"Charon" wrote in message
...
On Apr 19, 5:55 pm, "Arfa Daily" wrote:
"Charon" wrote in message
...
On Apr 19, 10:16 am, "Arfa Daily" wrote:
"Charon" wrote in message
...
I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly
I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.
The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?
Doesn't electric charge flow on C and E if there is no flow on base ?
Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?
Just trying to figure out how this works and why only C and E of this
transistor are used ?
Thanks if anyone can shed some light on this for me.
What makes you think that the device in question is a PNP transistor ?
I tracked the part down tohttp://www.diodes.com/datasheets/FZT789A.pdf
Silk screen on the part is FZT 789A
Does
it actually *have* a third leg that isn't connected anywhere ? The
reason
I
ask this is that there is a range of wire-ended fuses which are in a
TO92
package, just like a transistor, but they physically have only two legs
at
the two 'corners' or the package. These devices are typically marked
"Nx"
such as "N10" for instance. What is the descriptor silk screened on the
PCB
for this device ? If it really is a transistor, it is likely to be
something
like "Qx" or "Trx", but if it is a fuse of this type, it will likely be
"ICPx"
Arfa
The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.
The device having only collector and emitter connected, makes no sense at
all. A transistor connected in this configuration, would represent
essentially an open circuit (not a diode as someone else suggested) and no
current would (or even *could*) flow between these terminals.
I see from the data sheet that it is in fact a surface mount device, so
presumably all three terminals and the tab are at least soldered down to
pads ? Could it be that the base connection is actually underneath the
device - maybe even via a thru' plated hole ?
Arfa
I started to trace this out, and found what you had said. There was a
trace under the transistor that was painted over. And was very hard to
see.
I made this trace diagram of the circuit.
http://bauld.com/~eric/pics/trace.jpeg
By jumping the C and E on the transistor it works as expected when not
plugged into the dc power(12v) which is expected as then it is
connected just as the DC 12v power is. The only thing it appears
stopping the battery power from reaching the device load is the
transistor and the base resistor.
Looking at your schematic, which maybe doesn't look *quite* right, then if
R3 is good, the transistor should be on. You could try measuring it
in-circuit with the battery disconnected, and you should see sensible
readings. Failing that, remove it and read it. I don't know how much
experience you have of getting devices like this off a board, but if you
only have access to 'conventional' soldering equipment rather than hot air
rework equipment, you need to be careful that you don't 'lose' any traces or
pads.
Use a good quality desolder braid with a good sized iron - preferably a
temperature controlled one of perhaps 50 watts max - to remove as much
solder as you can with the braid, from both the pins and the tab. Then heat
the tab alone with a small scalpel under its edge, until the solder flows
enough to be able to slightly twist the blade to lift the tab no more than
1mm off the board. Then heat the pins, all at once if possible, and repeat
the blade twisting under the device body, to lift them up from the board a
little. This can be difficult to achieve, if the manufacturer has kindly
glued the device to the board, before flowing it ...
You should now be able to use a fresh piece of desolder braid to get the
remaining solder under the pins and tab, and the device should come off the
board cleanly, and with no damage. It's not easy, but if you can solder ok,
and understand about not overheating solder pads and causing them to
delaminate from the board, then it's not overly difficult, either.
Arfa