From: andrew@cucumber.demon.co.uk   
      
   In article <7de51a62-9931-4069-b30d-a0c175f95815@googlegroups.com>,   
   	jjk  writes:   
   > On Thursday, October 31, 2013 4:42:08 PM UTC-4, Andrew Gabriel wrote:   
   >>   
   >> This exactly matches the datasheet, which says it needs at least 76mA   
   >> in order to act as a zener voltage reference.   
   >>   
   >> This is a common issue with low voltage zeners - the current they   
   >> need to operate is high (compared with higher voltage zeners).   
   >>   
   >> If you want a low voltage reference at low current (e.g. for battery   
   >> operated equipment), an LED can often be used (forward biased) at a   
   >> much lower operating current (1mA or less), although not quite as   
   >> stable a reference.   
   >   
   > Thanks for your comments Andrew.   
   > I'm wondering why the zener would conduct at all at voltages as low as 1.4v?   
      
   Zener voltage regulation happens in reverse breakdown mode.   
   All zeners leak reverse current at voltages below the breakdown voltage.   
   With low voltage zeners, this leakage current is very high, up to 76mA   
   for this part number. This makes low voltage zeners unsuitable for   
   many applications, where you might expect a theoretically perfect   
   zener to work.   
      
   > My goal was not to use the zener as a regulator, but to protect 3.3v devices   
   from potentially external over voltage events (microcontroller driving 5v   
   circuits).   
      
   If the 50mA or so leakage at 3V is an unacceptable issue for you,   
   you might instead consider a potential divider across the 3V supply,   
   with the centerpoint connected to an SCR gate to crowbar the supply.   
   Adjust the potential divider ratio to give the right tripping voltage.   
   A red led (forward biased) added at the top of the potential divider   
   might make it more accurate/sensitive.   
      
   I haven't actually tried this at such a low voltage, but I suspect it   
   may work better than a zener.   
      
   --   
   Andrew Gabriel   
   [email address is not usable -- followup in the newsgroup]   
      
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