From: John Popelish
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Subject: Re: Why zener diode can't make at 1.5v/4.5v/5.0v/6.0v/7.5v/9.0v ?
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Date: Fri, 27 Dec 2002 16:15:46 GMT
NNTP-Posting-Date: Fri, 27 Dec 2002 11:15:46 EST
> I see. Thank you very much.
> But I think, sell the value in series is not good for zener diode
> and doesn't make sense.
> Why they doesn't sell every 0.5V value and/or popular voltage like
> 3.3v 5v 6v 7.5v 9v 12v - 5%
If they made 3.0, and 3.5 volt 5% tolerance devices, there would be a
range between them that no device might cover. If they sold 15.0 and
15.5 volt 5% devices, some of one type would produce voltage drops
that would be included on the other type spec.
> So technology can't make 1% today ?
Zeners can be selected with narrow tolerances, but that selection adds
to the cost. Also, for any zener that is not around 6.2 volts drop,
there is a significant temperature coefficient of voltage drop
(positive for higher voltages, and negative for lower voltages) so the
tolerance has to cover that effect, also, unless the device is speced
to operate at only a narrow range of junction temperatures.
> Does zener diode use in the imprecise function only ?
The zener voltage depends on how the dopant atoms are distributed in
the PN junction area, and this is is difficult to control perfectly.
Batches are made on a wafer, and then the wafer is tested to sort the
devices that have happened. So, yes, the process involves a lot of
art, and a bit of luck. It is a bit like cooking. Sometimes it just
doesn't come out exactly as you remembered it tasting the last time.
Like I said before, if you need some precise zener voltage, use an
integrated shunt regulator that includes a trim pot connection.