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.

enter image description here

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

enter image description here enter image description here

Measuring the Voltage between the rails:

enter image description here enter image description here

Measuring the output voltage of the regulator:

enter image description here enter image description here

Measuring the voltage drop across the 18KOhm resistor:

enter image description here enter image description here

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

enter image description here

These are the results: enter image description here

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:

enter image description here

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

enter image description here

These are the oscilloscope readings when the switch is closed:

enter image description here

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.

  • \$\begingroup\$ voltage across a resistor in an open circuit is zero. are you sure you are not measuring it in proportion to ground ? \$\endgroup\$ May 24, 2020 at 10:26
  • \$\begingroup\$ @TirdadSadriNejad Yes I am sure, I am measuring the voltage across the R1 resistor \$\endgroup\$ May 24, 2020 at 10:32
  • \$\begingroup\$ Do you get the same result by measuring the voltage with a multimeter? \$\endgroup\$
    – Ali Nategh
    May 24, 2020 at 10:40
  • 1
    \$\begingroup\$ 4.32 volts coming from a 7805 - that doesn't sound right to me. Fix that first is my advice. \$\endgroup\$
    – Andy aka
    May 24, 2020 at 10:46
  • 7
    \$\begingroup\$ 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. \$\endgroup\$
    – carloc
    May 24, 2020 at 10:57

1 Answer 1


enter image description here

Photo 1. (1) 5 V regulator with no decoupling capacitors! (2) Jumpers to (3) supply from a PCB.

enter image description here

Photo 2. (1) 5V and (2) GND. The guys in forensics lab reckon that the 5 V regulator is being fed from a 5 V supply. This won't work!

Your regulator needs "headroom" to work. If you check the datasheet for your regulator you'll find that you need a minimum of about 7 V for the regulator to work. You've also omitted the decoupling capacitors (which for some reason many beginners feel are an optional extra) so the regulator may go into oscillation. Solution: remove the regulator and supply the breadboard directly from the PCB's 5V and GND terminals.

I can see a USB cable plugged into the board. This means that your breadboard has a ground reference to some external power-supply. If your 'scope is using the same ground reference then you must not connect the ground clip to anywhere other than the 0 V rail of your breadboard.

  • \$\begingroup\$ Yes I do see ripples on my scope and it needs decoupling capacitors, but how is that related to my problem, where Vmicro = 5.12V, V18kohm = 4.16V (when open or close). Also, I tried it without the regulator, I got the same problem. I tried it again but the ground clip is connected to the ground V18Kohm -> GND = 4.32V, Vsw->GND = 4.32V, but when I measure voltage drop across V18Kohm I get 4.16V. This problem happened to me when I use the power supply of the microcontroller, and when I connect the scope's GND clip to somewhere other than the ground \$\endgroup\$ May 27, 2020 at 0:18

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