From: gherold@teachspin.com   
      
   On Tuesday, April 9, 2019 at 7:32:10 AM UTC-4, default wrote:   
   > I'm using some cmos IC gates in a project.  All seem to have an   
   > internal protection network to insure that the inputs can't exceed   
   > supply or go more negative than ground with internal diodes.   
   >   
   > My question has to do with those protection diode ratings.  How much   
   > current can they clamp to prevent exceeding the input voltage rating?   
   >   
   > For instance, I'm running one part of the circuit at 8 volts and   
   > another at 12.  It is necessary to input a 12volt output to a 8 volt   
   > input.  (that's a no-no according to absolute maximum ratings)   
   > HOWEVER, the 12V side is an open collector output with a 10K pull-up   
   > resistor.  It works... so apparently the internals diodes don't have a   
   > problem clamping ~400 microamps, but I'd feel better knowing what the   
   > spec. is and haven't found one.   
   >   
   > The datasheet shows two sets of protection diodes for every input with   
   > a current limiting resistor to further protect the inputs.   
      
   How about a link to data sheet, or some part numbers.   
   Did you measure the 400 uA?  As a WAG you can probably dissipate   
   ~100mW in the diode/series R.  Try blowing some up maybe?   
   (How about a resistor divider to drop the 12 volts to 8?)   
      
   George H.   
      
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