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Topic: [SOLVED]What recourse when electronic components are out of tolerance? (Read 2528 times) previous topic - next topic

allanhurst

No - one prefers not to have to use aot/sot or a pot unless really necessary - it slows down production. And I don't like pots because their temperature drift isn't very good - and idiots can tweak them when they shouldn't.

 But if they're only used for say +/- 5% of the range that can be tolerated.

You can buy 0.1% references.. but they're dear

It's all a trade-off between production time , stabilty , and the cost of high precision components.


regards

Allan

Paul_KD7HB

Hi,

I recently had 100 MCPCB's made that had Vishay 15V zener P.no. BZG03C15 on board.  The zener is passing 25mA and does not even get warm to touch, yet the voltage across them is 15.6V or more.  According to the data sheet, 15.6V is the tolerance maximum with a test current of 50mA, I'm only passing half that.  As such my boards, which cost me $3500 are scrap.  What recourse is available to consumers when components are supplied out of tolerance?

Cheers.
Did your BOM specify the manufacturer and distributor? We require that for all boards we build where we buy the components.

Paul

Boardburner2

You can buy 0.1% references.. but they're dear

That is all we used on one pcb, and every component bar one on that board was AOT.

boggydew

Did your BOM specify the manufacturer and distributor? We require that for all boards we build where we buy the components.

Paul
Yes the BOM was specific to the part number. 

As was mentioned in an earlier reply the tolerance values relate to pulse testing so heat is taken out of the equation.  This is pretty frustrating because I imagine the majority of implementations for zeners are under constant conduction, in which case you can't really use the tolerance specifications for design.  (Unless of course you have a Ph.D.)

The irony is that my work around for my 100 boards is to put another 5% zener across the battery.  This is counter intuitive because the original problem was that 5% is too broad a tolerance for a regulator.  But my experience with zeners is that despite being rated at 5% they are usually much closer in tolerance within a particular batch.  For example during prototyping, I bought ten 15V zeners and they were all +/-1% nominal voltage at 25mA.  But my production boards are all 15.6V +/-1% at 25mA.  The 5W 13V 5% zeners I am now using as a bandaid all zener within 0.1V of each other when passing 100mA, closer to 1% than 5%.  So I wonder why they just don't bin them into tighter tolerances?

The confidence I had in my design was due to my experience, not the data sheet, and it came back to bite me.  Live and learn. 

Wawa

I think you're missing the point that a TVS diode is not meant to be used as a zener.
A 15volt TVS diode, as you have used, is made to NOT conduct at the rated 15volt.
It is made to protect a 15volt supply rail, and not use too much current from that rail in the process.

Can't you solder a common 15volt/1watt zener across that TVS diode?
Leo..

raschemmel

Quote
A 15volt TVS diode, as you have used, is made to NOT conduct at the rated 15volt. 
That is the whole point.
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