The purpose of these resistors is to force an output or input to go to a defined state. This may happen for lots of different reasons. The alternative is to let that point in the circuit float. If it is an input then sometimes it will be a zero and somtimes it will be a one. Clearly an undesireable situation.Originally posted by Surendar@Apr 25 2006, 02:45 AM
What is the purpose of Pull up and Pull down resistors and how the absence of
these will affect the circuits!!
Thanks
Surendar
[post=16502]Quoted post[/post]
Sounds great... but I have two questions if you don't mind SparkManOriginally posted by SparkMan@Apr 27 2006, 12:41 AM
But that might not happen. Because the switches are normally open, you have left both inputs in an open circuit, thus they are free to go to the voltages they float at.
So if one input happens to float at 4.2 volts, then it is high, even without pussing the button, thus the logic you want, will not be what you want.
So you use "Pull-Down" resistors, and you connect them between the inputs and ground. They close the circuit and drop the voltage on the inputs to less than .8 volts, or a low.
[post=16596]Quoted post[/post]
No. A TTL input is actually the emitter of a multiple emitter transistor. It wants to source current into some kind of load. The amount of current is a "standard TTL load" of 1.6 mA. The biggest pulldown resistor you could use would be 0.8V/1.6 mA or 500 ohms. A smaller resistor would pull the voltage to a lower value. A larger resistor would allow the input to exist in the no-mans land between 0.8V and 2.0V Another kind of current sinking load would be an NPN transistor in an open collector gate or the NPN transistor in the bottom of a totem pole output. One problem with TTL totem pole outputs is that they have a hard time sourcing current and the have a hard time getting all the way to the 5V rail.Originally posted by Mazaag@Apr 27 2006, 12:51 PM
Sounds great... but I have two questions if you don't mind SparkMan
First, you say that when there are no inputs, the voltages they float at. I don't quite understand what you mean by that.. ? could you give me a schematic as an example.
Secondly, (lets just condiser one input) lets say that the "floating" voltage of one of the inputs is around 3 volts. How does connecting this node to a resistor and to ground cause the voltage at tha tnode to drop down to below 0.8 ? wouldn't it just cause whatever floating voltage we had to drop across the resistor and thats it?
Thanks
[post=16622]Quoted post[/post]
The pullup/down resistor is just as important here (or more in fact) to define the logic level. If you let a cmos input float it may do that to the midpoint between + and -. In this state it 'opens' both the positive and negative drive path inside the circuit, resulting in a much higher than normal current flowing through it.Originally posted by Papabravo@Apr 27 2006, 08:55 PM
It's all different with CMOS. Those inputs are high impeadance; they neither sink nor source any appreciable current.
[post=16626]Quoted post[/post]
Originally posted by Mazaag+--><div class='quotetop'>QUOTE(Mazaag)</div><div class='quotemain'>Sounds great... but I have two questions if you don't mind SparkMan
First, you say that when there are no inputs, the voltages they float at. I don't quite understand what you mean by that.. ? could you give me a schematic as an example.
Secondly, (lets just condiser one input) lets say that the "floating" voltage of one of the inputs is around 3 volts. How does connecting this node to a resistor and to ground cause the voltage at tha tnode to drop down to below 0.8 ? wouldn't it just cause whatever floating voltage we had to drop across the resistor and thats it?
Thanks
[post=16622]Quoted post[/post][/b]
How can you have two different values for each current direction? How do you set the direction? Resistors won't do that.Originally posted by SparkMan@Apr 28 2006, 03:30 AM
If I remember correctly, sinking means current flows into an input/output, and sourcing means current flows out of the input/output.
Pull resistors are properly calculated using these values.
For inputs, 1.6mA is the sinking, and 40uA is sourcing
For outputs, 800uA is sinking, 16mA is sourcing.
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