Re: Repairing/understanding PNP in charging circuit

Arfa Daily wrote:
"Eric Bauld" <eric_newsgroup@xxxxxxxxx> wrote in message news:ab5Hl.82407$9t6.48872@xxxxxxxxxxxxxxx
Arfa Daily wrote:
"Charon" <ebauld@xxxxxxxxx> wrote in message news:ecddfa6e-7654-4784-a644-f00b85b58dd5@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Apr 19, 5:55 pm, "Arfa Daily" <arfa.da...@xxxxxxxxxxxx> wrote:
"Charon" <eba...@xxxxxxxxx> wrote in message

news:85208e9c-0784-4b8e-bbf1-f065aac6670c@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Apr 19, 10:16 am, "Arfa Daily" <arfa.da...@xxxxxxxxxxxx> wrote:

"Charon" <eba...@xxxxxxxxx> wrote in message
news:b364a500-3580-40a8-830f-786142453887@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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 *** 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
It is entirely possible that I missed something, the traces are hard to see and hide under a few components. I spent quite a bit of time double checking with a multi meter for no resistance to help find beginning and end of traces. only if they made sense and would not have crossed another trace.

Testing the resistance on the PNP in circuit no battery it seems to change every time I test it.

To get a proper reading I will have to pull it out.

With battery in circuit, there is no resistance reading between C and E
I get 10.5 volts to B
11.9 volts to E
2.4 volts on C (not sure where the 2.4 volts is coming from)

This should be letting the 11.9 volts through to C but its not

When this was working "properly" there was a parasitic load on the battery that would drain it within a a few days. which was the original reason for opening this up. Was to put a switch on the battery connection to disable the battery when not in use.

So there would be two switches.
Original to turn device on.
New switch to disconnect battery.

If the transistor problem eludes me too long, just removing it and using the switches to control it manually would suffice.
Although if the device is on while charging the battery will not be disconnected from the circuit via the pnp.



OK. You should not read any resistance between collector and emitter i.e. it should be open circuit.

I should have clarified my post better. By no resistance reading between collector and emitter I meant it was closed circuit. I was expecting open circuit. But then everything would be working.

For measuring junction resistances on bipolar semis, I have always preferred an 'old fashioned' 20k ohms per volt analogue multimeter - in my case an AVO 8 MkIV. Digital meters can give very misleading resistance readings across transistor junctions. With an analogue ohm meter, with the red probe to the base pin, you should read around 700 or 800 ohms to the collector and emitter, with the black probe, for a silicon PNP transistor. All other combinations of test probe polarity and pins, should read open circuit.

As far as the voltage readings that you are getting, they would seem to be wrong. With 11.9v to the collector, the base voltage should be about 0.6 to 0.7v below that - i.e. around 11.3v, not 10.5v. Assuming that R3 is in good order, this voltage difference is a fixed function of a silicon PN junction, so any deviation from that figure would tend to indicate a faulty device. The 2.4v 'output' that you are seeing on the collector, is probably just a voltage that is 'battering its way through' the device from the emitter input voltage. My next move would be to get some proper resistance readings for the device junctions, removing it if necessary. From the information you have given, I think that you are going to find that the transistor is indeed faulty.

Arfa



Thanks for the assistance on this. And I can relate to the advantages of using a analog meter. I once trouble shot a wiring harness in a car. And was getting a rogue voltage reading on a DMM when there should have been none. An anolog meter drained the residual charge it was getting from surrounding wires and then it would read as expected(no voltage). But until this was pointed out to me I was confounded. As it was just enough to light a dash led when the harness was unplugged.

Ill track down an analog and try to pull this out this week.
.