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Kai Klaas
09/10/12 04:27
  09/10/12 04:29

Read: 484 times

Msg Score: +1
 +1 Informative
#188285 - contacts...
Responding to: Michael Karas's previous message
Michael said:
Also depends upon the type of switch. Some switches actually require a certain minimum amount of current through the contacts to permit them to operate correctly. To low of current and the contacts may fail to make a low impedance connection when contacts come together.

That's true. Even good contacts need a minimum current to operate properly. Bourns stated in some application notes, that across metallic contacs at least 100µA should flow. They mean cermet trimmer by this, but this is also true for mini switches. Only massive gold contacts can switch even lower currents. They are ideal for so called "dry switchings".

On the other hand, too high currents can also damage the contacts. No more than about 10mA should be allowed to flow across a mini switch. So, discharging a cap directly through a mini switch without any current limiting resistor is no good design practise.

For a typical 80c51 µC with its bidirectional port topology the logic 0 input current can be less than 100µA. So, it's a good idea to add a pull-up when a mini switch is connected to a port. I prefer something between 4k7 and 22k: 4k7 when considerable noise and interference is to expected and 22k if noise is not an issue. So, a 10k pull-up is a good cpmpromise which works well in most applications.

When pull-ups in arrays are used they do not waste relevant board space.

Kai Klaas

List of 4 messages in thread
Regarding pull up resistors      Sai Siddharth      09/08/12 23:08      
   Depends      Per Westermark      09/09/12 03:35      
      ++ Depends      Michael Karas      09/09/12 13:31      
         contacts...        Kai Klaas      09/10/12 04:27      

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