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Kai Klaas
02/22/06 09:13
Read: 510 times

#110505 - Using a cap is enough
Responding to: Sun Din's previous message
Sun said:
If the correct resistor values are chosen, the voltage devider will "clap" the output at a voltage lower than the total supply but higher than Vz so that the zener will switch on.

I don't think, that you need this additional resistor, if you have a cap in parallel to the zener. The additional resistor would only consume avoidable supply current.

Let's have an example:
You want to supply a TTL gate drawing 20mA from the supply. Then a current of 20mA + 5mA (at least, to maintain 'regulation') must flow through the upper resistor. So, the upper resistor should be (12V - 5V) / 25mA = 280 Ohm. We take 270 Ohm.
Paralleling a cap of even only 100nF would cause a time constant of 270*100nsec = 27Ásec, which is way enough to allow the zener to settle.

If you additionally take a lower resistor, means one which is in parallel to zener, then some more current is drawn: If we want to limit the voltage to let's say 6V, then you need an upper resistor of 240 Ohm and a lower resistor of 1200 Ohm. Then, 4mA more current is drawn.

By thw way, why don't you take a 7805 for the purpose?


List of 21 messages in thread
Zener Turn-on-time      Sun Din      02/20/06 01:16      
   Remedy        Kai Klaas      02/20/06 07:08      
      Thank You Kai      Sun Din      02/22/06 06:18      
         Divider      Kai Klaas      02/22/06 06:34      
            Resistor values      Sun Din      02/23/06 03:27      
               Read again my calculation example      Kai Klaas      02/23/06 05:58      
               drop that stupid resistor to ground      Erik Malund      02/23/06 07:04      
         can you draw the schematic?      Abhishek Singh      02/22/06 07:21      
            Assistance required to post Sch      Sun Din      02/23/06 03:38      
               If using "Windows"        Jon Ledbetter      02/23/06 07:57      
                  Typo Oops      Jon Ledbetter      02/24/06 08:19      
         Using a cap is enough      Kai Klaas      02/22/06 09:13      
            actually for the "typical" use it is not      Erik Malund      02/22/06 09:53      
               Less than 1nsec      Kai Klaas      02/22/06 10:12      
            Resistor instead of the cap      Sun Din      02/23/06 03:32      
   Sch      Sun Din      02/24/06 04:48      
      ALWAYS design to worst case.      Erik Malund      02/24/06 07:42      
      Homepage      Jon Ledbetter      02/24/06 08:13      
         why is that 1k still there      Erik Malund      02/24/06 08:52      
      Aha!      Kai Klaas      02/25/06 06:47      
         Thank you all for your help.      Sun Din      02/27/06 08:42      

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