# Arduino Forum

## Using Arduino => General Electronics => Topic started by: pedroabs on Sep 25, 2015, 10:16 pm

Title: series or parallel resistors
Post by: pedroabs on Sep 25, 2015, 10:16 pm
Could you please explain me if I have series or parallel resistors in the image below?

first I believed series because when I cover the LDR I have less tension but it does not make sense because the energy can flow through green cable or through the LDR if the resistance is low (have light).

I´m a bit confused ...

Thanks again !

Title: Re: series or parallel resistors
Post by: larryd on Sep 25, 2015, 10:53 pm
The 10K resistor is in series with the LDR.
The green wire is connected to an analog input which has a resistance of ~100M. (for your experiment this can be considered infinite)
.
Title: Re: series or parallel resistors
Post by: DVDdoug on Sep 25, 2015, 11:03 pm
The LDR & resistor are in series.    Essentially zero current flows through the green wire because the Arduino has very-high input impedance (resistance).    The Arduino's analog input can sense the voltage via the green wire, even though no current flows into it.

You've got a Voltage Divider (https://en.wikipedia.org/wiki/Voltage_divider) with one variable resistor.     You can measure the voltage divider's output with a multimeter and the multimeter won't have any effect on the voltage at the output, again because the multimeter has "almost infinite" input resistance/impedance.   If you put a motor at the voltage divider output (or try to power the Arduino from the voltage divider, etc.), the voltage will drop because the motor is in parallel with the bottom resistor and the voltage divider ratio changes.
Title: Re: series or parallel resistors
Post by: pedroabs on Sep 25, 2015, 11:25 pm
Thank for both answers.

One more doubt:

When I cover the LDR I notice that the tension at the green cable increases (I mesured with a multimeter) . I imagine that is because left more tension that does not pass through LDR. Am I right ?
Title: Re: series or parallel resistors
Post by: be80be on Sep 25, 2015, 11:37 pm
The voltage changes because the LDR changes resistance of the voltage divider.

Title: Re: series or parallel resistors
Post by: DrAzzy on Sep 25, 2015, 11:47 pm
"tension" is the wrong word. You mean "voltage" (I hope).

As the light on LDR goes down, the resistance of the LDR increases. This increases the total resistance between across the two resistors, so less current flows (ohm's law) through the two resistors. Since the 10k resistor is still 10k, that means the voltage drop across that resistor is smaller, while the voltage drop across the LDR is larger.

Take a look at Ohm's law - with some basic rearrangement, you can make all the relevant equations. The key thing to remember is that you can assume that no current flows into or out of an arduino pin set as input, so you can set up I = V/R for both resistors - and then they're equal to eachother, so V1/R1 = V2/R2 or V1/V2 = R1/R2, which is of course the basic design equation for a voltage divider.
Title: Re: series or parallel resistors
Post by: be80be on Sep 26, 2015, 12:11 am
He translated to English   tension is voltage
Title: Re: series or parallel resistors
Post by: MarkT on Sep 26, 2015, 12:57 am
"High-tension" is normal English usage for _very_ high voltage (50kV+) in electricity
distribution grids, but "tension" is not used for trivial voltages like 11kV or below!
Title: Re: series or parallel resistors
Post by: be80be on Sep 26, 2015, 01:24 am
The word voltage can become tension when translated to English. I'm sure the op means voltage
And I know people who call it tension. They call high tension lines or high voltage line take you pick.
But the op used tension for voltage.

What the OP is seeing is the voltage divider changing center voltage because he covered the LDR which makes it read higher where as with light on it the reading will be lower.

Now if the OP wants the reading to go high in light and low in dark just swap the LDR and resistor around.
Title: Re: series or parallel resistors
Post by: larryd on Sep 26, 2015, 01:36 am
Use to work with 500KV
That made you full of tension ;)
Title: Re: series or parallel resistors
Post by: be80be on Sep 26, 2015, 01:58 am
I've worked with 8400 that's all the tension I need  :o  But the worst shock I ever got was phone lines 90 volt DC why I didn't think it could shock me I don't know. But never agin.

Here's a pic for the OP

Title: Re: series or parallel resistors
Post by: jack wp on Sep 26, 2015, 02:33 am
The 10K resistor is in series with the LDR.
The green wire is connected to an analog input which has a resistance of ~100M. (for your experiment this can be considered infinite)
.
Even tho I could not view the original image from the OP, I assume they are using an analog input.
Even tho you can assume a digital input has 100M ohms impedence, you can't assume that for an analog input pin. Analog input pins use a capacitor, sample and hold, which is designed for about 10k input impedance. A small capacitor (0.1uf) on the analog pin, to ground may be a help.

Title: Re: series or parallel resistors
Post by: Grumpy_Mike on Sep 26, 2015, 05:34 am
Or there is this
Title: Re: series or parallel resistors
Post by: jack wp on Sep 26, 2015, 07:08 pm
@ Grumpy, Was that you in that youtube video? Which one?  LOL.
Title: Re: series or parallel resistors
Post by: larryd on Sep 26, 2015, 07:23 pm
He installed the glow hardware.
Title: Re: series or parallel resistors
Post by: be80be on Sep 26, 2015, 07:53 pm
He installed the glow hardware.
LOL he can keep that Job.
Title: Re: series or parallel resistors
Post by: Grumpy_Mike on Sep 26, 2015, 07:59 pm
@ Grumpy, Was that you in that youtube video? Which one?  LOL.

No I'm not that young.  :)
Title: Re: series or parallel resistors
Post by: Paul__B on Sep 28, 2015, 04:42 am
But the worst shock I ever got was phone lines 90 volt DC why I didn't think it could shock me I don't know. But never agin.
Perhaps because it was a very non-standard phone line?

Telephone lines are 50 V DC.  (Ring current is 80 V and rather nasty.)
Title: Re: series or parallel resistors
Post by: be80be on Sep 28, 2015, 09:06 am
They bump up the voltage it was a main trunk of 10,000 pairs and the old terminal blocks not puch down like they use now the plant has 5,000 phone lines in it. I was turning on the office and their bank phones and you don't have much room I rub a block that the cover was off I thought it had killed me LOL..

Title: Re: series or parallel resistors
Post by: jack wp on Sep 28, 2015, 06:48 pm
"Telephone lines are 50 V DC.  (Ring current is 80 V and rather nasty.)"

Did you mean to say "Ring current is 80 Amps and rather nasty."?

Wow, that's a lot of current.

Am I to picky?
Title: Re: series or parallel resistors
Post by: polymorph on Sep 29, 2015, 12:38 am
No, ring voltage is about 80Vac in the USA and can give you quite a shock.

Don't ask how I know....
Title: Re: series or parallel resistors
Post by: be80be on Sep 29, 2015, 12:40 am
A T1 span can have up to +-130 Volts of DC power superimposed on It. It will wake you up real fast.
So where all the phone lines of the plant I worked at some how had the same voltage of the 3 t1 lines. I guess I tested them. The guy who worked in the room run the servers and did daily backups said he would never go where I had to work I was uses to normal phone lines. I hooked my wire tracer in the office was trying to find it where the main trunk lines come in there's a wall with 10,000 lines on old lugs you use a nut driver not punch down. And it was hot the sweat on my hand and my side hit some that didn't have covers It's a shock you'll remember it hangs on to you LOL I thought It stopped my heart.
I later got to talk to the guy that handled the phones and t1 lines he said don't let it fool you these lines could kill you. I laughed I know I know it dang near did.
Title: Re: series or parallel resistors
Post by: jack wp on Sep 29, 2015, 12:49 am
A T1 span can have up to +-130 Volts of DC power superimposed on It. It will wake you up real fast.

Yes, it is not just dc, but also ac superimposed on it. So ~80v ac, on top of ~48v dc, will not be what you may expect to feel.

I think we have drifted off topic. I am sorry. Does the OP have more questions?