What Causes Corner of BiPolar Transistor to "Chip Off"?

Thread Starter

technicom

Joined Jun 29, 2010
5
Hi,
We are using an MPSA42 NPN transistor (TO-92) package in a product to drive a string of 30 blue LEDs from 120 VDC through a dropping resistor. Occasionally, one of our products is returned to us by our customer with the failure mode "blue LEDs not illuminating." When we inspect the unit, we see that the corner of the TO-92 package that is closest to the emitter has broken off (see photo). Further, the transistor is "intermittent" in that we can press on it (i.e. physically move the transistor very slightly) and the circuit will begin to work again. I think that the customer is applying too much voltage to our circuit and that is causing the case to "pop". Does this make sense? It certainly appears to me to be caused by electrical stress (exceeding the Absolute Maximum Ratings of the MPSA42) and not physical stress. Any comments would be appreciated!

Thanks,
Mike
Q1 Cracked By Emitter Lead.jpg
 

WBahn

Joined Mar 31, 2012
25,751
Uhm.... it's rather hard to guess how a circuit might be causing the transistors to fail without seeing the circuit.

Two obvious candidates -- physical stress or thermal stress. I'm voting for the latter, but without seeing a schematic for that part of the circuit that's just a wild guess.
 

ScottWang

Joined Aug 23, 2012
6,893
Are you used the tranformerless power supply for leds?
Please just using the normal font size, unless you want to emphasize something, the big font already modified to the normal.
 

Hypatia's Protege

Joined Mar 1, 2015
3,219
No need to blame the customer;) -- it's almost certainly a thermal (and, hence, design) issue -- especially if, as you say, there have been multiple (similar) failures...

As an aside; please be very, very careful with non-isolated line operated products...

Best Regards
HP
 
Last edited:

Thread Starter

technicom

Joined Jun 29, 2010
5
Are you used the tranformerless power supply for leds?
Please just using the normal font size, unless you want to emphasize something, the big font already modified to the normal.
Sorry about the font... I just signed on and that was the font that came up!
 

Thread Starter

technicom

Joined Jun 29, 2010
5
Uhm.... it's rather hard to guess how a circuit might be causing the transistors to fail without seeing the circuit.

To obvious candidates -- physical stress or thermal stress. I'm voting for the latter, but without seeing a schematic for that part of the circuit that's just a wild guess.
The product is used by electric utilities and they have both 125 VAC and 277 VAC in the test lab.... We believe that they are accidentally connecting 277 VAC to the circuit instead of the 125 VAC for which it is designed. My purpose in posting the photo was to get some opinions about the failure being caused by an electrical stress (over-voltage, over-current, etc.) and not by an assembly process (mechanical damage during assembly and handling). If I know that the chip fractured off the case as a result of electrical stress, we can focus on that and not on assembly issues. It doesn't look like any type of mechanical failure to me.....

Thanks all!
Mike
 

Lestraveled

Joined May 19, 2014
1,946
Ta Ta for Now! HP you are such a rogue.

The MPSA42 is a high voltage transistor. Apparently it is being subjected to excessive voltage or current. It is interesting that you can press on it and it begins working. Suggest you put a zener, or tran-sorb diode in parallel with the part.
 

WBahn

Joined Mar 31, 2012
25,751
The product is used by electric utilities and they have both 125 VAC and 277 VAC in the test lab.... We believe that they are accidentally connecting 277 VAC to the circuit instead of the 125 VAC for which it is designed. My purpose in posting the photo was to get some opinions about the failure being caused by an electrical stress (over-voltage, over-current, etc.) and not by an assembly process (mechanical damage during assembly and handling). If I know that the chip fractured off the case as a result of electrical stress, we can focus on that and not on assembly issues. It doesn't look like any type of mechanical failure to me.....

Thanks all!
Mike
I don't care who it's used by. How can we possibly do any kind of analysis on whether it is being stressed unless we know what it's operating conditions are. Just telling us that it's used to drive 30 blue LEDs through resistor from a 120 VDC supply doesn't tell us diddly about the operating conditions of the diode.

But it appears that that is some deep secret that you aren't going to share.
 

ScottWang

Joined Aug 23, 2012
6,893
Give us a proof that this is a cuircuit through transformer and diodes rectifier, otherwise we may treat it as a topic about transformerless power supply and close the thread which is not allowed by the TOS.
 

ian field

Joined Oct 27, 2012
6,539
The product is used by electric utilities and they have both 125 VAC and 277 VAC in the test lab.... We believe that they are accidentally connecting 277 VAC to the circuit instead of the 125 VAC for which it is designed.

Thanks all!
Mike
The obvious answer to that is a sidac and a fuse.
 

ian field

Joined Oct 27, 2012
6,539
Give us a proof that this is a cuircuit through transformer and diodes rectifier, otherwise we may treat it as a topic about transformerless power supply and close the thread which is not allowed by the TOS.
Pretty sure there'll be a transformer of some description if you follow the cables back to the substation.................
 

#12

Joined Nov 30, 2010
18,210
It is common enough to have an air pocket in a transistor which allows "wiggling" to cause intermittent operation, but THAT air pocket is not a manufacturing defect. It is also common for Our Father who Art in Heaven to slap down lightning bolts on the electric utilities. I am intimately familiar with the crackle of exploding thermoplastic, and that's what your photo looks like. Either blame the designer for not protecting that transistor or blame Dog for striking it down.
 

ian field

Joined Oct 27, 2012
6,539
Else 33kV directly off the 'Genny'? At an Ifault[max] of Ca. 45kA that could get nasty!:eek::eek::eek::D
AFAICR: 11kV is the highest I've actually seen cabled into factory premises, but I do know its substantially higher in the bigger smelting foundries.

In the days before the old Rugby transmitter became famed for the MSF60 time signal, it was a major world wide wireless communications facility - I read somewhere there power was direct from overhead cables (no substation transformer) - not sure, but I think I vaguely remember reading 11kV for that.

They had a power house full of rotary transformers, most of them were mounted on insulating pillars so the outputs could be any combination of series/parallel.

The 400kV pylons are almost a stones throw from my flat, the main substation for the surrounding towns & villages is less than a mile from me.
 

ScottWang

Joined Aug 23, 2012
6,893
Pretty sure there'll be a transformer of some description if you follow the cables back to the substation.................
:D

He didn't answer my first question, I'm not sure he is ignore it or avoid to answer it, I started to concerned this thread may closed soon.
 

ian field

Joined Oct 27, 2012
6,539
:D

He didn't answer my first question, I'm not sure he is ignore it or avoid to answer it, I started to concerned this thread may closed soon.
The TS appears to be describing a commercial product that they have some responsibility for.

From what I've read so far, the distinction between professional and amateur seems somewhat blurred in this instance.
 
AFAICR: 11kV is the highest I've actually seen cabled into factory premises, but I do know its substantially higher in the bigger smelting foundries.
My citation was in reference to a 'generating station' I am (was) familiar with (To wit: Prairie Island -- *NOT* to be confused with TMI:mad:;))

The 400kV pylons are almost a stones throw from my flat
All the better to 'tap' that stray EMR;););)

TTFN
HP
 

takao21203

Joined Apr 28, 2012
3,695
always interesting to encounter commercial products failing, engineer with degree consulted datasheet but, not enough experience, taking it literally, fail.

Makes me think of my 3v 20 LED circuit, safe to touch, and just the first throw, for sure, much more could be archieved.

Makes me think of that General Electric mini fluoro nightlight- after a short while, arcing inside and holes + molten plastic visible on outside. Total fail.

Capacitive mains supplies are serious issue/source of risks even when done professionally.

For a couple of reasons...

Accepting bids from investors for the blueprints for my LED driving circuit btw.
Its not difficult but apparently, PNP based, works with plain low voltage transistor.
 
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