Math help

Community Education
1

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?

Thanks for reading! :slight_smile:
//Mikael

2

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

3

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 doesn´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... :sleeping:

//Mikael

4

Mikae1:
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.

5

ah great, thanks!