Diode logic doubt (digital circuit)

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Discussion Overview

The discussion revolves around understanding the behavior of diode logic circuits, specifically focusing on the output conditions based on different input states. Participants explore the implications of input voltages and the role of resistors in determining output logic levels.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why the output is 1 when both inputs are 1, suggesting that the resistor R should cause a voltage drop.
  • Another participant explains that if either input is 1, current flows through the diode, resulting in a voltage drop across the resistor, which leads to an output of 1.
  • A participant expresses confusion about the relationship between current flow through the diode and the output being 1.
  • It is noted that if both inputs are 0, no current flows through the diodes, and the resistor pulls the output voltage down to ground, leading to an output of 0.
  • One participant suggests considering the logic levels as 5V for logic 1 and grounding for logic 0, explaining the voltage drop across the diode and how it affects the output.
  • A later reply introduces the concept of ideal diodes to clarify the relationship between input and output nodes in the circuit.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the interaction between input states, diode behavior, and resistor effects, indicating that multiple competing views remain on the topic.

Contextual Notes

Some participants highlight potential confusion arising from mixing voltage levels with logic levels, and there is an emphasis on the importance of understanding how voltage drops affect output in the context of diode logic.

rehcarlos
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Hello guys,

I'm not getting the output of the diode logic circuits:

http://upload.wikimedia.org/wikipedia/commons/1/1f/Diode-OR2.png

If both inputs are 1, then there is current across the diodes then the output is 1, but shouldn't it drop because of the resistor R?

If both inputs are 0, then why is there any kind of output?

I'm really lost,
Thanks in advance
 
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rehcarlos said:
Hello guys,

I'm not getting the output of the diode logic circuits:

http://upload.wikimedia.org/wikipedia/commons/1/1f/Diode-OR2.png

If both inputs are 1, then there is current across the diodes then the output is 1, but shouldn't it drop because of the resistor R?

If both inputs are 0, then why is there any kind of output?

I'm really lost,
Thanks in advance

If either input is 1 (or if both are 1), then forward current through the diode will cause a voltage drop across the resistor, hence the output is also a 1.

If neither diode has a forward current (both inputs are 0), then the resistor pulls the output voltage down to ground, and hence the output is a 0.

Does that make sense?
 
Hi berkeman,

If either input is 1 (or if both are 1), then forward current through the diode will cause a voltage drop across the resistor, hence the output is also a 1.

If either input is 1, then current will flow through the diode and there will be a voltage drop across the resistor. Ok I get that... but I don't understand the link between that and the output being 1

If neither diode has a forward current (both inputs are 0), then the resistor pulls the output voltage down to ground, and hence the output is a 0.

If both input are zero then no current will flow through the diodes. I don't get why the resistor pulls the output voltage down to ground
 
Okay, maybe the mixing of voltage levels and logic levels is what is causing the problem. Think of the "logic 1" inputs as 5V voltage supplies, and logic 0 inputs as grounding out the inputs. When you apply 5V to either input, that causes a current to flow through the diode and through the resistor to ground. There is about a 0.6V drop across the diode, so that means that there is a 4.4V drop across the output resistor. With normal logic levels, that 4.4V still qualifies as a logic 1.

If there is no voltage source driving the resistor through the diodes, then there will be no voltage drop across the output resistor, and that will be seen as a logic 0.
 
The output node comes before the voltage drop across the resistor.

If it helps, consider ideal diodes. So if the input is 5v, the diodes are replaced with a short. Now your output and input nodes are the same node and you have...

out/in =5v -----/\/\/\/\/------Gnd
 

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