Weird Op Amp thing.........

[TrickyNekro]

Hello guys,

I'm building an op amp circuit for a current sensor...
I'm pulling up the output pin with a 1k resistor...

I just noticed that when the input is more than a certain value the
output becomes higher than 5V!!!! And I'm powering in with only 5V!!!!

How the..... is this possible??

Is a zener diode a good way to prevent this from happening, cause I'll be measuring the
output with an ADC?


Thanks in advance,
Lefteris
Greece

[galannthegreat]

Try a general diode, such as a 1N4001, and test the voltage drop, that should work.

[chelmi]

Please post a schematic of circuit, there are many different ways of using an op-amp.

An op-amp amplifies, an output voltage higher than its input is not surprising...

Chelmi.

[TrickyNekro]

a diode in series is a little bit bad for the accuracy I want...
I only measure 0 Amps to 1-1.5Amps so I don't have a problem with the zener...
Cause this happens after 3Amps are being drawn...

It;s somethings I lack...
Like if I need a current limiting resistor with my zener and such...

And basically I need to know why this happens....


Thanks for your reply,
Lefteris,
Greece

[TrickyNekro]

Please post a schematic of circuit, there are many different ways of using an op-amp.

An op-amp amplifies, an output voltage higher than its input is not surprising...

Chelmi.

Higher than input yes....
Higher than supply no...

The supply is 5V and the output goes to ~5V6!!!!

That's confusing...
I'll post a schematic right away...

[chelmi]

Please post a schematic of circuit, there are many different ways of using an op-amp.

An op-amp amplifies, an output voltage higher than its input is not surprising...

Chelmi.

Higher than input yes....
Higher than supply no...

The supply is 5V and the output goes to ~5V6!!!!

That's confusing...
I'll post a schematic right away...

Oops, I misread your first post

[TrickyNekro]

Here are the schematics.... hope this helps....

[Soeren]

Hi,

I'm a bit puzzled over your schematic.
Is the Vref supposed to be the input?
Why did you connect R2 and R3 to pin 1? Shouldn't they connect to pin 2 instead?
What kind of circuit are you trying to build? An amplifier or...?

As far as I see it, you have connected it in a wrong way, you have a feedback resistor to the inverting input and a voltage divider on the supply with the tap on the output. I don't see what you want to accomplish with that.

Remake the circuit first and if it still goes higher than supply, then add a 10..100nF cap directly over the supply pins. Next step is decoupling the input(s) with small caps (in the pF range).

If you look at the output with a 'scope, I have a hunch that you'll find it oscillating wildly.

[TrickyNekro]

It's an non-inverting schematic....

If I don't use the R3 the output never goes over 3V8 volts and that's a problem....
It's to boost the output....
It's not something that I really got out of my head.... I once used a resistor in the same configuration
for an voltage comparator circuit.... to boost the output.... So I guessed that would be ok.....

I get the feeling that it is oscillating badly too.....but I don't have a scope to see it..... (BAD!!!!)
Usually, in current sensors I see a 100nF between inverting and non-inverting inputs....
And of course one for decoupling the power pins.... (Which I include in the real circuit)


Thanks for your reply,
Lefteris,
Greece

[TrickyNekro]

Ok... Problem solved..... new created.... but that's another thing.....

Thanks for the help guys!!!!!

[Soeren]

Hi,

It's an non-inverting schematic....
[...
]It's not something that I really got out of my head.... I once used a resistor in the same configuration
for an voltage comparator circuit.... to boost the output.... So I guessed that would be ok.....

It is not an inverting amp.
I rearranged your circuit to make it more clear. Your circuit on the left, the real inverting amp on the right:


When using a comparator (not an op-amp) you sometimes have to use a pull up, since some comparators has got an open collector output (same as if you had a loose NPN transistor with the emitter to Gnd, driven from the base and with the collector just sitting there).
On an op-amp, you just load the output by placing a pull up (or pull down) resistor.

What did you do to solve it and what is the new problem?


Admin <- why are images shrinked a wee bit in the posts, making the semi-unintelligible?

[TrickyNekro]

I know that the real circuit is at the right....
But the output won't go more than 3V8...
And that is problematic.....

[Soeren]

Hi,

You'll need an op-amp with rail to rail output.

[TrickyNekro]

Any recommendation?

[Soeren]

Hi,

One of the MAX416x would do
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1696

They come in single, dual and quads in PDIP (Plastic DIP, I guess they are what you want).

[Admin]

If you go to mouser.com and search for op-amps, there will be an option to filter by rail-to-rail output.