# Measuring the voltage drop across electrical components doesn't match the calculated values

I have a newbie question regarding circuit analysis.

I have a simple circuit with a voltage source of +4.32V (output of a u7805C voltage regulator), an 18K Ohm resistor, a normally open push button, and a 39K Ohm resistor.

The schematic of the circuit is shown below:

simulate this circuit – Schematic created using CircuitLab

Below is a table comparing the calculated and simulated values vs the values measured with an oscilloscope.

When I measure the value across the 18KOhm resistor I get a 4.16V when the switch is open and closed. Shouldn't the measured voltage drop across the 18KOhm be the same as that in the simulation?

EDIT and new findings

This is the circuit, I am using a solderless breadboard. I am using a microcontroller development board's (MSP430FR6989) to power the circuit, the 5V and GND pins are connected to a uA7805C voltage regulator to

The schematic of the circuit is shown below

simulate this circuit

Here is a picture of the circuit

Measuring the Voltage between the rails:

Measuring the output voltage of the regulator:

Measuring the voltage drop across the 18KOhm resistor:

However, I replaced development board power and replaced it with a 9V Alkaline Battery and I got different results get any issues:

These are the results:

I think the problem was with MSP430 powering the circuit, but that is what I don't understand I am using is only to power the circuit (5V power supply), and the voltage between the 5V pin and the GND pin was 5.12V

I think I figured what the problem was

I think the problem was the ground clip of the oscilloscope was connected to a point other than the ground of the circuit.

I connected it like that:

and I got these oscilloscope readings when the switch is open:

These are the oscilloscope readings when the switch is closed:

As mentioned the Oscilloscope's ground probe should only be connected to the ground, it is not like a multimeter where we can probe a voltage with reference to any other voltage point.

• voltage across a resistor in an open circuit is zero. are you sure you are not measuring it in proportion to ground ? May 24, 2020 at 10:26
• @TirdadSadriNejad Yes I am sure, I am measuring the voltage across the R1 resistor May 24, 2020 at 10:32
• Do you get the same result by measuring the voltage with a multimeter? May 24, 2020 at 10:40
• 4.32 volts coming from a 7805 - that doesn't sound right to me. Fix that first is my advice. May 24, 2020 at 10:46
• First rule of scope measurements: never ever move ground clip from circuit ground. While measuring across 18k you are shorting scope ground and PSU ground via mains earth PE. May 24, 2020 at 10:57