From: jjk439@gmail.com   
      
   On Friday, November 1, 2013 4:25:59 PM UTC-4, Andrew Gabriel wrote:   
   > 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]   
      
   Thanks again Andrew. this is making more sense. I never used 3.3v zeners   
   before.   
   The 3.3v protection I was talking about is to protect output pins of 3.3v   
   devices. The gpio pins on a Broadcom processor on the Raspberry Pi is one   
   example. If an external source that is attached to the pin fails and applies a   
   voltage greater than 3.3v,    
   the Broadcom device can be damaged. There are articles on the net describing   
   the use of a zener for protection and that's what I attempted to do. I guess   
   the safest thing to do is add buffers in between the processor and external   
   devices.   
      
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