Help with Op-Amps Design

Thread Starter

TedOlsen

Joined Oct 24, 2020
14
I'm helping some friends build a device. My part of the job is to turn on the transistor when the UV Output <6< gets up to .2v.
I tried an MCP6231 and it worked. The problem was their device uses 12v. and the max on that Op-Amp supply is 6v. I made it
work with a voltage divider but wanted to simplify this to the max. I thought using an LF356N might work like the MCP6231 but
it doesn't. I'm running the +12v right to pin 7 vss. I thought Op-Amps would adjust the output so that the voltage on pin 2 would
be the same as on pin 3. If that were the case, I would think the output at pin 6 would go to 1v. to get the voltage on pin 2 at .2v.
Then as long as the flow switch is closed, the 1v. should turn on the 2N2222.
I'm missing something. Help would be greatly appreciated.

1603573109858.png
 
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Ian0

Joined Aug 7, 2020
759
Input Common Mode Voltage is the important parameter here.
Specified at a minimum of -12V for a -15V supply on pin 4.
In other words, if the inputs are less than 3V above the pin 4 supply, it doesn't work.
An LM358 will work all the way down to 0.3V below the pin 4 supply.
 

Thread Starter

TedOlsen

Joined Oct 24, 2020
14
Thank you! So am I understanding right that if I get LM358's and I have pin 4 at ground, it will work with a .2v input on pin 3?
 

Thread Starter

TedOlsen

Joined Oct 24, 2020
14
I see the pinouts on the 358 are different because it's a dual circuit. I have some pcb's already created so that is a problem for right now.
 

Ian0

Joined Aug 7, 2020
759
Yes, it’s a dual, but it’s cheap!
there are plenty more singles.

If this Link works it should give you all TI’s Op-amps That work with the inputs at the negative supply, and will work with a supply of 12 Volts.
TLV271 looks like a good candidate.
Check that input common mode voltage.
 

Thread Starter

TedOlsen

Joined Oct 24, 2020
14
My next step is to create the Gerber files. I'm using Autodesk Eagle. I'm stuck on finding the TLV271IP in a library.
 

Audioguru again

Joined Oct 21, 2019
2,088
The 741 opamp design is 52 years old. Its inputs do not work if an input gets within 3V from a supply voltage so in this circuit it will do nothing.
 

Ioannis66

Joined Nov 7, 2012
39
My comment was to select a library part for the eagle. Sorry if it was confusing. Sure I never meant to use a 741 and have some serious common mode voltage range...
 

Thread Starter

TedOlsen

Joined Oct 24, 2020
14
Op-Amp Output. I hadn't used Op-Amps before this project. I was expecting that if I put a solid smooth 200 mv on the input, I would get a solid smooth 1v output. (with the voltage divider resistors feeding back). I just need to turn on a transistor with the output and the transistor will energize a relay. My question is the output. On the scope, I'm seeing the attached waveform on the output.
Is this the way an Op-Amp works? I assume you'd like to have the frequency of the output and the voltages but my question is just generic. Is the output something like the attached photo vs. a solid DC voltage? I can stick a capacitor on it and smooth it out but I'd rather not add components. It does work and energize the relay but I'm just bothered at what the output looks like. If it's normal, I'll just relax. :)
 

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OBW0549

Joined Mar 2, 2015
3,566
I was expecting that if I put a solid smooth 200 mv on the input, I would get a solid smooth 1v output. (with the voltage divider resistors feeding back).
Yes, that's exactly what you should expect.

On the scope, I'm seeing the attached waveform on the output.
Is this the way an Op-Amp works?
Something is terribly, horribly, catastrophically wrong if that's what you're seeing. If op amps normally worked that way, we wouldn't use them.

If it's normal, I'll just relax. :)
No, it is not. It looks like possibly a combination of oscillations of the op amp itself (i.e., instability) and external interference such as from lighting or nearby equipment.

See this comment in a somewhat related thread.
 

RPLaJeunesse

Joined Jul 29, 2018
124
General rule of thumb: never bring an unprotected IC pin off board for any distance, and definitely never EVER bring an unprotected IC pin out of a box. What do you need to protect against? 1) Electrostatic discharge. 2) Any DC or AC voltage that may exceed the input ratings of the IC - especially when IC power is off! 3) Unwanted RF signals that the IC might decide to have influence its output. Protection methods include shunt clamping devices, series resistor for current limiting, and an RC lowpass filter. Ferrite beads at the box wall connector might also be needed to keep RF out of the box.
 

Audioguru again

Joined Oct 21, 2019
2,088
I think the very messed-up 'scope waveform shows that the circuit was built on a solderless breadboard with stray capacitance coupling and interference picked up by the many rows of contacts and messy long jumper wires all over the place.
Maybe the input cable is unshielded and picks up interference?
Maybe the power supply is a switching type?
Maybe the opamps are fake or defective from ebay?

Also, your schematic is missing an important supply bypass capacitor very close to the opamp.
 

swr999

Joined Mar 30, 2011
30
Try a 1 uF or 0.1 uF or 0.01uF ceramic capacitor on the +supply pin of the opamp. Put the capacitor physically as close as you can (short leads) to the opamp. In addition maybe try a 10-ohm series resistor in the supply line to the opamp from "alternate power supply".
 
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Thread Starter

TedOlsen

Joined Oct 24, 2020
14
I think the very messed-up 'scope waveform shows that the circuit was built on a solderless breadboard with stray capacitance coupling and interference picked up by the many rows of contacts and messy long jumper wires all over the place.
Maybe the input cable is unshielded and picks up interference?
Maybe the power supply is a switching type?
Maybe the opamps are fake or defective from ebay?

Also, your schematic is missing an important supply bypass capacitor very close to the opamp.
Hello Audio guru. I think you'll get a laugh when you see the attached picture. Messy long jumper wires? Me? I don't know if the power supply is a switching type but I built it in school in 1968.
 

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