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Topic: How to read a negative voltage (Read 8 times) previous topic - next topic

Jassper

Ok, I understand the concept here, but would anyone want to explain to me what is actually happening with this divider on an elementury level? I like to understand the "why" instead of just "because".

Thanks, and thanks for both youe help.

Pepe34

Voltage divider is simple :
Let have two voltages : Vminus and Vplus, if I understand you have them as -5V and +5V. Mark resistors as Ra and Rb.
A current through divider is I = (Vplus - Vminus) / (Ra + Rb)
It creates a voltage drop on Ra resistor : Vdrop = I * Ra
So you have on analog input pin : Vminus + I * Ra

Pepe34

Practical example, with your values -5V and +5V and let say both 10 kOhm resistors :
I = (5 - - 5) / (10 + 10) = 10 / 20 = 0.5mA
Voltage on resistor V = 0.5mA * 10 kOhm = 5V
Added to -5V you have  -5 + 5 = 0 V
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Let your input voltage is 0V :
I = (5 - 0) / (10 + 10) kOhm = 0.25mA
On resistor V = 0.25 mA * 10 kOhm = 2.5V
Added to OV : 0 + 2.5 = 2.5V

Pepe34

Practical remark : to have stable results, +5V supply must be stabilized nad 2.5V reference voltage (TL 431, LM336 ...) is recomended for full range conversion result.

Jassper

Thanks Pepe34, single voltage dividers I know, but I couldn't get the math right with the neg voltage.

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