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Topic: What if I use 230 ohm resistor with 5V input to Ground? (Read 5115 times) previous topic - next topic

Wawa

You're misinformed.

The definition of input impedance is the resistance seen to GND . The resistance seen from the input pin to ground is about 100 Mohms.

:smiley-sad: I'm with jack wp here.

I think 100Mohm is the estimated leakage impedance of the chip's input circuit.
A diode to supply, a diode to ground, a parasitic input cap to ground, a pullup resistor with mosfet switch to supply, and whatnot.
But no resistor to ground.
The datasheet only mentions a switching circuit to ground in sleep mode.

And then there is the leakage of the circuit board, solder paste etc.
The total leakage to ground might well be more than the leakage to supply.
Leo..

raschemmel

#31
Aug 06, 2015, 03:35 pm Last Edit: Aug 06, 2015, 04:32 pm by raschemmel
Let's review,

Here's Jack wp's post from Reply #22:
Quote
The input pin represents a 100 Mohm resistance, not necessarily to ground, but to input signals. If this ls left floating, there can be triggers above and below the threshold to represent a high, and a low signal.

The pullup/pull down resistor is only out of the circuit when the button is pressed.

The pullup/pulldown resistors  are providing the voltage level only while the button is not pressed. Then the button overrides them once pressed.
Let's take those statements separately:
A-
Quote
The input pin represents a 100 Mohm resistance, not necessarily to ground, but to input signals. If this ls left floating, there can be triggers above and below the threshold to represent a high, and a low signal.
Here is my statement from Reply#21:
Quote
An input should never be left floating. A pullup or pulldown resistor (10k or larger) should be used.
I don't see any conflict between those two statements. Whether the 100 Mohms is to ground or to the input is irrelevant. The point is it too high a resistance to be concerned about how much power is dissipated by a pullup/pulldown resistor because those are across the supply when the button is pressed.


B-
Quote
The pullup/pull down resistor is only out of the circuit when the button is pressed.
Obviously this is false because pressing the button shorts the pullup/pulldown resistor to the other supply rail resulting in the pullup/pulldown resistor being placed across the supply , which , in my mind is anything but "out of the circuit" , unless you mean out of the input impedance circuit.

C-
Quote
The pullup/pulldown resistors  are providing the voltage level only while the button is not pressed. Then the button overrides them once pressed
I don't see the point of this statement.  So what ?
"providing the voltage level" is the sole purpose of a pullup/pulldown resistor. What's your point ?


When you consider the high resistance of 100 Mohms, whether it represents an impedance to ground or an impedance to the input , none of the above comments have any relevance because the value of a pullup/pulldown resistor is NORMALLY chosen to be in the 10k to 50k range. I fail to see what difference it makes which is correct "100 Mohms to ground" OR "100 Mohms presented to the input".  If I conceded you were right on that point, what have we accomplished ?

Quote
The definition of input impedance is the resistance seen to GND . The resistance seen from the input pin to ground is about 100 Mohms. 
So, what ? (difference does it make ?)
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Paul__B


raschemmel

Quote
I'm glad I kept out of this discussion! 
That was wise, I must admit.  ;D
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

jack wp

So I've build up a button like in this example but instead a 10k Ohm resistor I used a 230 Ohm one. The board run fine and the button example works correctly but I'm still worried if there will be any long-time consequences from this. I'm a total newbie at electronics.

Also when the button is pressed for a long time the resistor get a little hot (not like to hurt your skin but still).
@bsld, did you get your question answered? Do you have further questions? Sorry for all the confusion.

raschemmel

#35
Aug 06, 2015, 06:18 pm Last Edit: Aug 06, 2015, 06:18 pm by raschemmel
Quote
but instead a 10k Ohm resistor I used a 230 Ohm one. The board run fine and the button example works correctly but I'm still worried if there will be any long-time consequences from this. I'm a total newbie at electronics.  
Quote
but I'm still worried if there will be any long-time consequences from this
If you held the button down continuously, you would need to hold it down for 10000 hours to consume 1 kW of electricity (@0.1W )  (about $ 0.15/per kW in CA, USA).

10000 hours / 1 second per button press equates to 36 million button presses to consume 1 kW of electricity.

I think the operative phrase here is "long time "...

Is that long enough for you ?
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Wawa

I think we all understand the need for a pull-up or pull-down resistor when connecting a switch.
And that the value is not that critical.
The tiny switch might not like values below 100ohms (current), and values above 1Megohm (leakage, hum pickup etc.).
I think 10k is a good value for tact switches.

My point in post#30 was that there is NO 100Megohm input resistor to ground.

How to measure?
Take a DMM set to volt. Most of them have a 10Megohm impedance.
If you would measure between +5volt and input pin, you would make a 1:10 voltage divider, and measure 1/11 of 5volt.
That is not the case.
I measured about the same (hum) between in and +5 as between in and ground.
The resistance I measured this way on an UNO was more than 1000Megohm.
Leo..
 

raschemmel

#37
Aug 06, 2015, 11:52 pm Last Edit: Aug 07, 2015, 12:09 am by raschemmel
Quote
My point in post#30 was that there is NO 100Megohm input resistor to ground.
Again, I ask, so what ? If I concede on that point, what difference does it make to the OP or anyone else if we all agree the input is High Impedance ? I was just taking a rough guess as to the impedance but I don't see how that changes anything. Everyone agrees it is High Impedance, and that it shouldn't be left floating. My point is that if it is 1000 Mohms as you say and I was off by 900 Mohms, at the end of the day is that going to change anyone's plans ?

Quote
If you would measure between +5volt and input pin, you would make a 1:10 voltage divider, and measure 1/11 of 5volt.
That is not the case.
I measured about the same (hum) between in and +5 as between in and ground.
The resistance I measured this way on an UNO was more than 1000Megohm.
I don't really follow this. You are not reporting resistance readings in your description of how to measure. You are discussing voltage
Quote
and measure 1/11 of 5volt.
and then at the end you report 1000 Mohms.
Did you or did you not set your meter to resistance and measure from the input pin to GND with the power off ? (I don't know any other way to do it and I didn't follow your instructions. If I were to learn how to measure the input impedance of such a high impedance input, I would consider that a consolation prize. I haven't got anything else useful out that whole discussion other than that I was wrong about the value.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

jack wp

We are into a discussion that is not on topic for what the OP ask.

If you would like to, start a new topic, and we can discuss it there.  Let us know the new topic.

I can imagine that the OP is now more confused, than when they started this thread.

raschemmel

Agreed. Let's hope he at least got the part about 10k resistors.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

jack wp

Enough guys on this thread.
If you want me to explain it to you, start a new thread.  LOL

Wawa

@raschemmel.
One more thing before I stop.

A DMM set to current is a voltmeter across a very small resistor.
A DMM set to volt is a voltmeter across a 10Megohm resistor.

So in both cases it's a current meter.

If you realise that, you will understand my post.
Leo..

raschemmel

Quote
A DMM set to current is a voltmeter across a very small resistor.
A DMM set to volt is a voltmeter across a 10Megohm resistor.

So in both cases it's a current meter.
 
Ok. That's useful information. Thanks.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Paul__B

A DMM set to current is a voltmeter across a very small resistor.
A DMM set to volt is a voltmeter across a 10Megohm resistor.

So in both cases it's a current meter.
I don't think so.

The first is true.

In the second case, you are trying to say that a voltmeter is a current meter in series with a large resistor - well, that is what an analog meter is.

A DVM is either an electrostatic voltmeter (with FET input and an inherent extremely high resistance) directly connected to the input terminals, or the same connected to a resistive voltage divider.  The current is relevant only to the action of the voltage divider.

MarkT

Off subject: but once I applied for a job with Linear (sorta a small scale IBM) about 40 years ago. I knew TTL logic circuits (7400 series ). They gave me a written test, about logic ttl ckts. I failed it. Come to find out, their low was -5v, and their high was ground (I never had a clue). I guess it's all relative.
That's probably ECL, not TTL, and its -5.1V, not -5.0V.  Something like 30mW per gate IIRC, so liquid cooling
becomes necessary when you build processors out of it.  TTL was sometimes used alongside ECL in
which case it would be run from the same voltage rails.  The reason the +ve rail was ground is that
ECL voltages are close to the positive supply rail so you get much better noise immunity having that as
a plane.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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