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OpAmps and Circuit type

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drkidd22

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Hello,

I have not worked with opAmps in a while and I'm trying to understand this circuit. Looks like an inverting op amp with a gain of 5. (100k/20k) and some scaling at the output. What type of opAmp circuit is this? differential with a fixed reference? So will my output be (10vref-Vin)*(-100k/20k)?

Capture.PNG
 

ericgibbs

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hi,
The OPB is a slotted photo detector.
When the slot is clear the OPA is low, when obscured the 10Vref switches the OPA output high.
E
 

drkidd22

Member
ok I understand that, but the object used in the detector's slot is about 50% transmission or it can vary.
So I'm trying to understand the circuit operation a little more. I may change R278 to a pot to make adjustment.
By the way you describe it it sounds like it's working more as a comparator.
 

ericgibbs

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If you told us what the product/unit the circuit is used in ie: its application, we could perhaps give a more meaningful rely.
 

ronsimpson

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I just saw something:
(+) input = 10V
Supplies = 24V and gnd
The LED is not driven from supply but the output of the op-amp!
So I think the op-amp's output will go up until the the LED current makes light so that the photo transistor's Emitter reaches 10 volts. (about 1/2 supply) That is a sweet spot for the photo transistor.

If the light is broken the photo transistor's Emitter voltage will drop to near ground. This causes the op-amp output to about 20 volts. (or as high as it can pull)
 

crutschow

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I think ronsimpson is correct.
The photodetector is biased in the active region to give maximum sensitivity for objects of varying opacity.
 

ericgibbs

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I would agree, Ron's take looks the most likely.
I hope the OP tells us in which application the circuit is being used.
 

spec

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Hello,

I have not worked with opAmps in a while and I'm trying to understand this circuit. Looks like an inverting op amp with a gain of 5. (100k/20k) and some scaling at the output. What type of opAmp circuit is this? differential with a fixed reference? So will my output be (10vref-Vin)*(-100k/20k)?

View attachment 100090
As drawn the circuit makes no sense. I would think that R264 and R265 left should connect to 24V.

The OPA2604 is a dual FET opamp made by Texas Instruments. It has a high input impedance and can drive a 600 Ohm load. The only thing distinctive about it is that it can operate from +-25V supply lines. It also has reasonably low distortion. http://www.ti.com/lit/ds/symlink/opa2604.pdf

spec
 
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ronsimpson

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I would think that R264 and R265 left should connect to 24V.
My thought exactly. Then why an op-amp (with gain) and not a comparitor?

On the other hand:
The slotted photo detector probably has a very wide gain range. AND With room light variations you might need to have this feedback that I see.
With feedback this thing balances at the same point no matter what is happening. (room light, temp, part to part variations)

I am sure there are circuits I designed, and people are looking at them and thinking "What the F&#* was he thinking." It probably solves a problem we might not see from our position.

Also: The "designer" has hours to get this running and now for the next 3 years we can thing of better ways. (that might not be better) lol
 

spec

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My thought exactly. Then why an op-amp (with gain) and not a comparitor?

On the other hand:
The slotted photo detector probably has a very wide gain range. AND With room light variations you might need to have this feedback that I see.
With feedback this thing balances at the same point no matter what is happening. (room light, temp, part to part variations)

I am sure there are circuits I designed, and people are looking at them and thinking "What the F&#* was he thinking." It probably solves a problem we might not see from our position.

Also: The "designer" has hours to get this running and now for the next 3 years we can thing of better ways. (that might not be better) lol
Yeah, some good solid observations- as usual. I did try to figure the circuit and did consider some kind of balancing act, but then what would be the purpose of the output, considering the integration function of the opamp. Had that have been a differentiation function, it would have of made more sense, to me anyway. :wideyed:

Also, the opto receiver sensitivity is not being adjusted it is the opto transmitter, so what would be the point of reducing the transmitter intensity under any circumstances. Much better to have the opto transmitter running at full brightness under all ambient light conditions, or is that not true.

There is the thought that the material in the sensor slot is translucent, and the output is a DC indicating how translucent.

Most of my circuits that I look at in retrospect, I think what the F&#, especially when they have been mangled by other departments.

It would be very interesting to learn what this circuit does if the schematic is correct.

spec
 
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drkidd22

Member
Thanks for all of your responses, they all clarify a lot of my questions. I took some measurement, but the output doesn't seem to match with the gain setting of the opAmp.
With the semi-transparent object placed in the slot of the detector the output is 7.75V and input at pin 2 is 9.95V.
With no object placed the output is 12.55V and input at pin 2 is still 9.95V.
This does not make sense.
 

MikeMl

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Is the Agnd the same voltage as the Bgnd?

Is Bgnd the return for the 24V supply?
 

ronsimpson

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When both inputs are 9.95V then the amp is in balance. (9.95v=10vref) If the two inputs are not the same then we know the output is near gnd or supply.
7.75V.....12.55V
I think if the slot is open then the voltage would be low, because light goes from LED to transistor very easy.
I think if 50% of the light was lost because of "semi-transparent" then there must be a higher voltage to get more light.
BUT
Maybe the "semi-transparent" reflects room light into the transistor.
 

spec

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9.95V.
With no object placed the output is 12.55V and input at pin 2 is still 9.95V.
This does not make sense.
Hy drkidd22,

That is correct, pin 2 is the inverting input of the opamp which, if the opamp is functioning as an opamp and not a comparator, will always be the same voltage as the non-inverting input, pin 3. In fact pin 2 is a virtual earth at 10V.

spec

(crossed threads :))
 

spec

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Just a thought- is this a yellow sensor, perhaps for yellow ink in a cartridge?

spec
 

AnalogKid

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This looks sorta kinda like a positioning servo. Signal polarities at opto coupler pins 1 and 4 are the same, so it really is negative feedback. We don't know what else the opamp output is used for, but having it vary the brightness of the LED does seem strange...

ak
 

spec

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Looks like an inverting op amp with a gain of 5. (100k/20k) and some scaling at the output.
It is an inverting op amp (virtual earth). The gain of the opamp circuit is R235/R45= 100K/330 Ohms = 303. R244 provides DC scaling but does not affect the voltage gain.

spec
 

spec

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the object used in the detector's slot is about 50% transmission or it can vary.
In view of this statement, which I missed, I now think that the function of the circuit is to measure how transmissive the object in the slot is, the more transmissive the lower the output voltage.

spec
 

drkidd22

Member
Got it, thanks. It is to sense fluid of different colors going through a tube. Not particularly ink, but yeah maybe it can be used for that too :).
 

spec

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Got it, thanks. It is to sense fluid of different colors going through a tube. Not particularly ink, but yeah maybe it can be used for that too :).
Ah right.:happy: In that case we know exactly how it works. Are you also now happy about how it works or would you like a brief description?

spec
 
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