go much over 0.7 volts difference from the emitter. This current must be sufficient to "saturate" the transistor. This means the base current multiplied by the Hfe (gain) of the transistor must be greater than the collector current. So a transistor can be switched on by a low voltage with respect to the emitter, but a non-zero current.

A MOSFET is switched by a voltage on the gate lead. The voltage must be well over the gate threshold voltage. This gate voltage produces and electric field in the device which enhances the current carrying ability of the semiconductor material between the source and drain leads. The higher the voltage, the lower the resistance between the source and drain. The gate draws no steady state current, however it behaves like a small capacitor and so when applying a turn on voltage or removing the turn on voltage - one must quickly charge and discharge that small gate capacitance. Some MOSFETS have very high gate thresholds, and some are made with especially low ones - check the specs. So a MOSFET can be switched on by a sufficiently large voltage over the gate threshold level, but once charged the gate will not consume any current.

Remember when I said that the voltage is rising to certain level, then quickly dropping at Q3? This sounds exactly like what's required of the mosfet, according to that quote. And it did in fact attempt to power on with the mosfet I tried. But didn't stay on. Perhaps I just needed a different mosfet.

[ReverendJones]

Re: Help ID a sot-23 device

The smd code you listed could be one of two different P-channel MOSFETs, the Si2301DS you tried or a 2SJ243.

With the Si2301DS in circuit, what is the voltage on the gate?
When the power button is pressed, what does the gate voltage drop to?

[budm]

Re: Help ID a sot-23 device

That voltage from the IC should change state, right now it sitting at HI.If it is P-CH, the output of the IC will also have to swing low to turn on the P-CH. Either it is transistor or MOSFET, it will require steady state at the Gate or Bias at the base, it is does act like a latch. You take away the drive signal then it will go off.

[theagent]

Re: Help ID a sot-23 device

The smd code you listed could be one of two different P-channel MOSFETs, the Si2301DS you tried or a 2SJ243.

With the Si2301DS in circuit, what is the voltage on the gate?
When the power button is pressed, what does the gate voltage drop to?

Ah, that 2SJ243 looks promising. With the Si2301 in circuit, the gate is 4.057V to ~3.0-3.2V with the power button pressed. I don't see it in sot23 package, which is what the part is however.

[theagent]

Re: Help ID a sot-23 device

That voltage from the IC should change state, right now it sitting at HI.If it is P-CH, the output of the IC will also have to swing low to turn on the P-CH. Either it is transistor or MOSFET, it will require steady state at the Gate or Bias at the base, it is does act like a latch. You take away the drive signal then it will go off.

Well, it is doing the switching part... but isn't acting like a latch. It is getting a steady voltage to the gate (or base). I don't really know what else to try. What is responsible for keeping it "latched?"

[budm]

Re: Help ID a sot-23 device

The transistor or MOSFET will only stay conduct as long as it have signal driven the base or the gate, you take that signal away, it will go back to the previous state, it is the output of the IC that is either at logic hi or logic low, IC is the controller that tel the transistor to turn or or turn off. Transistor or MOSFET are not latching device.
So what voltage are you getting at the Drain (output), it does not sound like it gets enough drive signal to ground to get the MOSFET to be fully on.

[theagent]

Re: Help ID a sot-23 device

At the Drain it's 0V before pressing power... and ~2.3-2.5V when pressing/holding the power button. This is with the Si2301 in circuit.

[budm]

Re: Help ID a sot-23 device

It is not fully on and the IC is not staying low to keep the transistor on, the IC is not latching on.

[theagent]

Re: Help ID a sot-23 device

But it must be... it powers on, and stays on with the original damaged component. The only problem is that it powers on by itself, and the menu button doesn't function (the menu button is the power button, with the device powered on. Same button).

You know what, I'm sorry, I was wrong... Q3 doesn't go to the IC. I just looked at it more closely and did some continuity tests... it just goes through the resistors, then to the battery. It looks like it's just contained in that little group that I traced out.

[budm]

Re: Help ID a sot-23 device

When you have shorted out Emitter to collector, the current just flow right through the E to C with disregard as to what is at the Base of the transistor, basically the transistor is stuck on, same as you put a jumper between E and C.
That device under the black blob is probably a micro controller, and it gets its power all the time from the batteries, just think of your TV, the remote receiver and the main processor has to on all the time and ready to get the command from the remote control or the power switch to turn the rest of the circuits, it is supply by the standby power supply.

[theagent]

Re: Help ID a sot-23 device

I see. So perhaps the SI2301 mosfet is the correct part, and something else isn't working correctly?

[budm]

Re: Help ID a sot-23 device

Yes, the gate/base drive is not working which is supply by the IC. If you connect a 470 Ohms resistor to that R1/R2 junction and ground, that should turn on the MOSFET, 470 Ohms will limit the current to be only around 10mA (5V/470 = 10.6mA), that will not damage anything.

[theagent]

Re: Help ID a sot-23 device

Ok, I can try that... do I connect the resistor in series? So like this? https://www.dropbox.com/s/7m1muloh6s...cewheelGND.jpg

[budm]

Re: Help ID a sot-23 device

No, the top end where One end of R1 (not the bottom end of R1 as shown in your picture) connected to R2 end, that is the junction point that goes to the IC.

[theagent]

Re: Help ID a sot-23 device

Ok, just had a chance to solder in the 470ohm resistor. It did power it on... which was great... but unfortunately it acts just like the burnt component. The menu and start buttons don't function, and it doesn't power itself off as it should.

[budm]

Re: Help ID a sot-23 device

Ok, now we know that the IC is not working for sure since we can turn on the MOSFET manually with the resistor, who know what other functions of this IC are bad. By the way, what voltage are you getting at the EC3?