Weird transistor failures

might be load dump and esd

For ESD, I'd put a 0.1 uF cap directly from the from the base to the emitter and another 0.1 uF cap from the collector to the emitter

and figure that the "+12V" can go to +80 volts during load dumps and surges and either choose a suitably rated transistor or put some other form of protection. I like a 100 Ohm 5 Watt resistor in sereis with the +12 volt feed to a 22 Volt zener and 100 uF cap to ground.

Mark

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.

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Boris Mohar

Where's the 12V coming from? Automotive?

Best regards, Spehro Pefhany

Perhaps transients are damaging the transistor.

Best regards, Spehro Pefhany

-- "it's the network..." "The Journey is the reward" snipped-for-privacy@interlog.com Info for manufacturers:

I've been following this thread on 'Outlook' where I can see numerous responses, but here on Google I see only your initial question for some reason.

Anyhow, FWIW I believe your problem lies in the fact that you are operating in an automative environment and, whilst the supply voltage is notionally 12V DC, it is subject to all manner of voltage spikes and reversals that rival even the most severe military environment. Temperature range is also far more onerous than most people realise.

As a starter, I would select one of the many 'Transorb' type components to strap across the supply to your electronics, (possibly in conjunction with some series element - this could be as simple as a few ferite beads on the supply leads to your circuit). Transorbs are capable of absorbing considerable amounts of short-term energy and can go a long way towards keeping circuit voltages within safe realms.

You may also need to look at your overall circuit configuration, in terms of wire lengths and what sensitive circuit nodes might be exposed to potentially damaging voltage levels - the odd capacitor connected in the appropriate spot can go a long way to providing immunity to damage.

It is not an exact science, but some basic approaches tend to solve most problems in most cases.

HTH

Regards

Ted

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

failures

So we know the cause isnt the trs themselves.

Also since you cant duplicate in the lab, again w know the cause is something in the auto app that isnt being duped in the lab.

The one things that sticks out like a thumb to me is the auto supply: theyre very rough. Expect peaks of +60v and even negative v transients. Use series element plus zener to deal with this, and 60 or 80v rated trs. Maybe that will be the end of your trouble. Of course youve also got vibration dirt and damp to deal with in autos.

John mentioned collector resonance: how does that work with resistance in the C load? (VHF is a bit beyond me). If that is a problem, presumably some R in series with the tr lead would stop it.

NT

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?

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

I read in sci.electronics.design that Tom R wrote (in ) about 'Weird transistor failures', on Fri, 8 Apr 2005:

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?

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

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

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

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

Thanks Spehro, yes its automotive. Tom

high

days -

Thanks Graham, I'm actually using 182L's but I have had identical failures with 3704's/3904's etc. Tom

I read in sci.electronics.design that Tom R wrote (in ) about 'Weird transistor failures', on Fri, 8 Apr 2005:

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