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02/09/14 17:00
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Msg Score: +1
 +1 Good Question
#190268 - Interfacing 12V relay with AT89C51
1. I have to interface 12V relay & used circuit used is:

I have used R2=1K & R3=4.7K. Relay is operating fine but I have seen my pin gets loaded. Link says R2 can be avoided also. but i used for safety.
When logic is +5V at AT89C51, due to loading voltage across it reduces to 1V or less.

What should be correct values I should select.

2. I have to connect relays to PORT0 also. Now for using port0, first I needed external 10K sip after which I will connect the circuit. Is this correct??

Because I think this created two pull-ups across port0. One is 10K from sip & other is 4.7K(R3). Should I remove 10K sip from port0 in this case.

3. What should be max IOH & IOL for AT89C51.
I was reading:

On Page 10, maximum Iol per 8 bit port is 26mA(port 0) & 15mA other ports.

What should be Ioh per port or per pin

List of 4 messages in thread
Interfacing 12V relay with AT89C51            01/01/70 00:00      
   Mostly there            01/01/70 00:00      
      Then what do to avoid loading??            01/01/70 00:00      
         Pin doesn't pull up            01/01/70 00:00      

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