# shunt resistor

#### mlkcampion

Joined Nov 16, 2006
59
The circuit i have consists of one circuit which can carry anything up to
50Adc at 12vDC. A shunt resistor (60mV = 100A) is placed in series with this
circuit and thus produces a voltage drop between 0mV and 60mV depending on the current. Iam tyring to read this voltage into a PLC and i have placed an
op-amp circuit giving me a gain of 50. This op-amp circuit is supplied via a 24vDC supply and i have commoned the two power supplies. My problem is when i measure the voltage across the shunt i get for example 10mV however when i measure across the 4.7uf capacitor (see circuit) i end up getting something like 16mV. What i dont understand is that the + and - of the shunt and the + and - of the capcitor are effectively the same points in the circuit?

With the mV signal i would have expected a voltage drop due to maybe the wiring which i've shortened as much as i can but there still seems to be this error? if anyone knows of another way of acheiving the same thing with the shunt i would appreciate it?

Thanks

#### Attachments

• 269.7 KB Views: 66

#### JoeJester

Joined Apr 26, 2005
4,390
MLKCampion,

It looks like you didn't ground the input properly.

#### Attachments

• 23.1 KB Views: 38

#### mlkcampion

Joined Nov 16, 2006
59
When i tried to download the .pdf file you attached it came up
with a corrupt error.

Thanks

#### JoeJester

Joined Apr 26, 2005
4,390
try a right mouse....save target as....

If that don't work, I'll post the three graphics

#### mlkcampion

Joined Nov 16, 2006
59
Hey Joe
Yea i've tried that but it still wont open. I just check the picture that i attached, can you please note that i actually have a ground (pspice ground) to the left of the picture, i just noiced that i cut it out by accident.

Thanks

#### Dave

Joined Nov 17, 2003
6,969
Hey Joe
Yea i've tried that but it still wont open. I just check the picture that i attached, can you please note that i actually have a ground (pspice ground) to the left of the picture, i just noiced that i cut it out by accident.

Thanks
Strange, you should be able to either open or download the file. You haven't got file downloads disabled have you? If not can you screenshot the corruption error?

Dave

#### mlkcampion

Joined Nov 16, 2006
59

#### Attachments

• 348.3 KB Views: 44

#### Dave

Joined Nov 17, 2003
6,969
Then it must be an issue with your PDF reader. I would suggest you update to the latest version; if you're using Adobe Reader update to version 7.0.9 - I can confirm it opens fine with version 7.0.9 - or version 8.

Dave

#### mlkcampion

Joined Nov 16, 2006
59
I've upgraded to 7.0.9 and i still get the same error. I s there another
way you could post it?

Thanks

#### thingmaker3

Joined May 16, 2005
5,083
Let's try this:

#### mlkcampion

Joined Nov 16, 2006
59
Hey
Thanks for that,
i was able to download these. I ran a few more simulations and even
though got slightly different results, i kinda get what you were sayin bout the
ground. I am still working on the problem because simulating is one thing but trying to find the problem on the physical wiring ain't shown up much.

Thanks