# If same voltage connect each other will output come 0V (More detail here)

#### Sornpraram Xu

Joined Feb 24, 2019
21
Hello
I am trying to make a switch to control LED
In this schematic below
I am trying to operate the Opto-Isolators so the D9 Led could light up.
Me and my friend asume that If same voltage connected(Yellow Circle) the output will be 0v(Blue Circle) Is that true??
My friend tell me that it must have Resistor connect to ground (Red Circle) It that neccesary ? If yes, Why?
Thank for help. #### wayneh

Joined Sep 9, 2010
16,114
I see no purpose to R19. The voltage at U3 will not necessarily be zero, it will depend on the output voltages of the op-amps. Are you sure that the maximum output voltage of the op-amp is high enough to exceed the Vf of the two diodes, the blue one plus the opto-isolator one?

• Sornpraram Xu

#### Tonyr1084

Joined Sep 24, 2015
3,649
Voltages at A, B and C:

A to C = 12 volts
B to C = 12 volts
A to B = 0 volts.

OR

A to C = 18 volts
B to C = 12 volts
A to B = 6 volts

If two potentials are the same then when you measure between them they will be zero volts. You can measure either potential in reference to ground (or negative) you will read the voltage. In the above examples let C = zero volts (ground or neutral). When measuring between A or B and C you get the potential difference. But when you measure between A AND B, if they are at the same potential you will have zero volts. IF A is higher than B then A will be positive in reference to B. If the difference between A and B is 3 volts (for example) then you will measure three volts. Whatever potential A is at, B will be three volts lower in reference to ground (C or negative).

• Sornpraram Xu

#### Sornpraram Xu

Joined Feb 24, 2019
21
I see no purpose to R19. The voltage at U3 will not necessarily be zero, it will depend on the output voltages of the op-amps. Are you sure that the maximum output voltage of the op-amp is high enough to exceed the Vf of the two diodes, the blue one plus the opto-isolator one?
Output Op-amp is 4.4V When connect to Opto with no other 2 Diode it drop to 1.9V and if I connect with 2 Diode and opto it drop to 1.2V
in the schematic Opto it still work. But I don't know if in real life will it work?

#### wayneh

Joined Sep 9, 2010
16,114
Output Op-amp is 4.4V When connect to Opto with no other 2 Diode it drop to 1.9V and if I connect with 2 Diode and opto it drop to 1.2V
in the schematic Opto it still work. But I don't know if in real life will it work?
Do you have the specs for the opto-isolator? It will provide the Vf.

#### Sornpraram Xu

Joined Feb 24, 2019
21
Do you have the specs for the opto-isolator? It will provide the Vf.
PC817 has Vf of 1.4V
Outcome from Op-amp is 4.5V (Does actually LM324 have headroom of 0.5V ??? )
With no 2 LED, After 4.5V operate Opto the voltage change to 1.5V (Isn't Vf should be 1.4V Why it be 3.0V ?????)

#### Alec_t

Joined Sep 17, 2013
10,444
Me and my friend asume that If same voltage connected(Yellow Circle) the output will be 0v(Blue Circle) Is that true??
No, unless both yellow points are also 0V. Take a close look at the probe voltages shown on your schematic.

#### wayneh

Joined Sep 9, 2010
16,114
PC817 has Vf of 1.4V
OK, that helps.
Outcome from Op-amp is 4.5V (Does actually LM324 have headroom of 0.5V ??? )
I think 4.5V is optimistic. The common-mode range only goes up to 3.5V on a 5V supply. I'm not sure the LM324 will output much more than that.
With no 2 LED, After 4.5V operate Opto the voltage change to 1.5V (Isn't Vf should be 1.4V Why it be 3.0V ?????)
I don't fully understand your question. The voltage will drop across the resistor (ex. R13), across an LED if you put one there, and finally across the diode in the onto-isolator. You probably don't have enough voltage to use the blue LED in series as drawn.

#### mvas

Joined Jun 19, 2017
538
Hello
I am trying to make a switch to control LED
In this schematic below
I am trying to operate the Opto-Isolators so the D9 Led could light up.
Me and my friend assume that If same voltage connected (Yellow Circle) the output will be 0v (Blue Circle) Is that true??
0 Volts ???
No, that cannot be true.
The voltage in both Yellow Rectangles and the Blue Oval must be the same, as per your diagram, all = 1.20227 Volts
All three points are connected by a wire.
All, three voltages are measured vs Ground.
So, how can the three points ever be different?
The three voltages will always be the voltage across the Photo-Diode, located inside the Opto-Isolator.