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Using Arduino => General Electronics => Topic started by: bsld on Aug 05, 2015, 07:02 pm

Title: What if I use 230 ohm resistor with 5V input to Ground?
Post by: bsld on Aug 05, 2015, 07:02 pm
So I've build up a button like in this example (https://www.arduino.cc/en/tutorial/button) 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).
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: ApexM0Eng on Aug 05, 2015, 07:31 pm
The resistor is there to make sure that the pin is connected and pulled-down to ground as its default state. When you close the switch it receives 5V instead. However because your resistor value is so low, it is conducting a fair amount of current. This resistor is meant to limit the amount of current can flow to ground.

As current flows through the resistor it heats up, which is a good indication it should be removed for a larger one.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: groundFungus on Aug 05, 2015, 07:39 pm
Or ditch the pull down and wire the switch from ground to the input and enable the internal pullup.

Code: [Select]
pinMode(pin, INPUT_PULLUP);
 

The switch will read LOW when pushed.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: ApexM0Eng on Aug 05, 2015, 07:49 pm
'Internal pull-ups' is only three tutorials past 'button', you'd think they could put them together to teach about using one or the other. Rather than messing around with external ones first, then telling you there are better ways for simple projects.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 05, 2015, 07:54 pm
If only the world was perfect. Then there would be no need for this forum.  LOL
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: groundFungus on Aug 05, 2015, 07:55 pm
I think it is because people associate LOW with off and have a hard time with off being on.  
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 05, 2015, 08:05 pm
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.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 05, 2015, 08:16 pm
Quote
have a hard time with off being on.   
That's understandable...
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 05, 2015, 08:28 pm
"but instead a 10k Ohm resistor I used a 230 Ohm one."
That substitution may draw lots more power than needed, but It will not hurt anything.
In most cases, a button is not held down for a long time, so if the button is only pressed for one second, then there is not much difference is current draw.

But, I suggest you pick up some more resistor values. Other circuits may not be so forgiving.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: bsld on Aug 05, 2015, 08:44 pm
Or ditch the pull down and wire the switch from ground to the input and enable the internal pullup.

Code: [Select]
pinMode(pin, INPUT_PULLUP);
 

The switch will read LOW when pushed.
What are the drawbacks of this method - it will consume more energy I guess?
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 05, 2015, 08:55 pm
Arduino GPIO pins have two modes (INPUT or OUTPUT).
When configured as an INPUT, the pin becomes a high impedance (100 Mohms) input)
If you are sensing a push button, the pin MUST be configured as an INPUT, therefore
you can short it directly to 5V or GND  without using any power to speak of . What you CAN'T do is connect the push button IN BETWEEN 5V and GND and then push the button because you will short your 5V power supply. Thus, ANY DISCUSSION OF POWER CONSUMPTION OF AN I/O PIN CONFIGURED AS AN INPUT is completely irrelevant and non valid. Such a discussion would only  be appropriate for a pin configured as an OUTPUT .

Do you understand the significance of a input impedance of 100 Mohms ?

If you are discussing the power consumption of a 10k ohm pullup resistor, it depends on WHAT you connect that input pin to. By itself , an INPUT pin with a 10k pullup is part of a voltage divider with 10k in the upper position and 100 Mohms in the lower position. Any discussion of input pin power consumption would have to be based on the circuit that you connect to that input pin.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 05, 2015, 09:00 pm
"What are the drawbacks of this method - it will consume more energy I guess?"

Depending on how often the button is pressed, and how long it is held down, is the processor run 24/7 ?

With the natural order of things, I think you would be right.  But I still suggest getting a varity pack of resistors.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: DVDdoug on Aug 05, 2015, 09:04 pm
Quote
What are the drawbacks of this method - it will consume more energy I guess?
Less power...  IIRC, the internal pull-up is 20k-50k Ohms.  

Note that power is only consumed when the switch is on.  (That's true with pull-ups or pull-downs.)

From Ohm's Law 5V/250 Ohms is 20mA, and that's about the same current as an LED.  No big deal, but pull-ups or pull-downs are usually in the range of 1K to 10K.

As far as energy, power is calculated as Voltage x Current, so 250 Ohms is 1/10th of a Watt.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 05, 2015, 09:06 pm
Quote
it will consume more energy I guess?"
How long do you plan to hold the button down ? (effectively connecting a 230 ohm resistor across 5V and GND.

I = V/R = 5V/230 ohms = 0.0217 A (21.7 mA)
P = I x V = 0.0217 A x 5V = 0.1086 W = 109 mW (while the button is held down)

Can you spare 100 mW ? (for 1 second ?)
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: Grumpy_Mike on Aug 05, 2015, 09:09 pm
For the full lowdown on this question read this:-
http://www.thebox.myzen.co.uk/Tutorial/Inputs.html (http://www.thebox.myzen.co.uk/Tutorial/Inputs.html)
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 05, 2015, 09:15 pm
"If you are sensing a push button, the pin MUST be configured as an INPUT, therefore
you can short it directly to 5V or GND "


I beg to differ:
If your switch will short it directly to 5V, then the other side of the coin is,
" short it to GND, leave it floating, or use a small pull down resistor".

The default being shorted to ground, nor leave it floating will not be a functional option.
So, use a small pull down resistor is the only viable option.
I could be mistaken tho.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: bsld on Aug 05, 2015, 09:17 pm
Arduino GPIO pins have two modes (INPUT or OUTPUT).
When configured as an INPUT, the pin becomes a high impedance (100 Mohms) input)
If you are sensing a push button, the pin MUST be configured as an INPUT, therefore
you can short it directly to 5V or GND  without using any power to speak of . What you CAN'T do is connect the push button IN BETWEEN 5V and GND and then push the button because you will short your 5V power supply. Thus, ANY DISCUSSION OF POWER CONSUMPTION OF AN I/O PIN CONFIGURED AS AN INPUT is completely irrelevant and non valid. Such a discussion would only  be appropriate for a pin configured as an OUTPUT .

Do you understand the significance of a input impedance of 100 Mohms ?

If you are discussing the power consumption of a 10k ohm pullup resistor, it depends on WHAT you connect that input pin to. By itself , an INPUT pin with a 10k pullup is part of a voltage divider with 10k in the upper position and 100 Mohms in the lower position. Any discussion of input pin power consumption would have to be based on the circuit that you connect to that input pin.
So what are you saying is that I can directly connect the 5V through switch into the Input pin because it have high resistance?

But why is the example linked here so complicated then?

Also what will happen if I press the button before the 'setup' function executes or if I mistakenly don't configure the pin as INPUT?
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: ApexM0Eng on Aug 05, 2015, 09:25 pm
Also what will happen if I press the button before the 'setup' function executes or if I mistakenly don't configure the pin as INPUT?
Arduino pins are defaulted to inputs, which is a safe state.

Consider wiring two inputs to each other. Nothing happens as both expect but don't receive.
Consider wiring two outputs to each other. The pins are now trying to source current into each other which is bad.

Default inputs is safe.

Also if your pushing the button prior to the setup function, the pin will sink current, but nothing will happen. The MCU is safe and will wait for your code to tell it to do something with it's current pin state.

https://www.arduino.cc/en/Tutorial/DigitalPins

This explains further on pins and why pull-ups/pull-downs are necessary.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 05, 2015, 09:31 pm
Quote
The default being shorted to ground, nor leave it floating will not be a functional option.
So, use a small pull down resistor is the only viable option.
I could be mistaken tho.
Think about it.
The input pin represents a 100 Mohm resistor to ground.
The pullup /pulldown resistor is out of the circuit UNLESS the button is pressed.
This means the pullup/pulldown resistors are not conducting any current unless you press the button.

CASE -A (230 ohm pullUP resistor)
button open => no current
button pressed => (230 ohm resistor across 5V and GND => 21mA/0.1W consumed


Quote
or use a small pull down resistor".
I think you mean use a LARGE pulldown resistor. (230 ohm is already way too SMALL)
A pullup/pulldown resistor should be  10k >= optimum <= 50k

230 is a value NO ONE with any electronics experience would ever consider using unless it was LITERALLY the ONLY OTHER resistor they had available. PERIOD

CASE-B(230 pullDOWN resistor)
button open => no current
button pressed => (230 ohm resistor across 5V and GND => 21mA/0.1W consumed

Quote
Consider wiring two outputs to each other. The pins are now trying to source current into each other which is bad.  
This topic is about an INPUT pin. The above comment is not relevant to this topic.


Quote
Also what will happen if I press the button before the 'setup' function executes or if I mistakenly don't configure the pin as INPUT?
That's EXACTLY WHY no one would ever consider using such a low value as a pullup/pulldown resistor.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: bsld on Aug 05, 2015, 09:38 pm
Think about it.
The input pin represents a 100 Mohm resistor to ground.
The pullup /pulldown resistor is out of the circuit UNLESS the button is pressed.
This means the pullup/pulldown resistors are not conducting any current unless you press the button.

CASE -A (230 ohm pullUP resistor)
button open => no current
button pressed => (230 ohm resistor across 5V and GND => 21mA/0.1W consumed


I think you mean use a LARGE pulldown resistor. (230 ohm is already way too SMALL)
A pullup/pulldown resistor should be  10k >= optimum <= 50k

230 is a value NO ONE with any electronics experience would ever consider using unless it was LITERALLY the ONLY OTHER resistor they had available. PERIOD

CASE-B(230 pullDOWN resistor)
button open => no current
button pressed => (230 ohm resistor across 5V and GND => 21mA/0.1W consumed

This topic is about an INPUT pin. The above comment is not relevant to this topic.


That's EXACTLY WHY no one would ever consider using such a low value as a pullup/pulldown resistor.
What about my first question - can I directly connect the 5V through switch into the Input pin and will it work that way?

Because you've said that the input pin have it's own resistors.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: ApexM0Eng on Aug 05, 2015, 09:43 pm
Assuming you haven't enabled the internal pull-up resistors? It will work, but not well. They have very high impedance as inputs but that also means they are very sensitive. So if you leave a pin "floating" (not connected to anything when the button isn't pressed) it will give off random values. Often messing with your program.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 05, 2015, 09:44 pm
Quote
Because you've said that the input pin have it's own resistors.  
That isn't exactly what I said. I said the input has an equivilent input impedance of 100 Mohms. If you use internal pullups they are 50k ohms.
With an input impedance of 100 Mohms, 5V connected directly to the pin would draw

I= V/R = 5V/100 Mohms = 0.00000005 A (50 nA)

Can you handle that ?

Quote
Assuming you haven't enabled the internal pull-up resistors? It will work, but not well. They have very high impedance as inputs but that also means they are very sensitive. So if you leave a pin "floating" (not connected to anything when the button isn't pressed) it will give off random values. Often messing with your program.  
A floating pin will be read as a "0" (LOW).
An input should never be left floating. A pullup or pulldown resistor (10k or larger) should be used.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 05, 2015, 09:57 pm
I beg to differ:
"Think about it.
The input pin represents a 100 Mohm resistor to ground.
The pullup /pulldown resistor is out of the circuit UNLESS the button is pressed.
This means the pullup/pulldown resistors are not conducting any current unless you press the button.
"


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.
As I see it.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 05, 2015, 10:03 pm
Quote
The input pin represents a 100 Mohm resistance, not necessarily to ground, but to input signals.
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.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 05, 2015, 10:10 pm
@raschemmel, then that would mean that a digital input that is not connected to anything would always read a low?   Not what I had understood before.  Is that right?
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: Grumpy_Mike on Aug 05, 2015, 10:30 pm
Quote
then that would mean that a digital input that is not connected to anything would always read a low? 
No that is not right. You have misunderstood what raschemmel said.  A floating input can read anything due to pickup.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 05, 2015, 10:42 pm
Quote
@raschemmel, then that would mean that a digital input that is not connected to anything would always read a low? 
There was a discussion about this in another thread about a year ago so out of curiousity
so I wrote a loop to read some floating inputs and print the values to the serial port and it was always low . That doesn't mean that would always be the case but that's what I saw. I think you have to keep in mind that 100 Mohms is not your typical pulldown resistor.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 05, 2015, 10:58 pm
There seems to be a lot of misunderstanding going on here. I am going to back away for now, and check back when the smoke clears. Good luck.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 05, 2015, 11:30 pm
There is no misunderstanding.
I have been working in electronics for over 30 yrs and Mike has over 40 yrs experience. The definition of input impedance hasn't changed  and it's still a bad idea to leave uC inputs floating. If there is any misunderstanding it is at your end.

Quote
To calculate the input impedance, short the input terminals together and reduce the circuit by determining the equivalent circuit with only one component.
In this case, the one component is a 100 Mohm resistor.  (ZL (from the pin to GND))

Input Impedance (https://en.wikipedia.org/wiki/Input_impedance)


Is it a bad idea to leave micro-controller inputs floating ?
Quote
Problem:
Leaving a pin configured as an input floating is dangerous simply because you cannot be sure of the state of the pin. Like you mentioned, because of your circuit, your pin was sometimes LOW or sometimes in no-man's land or could sometimes go to HIGH.

Result:
Essentially, the floating input WILL definitely cause erratic chip operation or unpredictable behaviour. I have noticed some chips froze by simply moving my hand closer to the board (I wasn't wearing a ESD wrist band) or some would have different startup behaviour each time the board would powerup.

Why:
This happens simply because if there is external noise on that pin, the pin would oscillate, which would drain power as CMOS logic gates drain power when they switch states.

Solution:
Most micros nowdays have internal pullups as well, so that could prevent this behaviour from occuring. Another option would be to configure the pin as an output so it does not affect the internals.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: runaway_pancake on Aug 06, 2015, 04:58 am
bsld,

1) in setup() you need:
  pinMode (X, INPUT_PULLUP);  // X is your input pin of choice
2) place your switch between that input pin and Ground.
3) Done

Assuming you have a Normally Open switch, you will look for a LOW when the switch is closed/activated.
That's how it works.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: Wawa on Aug 06, 2015, 11:14 am
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..
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 06, 2015, 03:35 pm
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 ?)
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: Paul__B on Aug 06, 2015, 04:10 pm
I'm glad I kept out of this discussion! :smiley-lol:
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 06, 2015, 04:15 pm
Quote
I'm glad I kept out of this discussion! 
That was wise, I must admit.  ;D
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 06, 2015, 05:11 pm
So I've build up a button like in this example (https://www.arduino.cc/en/tutorial/button) 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.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 06, 2015, 06:18 pm
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 ?
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: Wawa on Aug 06, 2015, 11:38 pm
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..
 
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 06, 2015, 11:52 pm
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.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 07, 2015, 12:01 am
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.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 07, 2015, 12:09 am
Agreed. Let's hope he at least got the part about 10k resistors.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: jack wp on Aug 07, 2015, 12:27 am
Enough guys on this thread.
If you want me to explain it to you, start a new thread.  LOL
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: Wawa on Aug 07, 2015, 12:32 am
@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..
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: raschemmel on Aug 07, 2015, 12:41 am
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.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: Paul__B on Aug 07, 2015, 10:03 am
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.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: MarkT on Aug 07, 2015, 01:41 pm
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.
Title: Re: What if I use 230 ohm resistor with 5V input to Ground?
Post by: Wawa on Aug 08, 2015, 01:38 am
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.
Call it whatever you like.
A display with infinite resistance across a 10Megohm resistor, or a display with zero resistance with a 10Megohm resistor in series.
Electrically it's just a 10Megohm resistor. Independent of the range you select.
Leo..