Post subject: How to tell the transistor or FET
Posted: Tue Jul 26, 2005 9:16 pm
I would like
to know how to tell TR or FET with SOT-23 Package in the PC Board.
If it is TR, how to tell NPN or PNP. If it was FET, Ntype MOSFET
or Ptype MOSFET.
Posted: Wed Jul 27, 2005 11:53 am
BJTs are easy: A BJT will
look like two diodes (Base-Emitter and Base -
the common lead. That's your base. If the
('+' to '-') from the base to the other two, it's
an NPN. If
it conducts the other way it's a PNP. If it conducts both
or neither, pitch it. Telling the difference between the emitter
and collector is a tad harder. If your multimeter has a "transistor
tester" the direction with the largest beta is correct.
The (forward-biased) resistance of the base-collector junction
lower than that for the base-emitter junction. The
difference is quite
them, be aware that some transistors have a diode
and emitter, while others have resistors between the
emitter. Then there are digital transistors ...
generally get a good idea of a MOSFET's condition by making a simple
resistance measurement between the gate and source leads. The
oxide is quite thin and fragile, so if the MOSFET
fails one of the
consequences will be that it will normally
damage the gate oxide (even if
the gate itself wasn't abused
by excessive voltage). A failed MOSFET will
normally measure <1k
ohms between gate and source. These failed MOSFETs
normally exhibit low resistance from drain to source as well. A
good MOSFET's gate to source resistance will measure somewhere
megohms to infinity range.
There are exceptions
to this however. Sometimes the failure (especially in
of power MOSFETs used in such a way that if they fail large
currents flow) is so catastrophic that internal fusing can occur.
enough transient power can be dissipated in the device
to physically blow
the plastic package apart, physically separating
one of the leads from the
die. In these types of failures the
above mentioned resistance test
read infinity even though the
device is definitely broken. In my experience
these types of
failures are relatively uncommon compared to more mundane
which simply result in low resistance shorts between the three
pins. When they do fail like this it is often obvious since
package has literally blown apart or has otherwise
suffered evident trauma.
In one case I found a 2N7000 MOSFET
that failed by means of a parametric
shift. The drain to source
became very leaky (comparable to a 100k ohm
resistor) even though
the gate was fully intact. The device still worked
the extra leakage. The leakage current could be reduced by
the gate with negative gate-source potential, but still not
completely turned off. I have no idea what might have happened to
device to cause this failure. Nevertheless I think this
is a very rare
failure mode which you shouldn't normally need
to worry about.