Resisters pull-up and pull down.

NewStamper

The reply to my previous post "Resister's specification" is not showing up on the forum. I am posting it again w/o the strings.

Ugha
writes:
Hopefully that helps a bit. If you need more info on the actual wiring of the resistors, let us know.

1. I am getting the gist of what all of you have contributed. Now I am applying what you and others have said to the example in the Student guide, "What's a Microcontroller?". The example on page 77-79 explains pull down and up resisters. Figure 3-7 is an example of pull down. "When the push button is not pressed, the resister connected to Vss pulls the voltage at pin 3 down to Vss (0 volts)".

Question 1: The default value at Vss was zero before pressing the push button. Why do we need this resister to pull it down to zero? If you let go the push button, should the voltage not go to zero w/o the resister? I think the only purpose of resistors is to avoid a short.

The example in the student guide, has some diagrams, but they have not used the pull terminology in earlier examples. It is a very simple code on P 48:

Question 2:
Do
High 14
Pause 500
Low 14
Loop

Diagram:
Pin 14___/\/\/\___LED____Vss

We have internally connected I/O Pin 14 to Vdd by setting it to High.
Would this be an example of a Pull up resister?

Please let me know what you think?
Thanks

r2d2_bot

·· When the switch is not pressed, only ground can go through the 220 ohm resistor.· However when the switch is pressed, there are now two paths for current flow, through the 220 and the 10K resistors.· Current will take the easiest route, so that is why the 10K resistor is so much larger then the 220 ohm resistor.· Some current will go through the 10K, but not very much.· If both resistors were the same value, then you would have roughly equal amounts of current going through both resistors, which you don't want.

[noparse][[/noparse]Tim]

NewStamper said...
The reply to my previous post "Resister's specification" is not showing up on the forum. I am posting it again w/o the strings.

Ugha
writes:
Hopefully that helps a bit. If you need more info on the actual wiring of the resistors, let us know.

1. I am getting the gist of what all of you have contributed. Now I am applying what you and others have said to the example in the Student guide, "What's a Microcontroller?". The example on page 77-79 explains pull down and up resisters. Figure 3-7 is an example of pull down. "When the push button is not pressed, the resister connected to Vss pulls the voltage at pin 3 down to Vss (0 volts)".

Question 1: The default value at Vss was zero before pressing the push button. Why do we need this resister to pull it down to zero? If you let go the push button, should the voltage not go to zero w/o the resister? I think the only purpose of resistors is to avoid a short.

The example in the student guide, has some diagrams, but they have not used the pull terminology in earlier examples. It is a very simple code on P 48:

Question 2:
Do
High 14
Pause 500
Low 14
Loop

Diagram:
Pin 14___/\/\/\___LED____Vss

We have internally connected I/O Pin 14 to Vdd by setting it to High.
Would this be an example of a Pull up resister?

Please let me know what you think?
Thanks

Amaral

well NewStamper , the reason for the pull down resistor is to PULL THE VOLTAGE DOWN, that may look stupit but its not, when you leave a wire FLOATING, witch is : NOT CONNECTED TO ANYTHING the voltage in this wire is not defined due to eletromagnetic influences and interferences, this may cause peak of voltages up to the limit of triggering the device(in this case Basic Stamp). When you use a Pull Down resistor you garantee that the voltage is LOW up to the value of the resistor that you are using.

In resume, Basic Stamp has a very high impedance on pin defined to be input ( about 1 mhoms = 1.000.000 ohm) that means that if you have anything generanting power to a wire connected to that you will be deviding your input voltage with that value , this means that a single 10K resistor will be more than enough to put the voltage up to levels where the stamp identify as logic 1. I think the best way is for you to LOOK FOR Voltage devider on the internet and try to understand that (is much better if you lookfor it than we teach you , belive me you will learn much more).

But never assume that a floating wire has Zero Volts on it ! it is safer and right to work with everithyng tide up.

Regards

Amaral

allanlane5

1. What Amaral said is correct -- a wire with NOTHING connected to it (except an input pin) is NOT sitting at zero volts. It's 'floating', subject to whatever RF noise is in the air.

2. No, your diagram shows a "current limiting" resistor, not a "pull-up" resistor. Because the purpose of that resistor in that circuit is not to 'pull-up' the voltage on the LED, but instead to limit the current through the LED.

NewStamper

allanalane5:
Wrote:
"2. No, your diagram shows a "current limiting" resistor, not a "pull-up" resistor. Because the purpose of that resistor in that circuit is not to 'pull-up' the voltage on the LED, but instead to limit the current through the LED."

Another question:
Is it how you wire the same resistor that makes it pull-up, pull-down or a limiting current resister as you have put it?

Amaral:
Thank you for pointing me to "Voltage devider". I will look it up.

terry_bear

Newstamper,

A resistor is a resistor. It is called pull-up, pull-down, current-limiting, biasing, whatever, according to the use that you put it to in-circuit... Some resistors are put in power supply circuits in order to provide a path for the massive capacitors to discharge through so that technicians don't receive rude surprises. Those resistors are known by none of the above names, but they are still resistors...

Terry

NewStamper

Thank you very much Terry and the rest of the gang. Now I will polish myself based on all the inputs that I have gathered.

After a few years, I may sign each of my posts as, "I have learnt it all here".