How sensitive is a BD139 transistor for static electricity?
Just blew one when touching it. No big load on it (used in a constant-current circuit for a LED).
Is this normal or was it a bad one just waiting to blow?
Please supply a link to the part.
Not very.
Well it wasn't the static charge that caused its demise.
Neither.
It was the circuit attached to it.
It doesn't have to be a big load, but it looks like the conductance or capacitance of your finger disturbed the feedback circuit of your constant current driver, forcing it to overload the current to the LED.
There was only power to the constant current circuit driving a LED. And I could feel the static electricity when touching it.
This is the circuit:
+44 Volt at top, GND at bottom
Do you really need a constant current driver for 150uA LED current?
A 150k resistor in series with the LED would do the same as that whole circuit.
Leo..
Input voltage may vary from 5V to 50V - that's why 
Ahh, but why the 200uA.
Up to ~2MA can be done with TO-92 transistors (2 x BC546).
R1 could also be 100k.
Leo..
People experienced with electronics ground themselves with a wrist strap or similar before touching circuitry. They also avoid touching any circuit elements while it is powered up.
In spite of feedback, any excess current into the base of Q2, also follows a low impedance path through the BE junction of Q1, allowing Q2 to saturate. The current is no longer limited by R2. Since Q2 collector current follows the same path, it's a 50/50 chance more or less, which transistor will blow. Actually I would have expected Q1 to blow because there is a lower limit on allowable BE current than the limit on Q2 CE current.
But why does the feedback not work when Q1 is turned on by that current?
Because the feedback bandwidth is much less than the bandwidth of the static surge?
I got one. But sometime you forget to use it - like this time. Beside, I could not image that static electricity could be that harmful. Lesson learned
If the static surge is that brief, I can't see that it is particularly likely to damage a forward biased junction let alone drive a transistor into saturation.
ESD damage is generally breakdown of insulating - non-conducting - layers, is it not? 
Obviously if a static surge in itself carries enough energy (Joules) to cause overheating of a conducting junction, it will do damage, but to suggest that it will cause one transistor to turn on and not the other is rather dubious. 
I was just playing devils advocate but looking at the component specs, the failure of Q2 from applied currents seems nearly impossible. For one thing, it's a fairly high capacity BJT. Q1 is the sink for any significant current into the junction of Q1 collector and Q2 base. So if anything, it should fail, not Q2. So far we are only looking at theoretical circuits, I would like to see an image of the circuit. When you touch a circuit, you are applying physical not just electrical force. But there are additional aspects that make it dubious. Such as, why the LED did not blow.
Forgot to say that the LED blow too. I barely touched Q2 when I felt the discharge and saw the LED flashed and died.
It's possible that your circuit isn't built identically to the diagram you showed. There are numerous alternate and not unlikely causes... for example a finger brushing a jumper wire and causing a short.
If this is a new circuit (rather than one that used to work and now doesn't) ...
Might the BD139 have been installed upside-down? That would swap B & E & could reverse bias Veb (design abs max 5V) which might kill it.
Which we might have a chance of noticing, if the requested images were posted.
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I'm quite sure it has been installed correctly. But if not, would not 50V input to the CC kills it?
Hi @thehardwareman sorry to contact you here i just find out the you had the same problem as mine and i supposed you can help me (Your old post was closed)
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Thanks a lot