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### Topic: series or parallel resistors (Read 2093 times)previous topic - next topic

#### pedroabs

##### 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 !

#### larryd

#1
##### Sep 25, 2015, 10:53 pmLast Edit: Sep 25, 2015, 10:54 pm by LarryD
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)
.
No technical PMs.
The last thing you did is where you should start looking.

#### DVDdoug

#2
##### Sep 25, 2015, 11:03 pmLast Edit: Sep 26, 2015, 12:13 am by DVDdoug
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 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.

#### pedroabs

#3
##### Sep 25, 2015, 11:25 pm

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 ?

#### be80be

#4
##### Sep 25, 2015, 11:37 pm
The voltage changes because the LDR changes resistance of the voltage divider.

#### DrAzzy

#5
##### 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.
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#### be80be

#6
##### Sep 26, 2015, 12:11 am
He translated to English   tension is voltage

#### MarkT

#7
##### 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!
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### be80be

#8
##### 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.

#### larryd

#9
##### Sep 26, 2015, 01:36 amLast Edit: Sep 26, 2015, 05:47 am by LarryD
Use to work with 500KV
That made you full of tension
No technical PMs.
The last thing you did is where you should start looking.

#### be80be

#10
##### Sep 26, 2015, 01:58 amLast Edit: Sep 26, 2015, 02:02 am by be80be
I've worked with 8400 that's all the tension I need    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

#### jack wp

#11
##### 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.

#12

#### jack wp

#13
##### Sep 26, 2015, 07:08 pm
@ Grumpy, Was that you in that youtube video? Which one?  LOL.

#### larryd

#14
##### Sep 26, 2015, 07:23 pm
He installed the glow hardware.
No technical PMs.
The last thing you did is where you should start looking.

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