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Author Topic: Weird Op Amp thing.........  (Read 2262 times)

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Offline TrickyNekroTopic starter

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Weird Op Amp thing.........
« on: February 13, 2009, 10:55:54 AM »
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
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline galannthegreat

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Re: Weird Op Amp thing.........
« Reply #1 on: February 13, 2009, 02:08:54 PM »
Try a general diode, such as a 1N4001, and test the voltage drop, that should work.
Kurt

Offline chelmi

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Re: Weird Op Amp thing.........
« Reply #2 on: February 13, 2009, 02:57:45 PM »
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.

Offline TrickyNekroTopic starter

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Re: Weird Op Amp thing.........
« Reply #3 on: February 13, 2009, 03:00:40 PM »
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
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline TrickyNekroTopic starter

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Re: Weird Op Amp thing.........
« Reply #4 on: February 13, 2009, 03:02:25 PM »
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...
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline chelmi

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Re: Weird Op Amp thing.........
« Reply #5 on: February 13, 2009, 03:26:08 PM »
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 :)

Offline TrickyNekroTopic starter

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Re: Weird Op Amp thing.........
« Reply #6 on: February 13, 2009, 05:06:31 PM »
Here are the schematics.... hope this helps....
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline Soeren

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Re: Weird Op Amp thing.........
« Reply #7 on: February 14, 2009, 01:45:49 AM »
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.
Regards,
Søren

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Offline TrickyNekroTopic starter

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Re: Weird Op Amp thing.........
« Reply #8 on: February 14, 2009, 03:52:41 AM »
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
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline TrickyNekroTopic starter

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Re: Weird Op Amp thing.........
« Reply #9 on: February 14, 2009, 05:16:51 AM »
Ok... Problem solved..... new created.... but that's another thing.....

Thanks for the help guys!!!!!
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline Soeren

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Re: Weird Op Amp thing.........
« Reply #10 on: February 14, 2009, 07:33:00 AM »
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?
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline TrickyNekroTopic starter

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Re: Weird Op Amp thing.........
« Reply #11 on: February 15, 2009, 07:26:20 AM »
I know that the real circuit is at the right....
But the output won't go more than 3V8...
And that is problematic.....
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline Soeren

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Re: Weird Op Amp thing.........
« Reply #12 on: February 15, 2009, 01:57:49 PM »
Hi,

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

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline TrickyNekroTopic starter

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Re: Weird Op Amp thing.........
« Reply #13 on: February 15, 2009, 05:57:07 PM »
Any recommendation?
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline Soeren

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Re: Weird Op Amp thing.........
« Reply #14 on: February 15, 2009, 07:06:23 PM »
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).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Admin

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Re: Weird Op Amp thing.........
« Reply #15 on: February 17, 2009, 12:17:05 PM »
If you go to mouser.com and search for op-amps, there will be an option to filter by rail-to-rail output.

 


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