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### Topic: Math help (Read 3265 times)previous topic - next topic

#### Mikae1

##### Mar 16, 2014, 03:27 pm
Hi!

I`m very bad at mathematics, and not so good in electronics either, but it feels like it should be possible by knowing what a pullup resistor have, calculating what the pulldown resistor must be, if I can measure the regulated voltage in the middle.
Is it?

//Mikael

#### robtillaart

#1
##### Mar 16, 2014, 07:47 pm
you are right!

if the resistors are  2:1  the voltage will be divided  2:1

example

+5V  ----[5K]----X ----[ 10K] ---- 0V

X = (5V-0V) / (5K + 10K) * 10K = 5/15*10 = 50/15 = 3.3V

+12V  ----[20K]----X ----[ 10K] ---- 0V

X = (12V-0V) / (20K + 10K) * 10K = 12/30*10 = 120/30 = 4.0V
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

#### Mikae1

#2
##### Mar 16, 2014, 11:44 pmLast Edit: Mar 17, 2014, 02:58 am by Mikae1 Reason: 1
Thanks for the math lesson, it was not to waste!

However the thing is I do not know the pulldown resistors value (variable resistance)
So if I put it like this:
+4.6V---[6.2K]---0.07v---[Y]---0V

Possible to know Y?
Your X is where my Arduino is measuring 0.07 volt.
In above example I know Y is about 109ohm, but Arduino doesnÂ´t... nor will I when the variable resistance changes.

But I guess its not possible as we need to know the current(!) current in the circuit to calculate that R...

//Mikael

#### robtillaart

#3
##### Mar 17, 2014, 06:40 pm

Thanks for the math lesson, it was not to waste!

However the thing is I do not know the pulldown resistors value (variable resistance)
So if I put it like this:
+4.6V---[6.2K]---0.07v---[Y]---0V

Possible to know Y?

Yes very well possible - Ohms law (2x).
- Voltage = Amps * resistance  or
- Amps = voltage / resistance or
- resistance = voltage/amps
there is a drop of 4.53 volt over 6200 ohm => 4.53/6200 = 0.73 milli ampere
That current flows also through -[ Y] -resulting in 0.07Volt
Y = 0.07 / 0.73e-3 = 95.8 ohm

check: the resistors are about  64.7  to 1   and the the voltages are ..............  64.7 : 1  yeah it 's correct.

Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

#### Mikae1

#4
##### Mar 18, 2014, 10:53 pm
ah great, thanks!

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