Transistor failure and protection.

[itsmoked]

The 2N2905A is blowing at the slightest provocation. I've been thru 8 on four out of 6 channels. Generally the transistor fails with the Base shorted to the Collector and the Emitter open (5). And, in two cases, all three pins open.

I've never seen the transistor even warm. I'm looking for any suggestions as to what might be giving that transistor such a glass jaw. I'm wondering if these are V_(BR)EBO failures?
In the spirit of hotrodding do you have any suggestions for protecting that transistor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

You are a brave man, Keith!

The thought that you do not want to consider is difficult to admit to oneself - and most guys would never admit it in public.

You and Abe 'did it with my ax' Lincoln.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[MacGyverS2000]

Wow, PTH components... you lucky dog! I'm usually probing pin 37 of 200 on a 0.5mm pitch part. You don't want to slip there, either.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[BrianG]

Keith,

Looking at your photos of the transistor with the lid off, please note it's the emitter wire that has vaporised, not the collector. On the 2N2905A in TO5 package the external collector lead is common to the metal case, and the collector area on the planar die is in direct contact with the case.

In the example photographed, is it possible that you have touched the can of the transistor with the scope ground? That would immediately pop the transistor emitter bond wire as in the photo because it would bypass the R-C network on the collector circuit and thus the transistor would be connected between 28V and ground without any current limiting.

 

[itsmoked]

Thanks Skogs. Foolish and brave.

Yeah Dan, ain't it great!

Brian; You're right! Emitter. Yes I get it now.

Today I installed the unit back in the furnace and fired it up. Instantly it was obvious it wasn't running correctly. It makes a grating dental-drill sound due to nasty harmonics. I broke out the unidirectional test-lamp. Sure enough one/half channel was missing.

Running the test points showed all the channels running correctly on that side. So, I ran down the transistor cases. They all showed correct waveforms too. Right up until I touched the &](^#^#%%@$ case of one with the probe ground-ring, pffft! That half channel was also now gone.

So yes Brian that certainly does it!

Turned out Fry's carried that transistor. So in 45 minutes we were back up. I very, VERY, carefully probed the transistors after wrapping the ground ring in electrical tape. All channel fine. (Skogs that's the only confirmed time I did it!)

I switched to looking at the final output to the SCRs on the control board. Every gate was running with 1.4V pulses except one. It was 7V.

I powered down and unplugged all the outputs. Powered up and re-measured. Now all the channels were 7V. Bingo.

The trigger line was open at the snubber board somewhere. Where? Get this.. The terminal block was failed. The internal pin that connects the flat screw surface to the circuit board was sheared. Sometimes it would make and lately it wasn't very often. Turned out for that line on that snubber board there was another unused terminal that was paralleled to that same signal. Moving the control wire to that screw fixed the problem.

Next!

Thanks for all the help folks.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[BrianG]

Well Done Keith! No amount of analysis will ever replace sheer dogged determination!