Weird transistor failures

Are you by chance using water soluble flux and not cleaning it up? Water
soluble flux is conductive. I also heard that no clean flux can leak.

 

You are aware that the BC182 TO-92 package is C-B-E ordering on the
leads instead of the more common E-B-C?

 

Tom R wrote...

That's a puzzle, Tom. I assume when the LED is supposed to be
off the PIC is powered on with its output at 0 volts? Which means
your 1k resistor is pulling over 0.5mA out of the base of the bc182
to insure it stays off? Note, if you've got a situation where the
PIC power is off, but the 12V is still present, you can add a 2.2k
resistor from the bc182 base to ground to provide a leakage path.

As for the bc182's strange leakage-current deterioration with time,
I wonder if the transistor is fully saturated to avoid overheating
when powering the LEDs. Have you measured the collector voltage?

 

I wouldn't choose a BC182 for 25 mA collector current, even if it's
operating within its ratings. There is also the possibility that it's
oscillating at VHF. How hot does it get?

 

Where's the 12V coming from? Automotive?


Best regards,
Spehro Pefhany

 

I have had a number of failures of simple transistor LED drives which I
really can't figure out why:

I am using a standard bc182 driven from 1 k directly from a PIC. The emitter
is grounded, the collector has two white leds (connected in parallel) , a
red LED in series and a 240 Ohm resistor. The drawn collecter current is
around 25mA.
The voltage is 12v dc and everything works perfectly for months
until...eventually (and not always) the transistor begins to leak such that
the LED's become just visible in the dark. The transistor will still turn on
fully but not fully off.

Anybody ?

Thanks
Tom

 

There's nothing wrong with your circuit - although you're using a pretty high
base current.. I'd tend to use 10k in the base connection in such an
application.

Where did you get the BC182s from ? Don't see them around much these days -
dodgy stock ?


Graham

 

Perhaps transients are damaging the transistor.


Best regards,
Spehro Pefhany

 

I think that's too big; it's a series tuned circuit even at 10 MHz. I
suggest 1 to 10 nF.

 

Do you have any appreciable wire lengths between
the transistor and the LEDs... ie, inductance,
with maybe enough flyback energy to wound the
transistor. It will be the total wire length
from the nearest 12v decoupling capacitor.

Long wires in the field and short wires in the
lab could be a confirmation of the inductance
suggestion.

 

The easiest (but not cleanest) way is to put a Zener diode across the
power input. Put one of the appropriate voltage and it would drop
anything above the given voltage.
However, there is a risk of "burning" the power supply.

A cleaner way would be to use a voltage regulator, I guess.

 

Voltage suppresor diodes?

By the way, voltage can be as high as 14.4V in the car, possibly a bit more.
Not that I can see how this could destroy your transistors.

A dodgy stock was suggested by someone, that's possible if you bought
them from surplus.

The only similar effect I know is from the RF field where a power bipolar
slowly builds a schottky diode across BE junction slowly deteriorating performance.

 

Are you by chance using water soluble flux and not cleaning it up? Water

Thanks Boris, I don't think flux is the problem as there are no other
strange effects on the circuit and the transistor is still damaged after
extraction.
Tom

 

FWIW - I have seen this behaviour in cheap transistors used in bulk
(e.g. C945, general purpose npn) - in any application it was necessary
to provide a good ground potential to the base to prevent it from
leaking. (it is amazing to see how little current is required to make
a LED glow visibly)
Sure the pic emits a solid "0V" in off position? Resistor directly
from base to ground (~3K3) added to series resistor is std issue with
these part ever since in my book.

 

You are aware that the BC182 TO-92 package is C-B-E ordering on the

Thanks Fred, yes I am actually using an bc182L but I have had the same
problem with a 2n3704.
Tom

 

Dirty fix- a zener or unipolar TVS across C-E on the transistor could
do the trick (dropping excess voltage across the resistors and LEDs).
You should be able to find an upper limit on typical transients from
various sources (load dump etc, both time and voltage) by searching on
the net, then calculate what kind of part you need. Gut feel-- a 3
cent 1W 30V zener (eg. 1N4751) will be more than good enough, but
that's worth about what you paid for it.. TVS parts can take a lot
bigger surges than regular zeners.

Now, that might start killing the LEDs (or resistors). A better
solution would replace the resistor with a surge-resistant type (if
it's not already), and connect a TVS from the resistor to ground. That
would kill transients without putting current through the LEDs. BTW,
I'm using 30V for the zener/TVS to deal with the situation where
people series two batteries to start the vehicle, so it might see
double the nominal input voltage for a significant period of time. I
hope you're using a regulator that is rated for automotive
applications, or otherwise protecting a more fragile regulator.

Don't lower the base drive, it's pretty good. You might consider using
a more rugged transistor such as a 2N4401, which probably wouldn't
cost any more, and will be even further from coming out of hard
saturation, even under transient conditions.


Best regards,
Spehro Pefhany

 

Unless you do something silly in the programming, the PIC output is
push-pull, with a fairly low resistance MOSFET to ground when "off",
so effectively he has a ~1K resistor from emitter to base when off.


Best regards,
Spehro Pefhany

 

That's a puzzle, Tom. I assume when the LED is supposed to be

Thanks Win, the PIC is powered from the same supply (via regulator) as the
LED drive.
I have deliberately stopped the pic to simulate a crash and the transistor
is quite happy.
I have used drive resistors from 1K thro to 10K and still have the effect.
I have used 2n3704's but still...
Not had any complete failures, just not quite shutting off failures.
Have not measured collector voltages as there appears no heat buildup and
sadly I cannot get the things to fail in the lab.
Tom

 

Thanks John, had not considered this. Sadly I have never managed to cause
the failures in the lab.Would a 0.1uf from base to emitter prevent
oscillation ?
Tom

 

http://www.speff.com

Thanks Spehro, yes its automotive.
Tom