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Non-inverting op amp malfunction; did I miswire it?

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rhombus

New Member
Here is a picture of the circuit in question.

I am running the op amp on 0 to 5 volts, in non-inverting mode

Put simply, the output is unchanging as I adjust the voltage at the non-inverting input. _The top 10k resistor and the blue 8.8k (measured) pot, produce a voltage on the yellow wire that swings between gnd and about 2.38 volts. _The blue strips are gnd, and the red strip is 5.15 volts.

With the 2k feedback, and 1k (other) resistor it should have a gain of 3, but as I adjust the input voltage, the output stays constant at about 4.68v. _If I remove the other resistor (between invert and gnd) it works as a voltage follower, swinging between 1.92v and about 4.68v. _I have tried this circuit with four op amps; three from Fairchild, and the one pictured from National.

Is there any reason why this is not working?

Thank you for your time and consideration.
 

ericgibbs

Well-Known Member
Most Helpful Member
Here is a picture of the circuit in question.

I am running the op amp on 0 to 5 volts, in non-inverting mode

Put simply, the output is unchanging as I adjust the voltage at the non-inverting input. _The top 10k resistor and the blue 8.8k (measured) pot, produce a voltage on the yellow wire that swings between gnd and about 2.38 volts. _The blue strips are gnd, and the red strip is 5.15 volts.

With the 2k feedback, and 1k (other) resistor it should have a gain of 3, but as I adjust the input voltage, the output stays constant at about 4.68v. _If I remove the other resistor (between invert and gnd) it works as a voltage follower, swinging between 1.92v and about 4.68v. _I have tried this circuit with four op amps; three from Fairchild, and the one pictured from National.

Is there any reason why this is not working?

Thank you for your time and consideration.
hi,
the wire that goes from the pot [near pin #1] to the right rail, is that at 0V for sure.?
 

rhombus

New Member
I'm pretty sure it is. I scoped the voltage on the breadboard rail at pin three on the op amp (that has the yellow wire in it); and it swings between something just over 0 volts to just under 2.5 volts (since the pot is a bit smaller than the fixed resistor).. I'm pretty sure I'm getting good voltage swing into the op amp.

I also measured the resistance of the resistors, and they are correct (within the expected tolerance)

-Thanks.
 

ericgibbs

Well-Known Member
Most Helpful Member
I'm pretty sure it is. I scoped the voltage on the breadboard rail at pin three on the op amp (that has the yellow wire in it); and it swings between something just over 0 volts to just under 2.5 volts (since the pot is a bit smaller than the fixed resistor).. I'm pretty sure I'm getting good voltage swing into the op amp.

I also measured the resistance of the resistors, and they are correct (within the expected tolerance)

-Thanks.
Ok,
Whats the OPA type.?

EDIT;
look at the right bottom corner , is this what you have.?
 

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rhombus

New Member
Shoot..I forgot that detail; the op amp is an LM 741CN

Yes, the schematic looks like what I was inteneding to build.

The picture on National's website shows the the inverting input on top, and non-inverting on the bottom (as is usually the case), so I think that part is correct.
http www national com /mpf/LM/LM741.html <--fix the link to see the page.
 
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audioguru

Well-Known Member
Most Helpful Member
A lousy old 741 opamp was designed to amplify DC and low frequencies when it has a +15V and -15v dual polarity supply. Its design is 41 years old.

It probably won't do anything with a supply as low as 5V.
Its inputs do not work unless they are 4V or more above the negative supply and 4V less than the positive supply.

Another lousy old opamp is the quad LM324 or the dual LM358.
They have a minimum allowed supply voltage of 3V and their inputs still work down to 0V in your circuit.

I use an MC34071 single, MC34072 dual or MC34074 quad opamps which are similar but are much better.
Their max allowed input voltage is 1.5V less than the positive supply and their unloaded output goes as high as 1.2V less than the positive supply. Try one.
 

rhombus

New Member
audioguru, thank you for the reply. I will look at the opamps you suggested.

I'm glad to finally know what the problem is.
It was more than a little disconcerting that such a simple circuit was failing to work as expected - with no obvious explanation.
 
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