Reverse engineering of LM358 opamp

Hi all,


Can anyone help me to find out the IC details from the attached circuit diagram..

Is it comparator???

Re: Reverse engineering

My guess:
It's an LM358 or similar opamp. When the pwm output goes below 5V the opamp output goes high; it latches because the reference level (that was 5V before) becomes 12V.
(Ignoring diode drops and Vout limits)

Re: Reverse engineering

What is this circuit supposed to be doing, and where did you get this diagram?

steveelliott said:

My guess:
It's an LM358 or similar opamp. When the pwm output goes below 5V the opamp output goes high; it latches because the reference level (that was 5V before) becomes 12V.
(Ignoring diode drops and Vout limits)

Click to expand...

But there's an unconnected pin; a dual op-amp package would have 2 power pins and 2x3 op amp pins, so if it were an op amp one of the amps would have an unconnected pin, which would be weird? Also fwiw the pinouts don't match the LM358. But I dunno...

Re: Reverse engineering

jasonc2 said:

What is this circuit supposed to be doing, and where did you get this diagram?



But there's an unconnected pin; a dual op-amp package would have 2 power pins and 2x3 op amp pins, so if it were an op amp one of the amps would have an unconnected pin, which would be weird? Also fwiw the pinouts don't match the LM358. But I dunno...

Click to expand...

Pin 8 + supply
Pin 4 - supply
Pin 7 output
Pin 6 inverting input
Pin 5 non-inverting input
Opamp #1 not used but inputs tied to known voltages so that the opamp doesn't flop around with floating inputs

Sure looks like a dual opamp pin compatible with lm358 to me.

Re: Reverse engineering

steveelliott said:

Opamp #1 not used but inputs tied to known voltages so that the opamp doesn't flop around with floating inputs

Click to expand...

Oh right good point!

--

Off topic isn't saturating an unused op amps output or applying consistent largeish voltage across the inputs (in this case 5) less than ideal for a lot of reasons?

Re: Reverse engineering

Off topic isn't saturating an unused op amps output or applying consistent largeish voltage across the inputs (in this case 5) less than ideal for a lot of reasons?

Click to expand...

No problem with LM358 and most general purpose OPs. Some bipolar OPs have limited differential input voltage rating however.

Re: Reverse engineering

LM358 allows differential input to both rails and senses near ground (V-)
Are pins 1& 2 connected elsewhere?
Are R2&R3 small Ω current sense power resistors or large and connected to a load?

There is a positive feedback loop from 2OUT(pin7) to 2IN+ (pin5) via diode D1,
such that if the output 2OUT goes high, it stays high at 12V.

1OUT (pin1) s not shown connected. (x) but is a bipolar logic level sensing the state of 2OUT (pin7) by comparing 2IN+(pin5) to Gnd at 1IN+(pin3) .

If the 2IN-(pin6) from (filtered) PWM signal drops below 2IN+(pin5) (~5V? minus load IR drop) then the Overvoltage indicator goes hi on 2OUT(pin7) = 12V otherwise it is -12V.

Not sure , but thats what I see.

Re: Reverse engineering

jasonc2 said:

Oh right good point!

--

Off topic isn't saturating an unused op amps output or applying consistent largeish voltage across the inputs (in this case 5) less than ideal for a lot of reasons?

Click to expand...

Jasonc2-
You are right, as a general goal. High differential voltage (> 2 Vbe) tends to degrade the bias current spec because over time the diff pair ages unevenly. Obviously this is only an issue at high temperature. And of course in this case the inputs are tied hard so they're not going to do anything anyway. As FvM implies above, some CMOS devices also have the same problem: Ibias is so small (sub-pA), it's relatively easy to mess up the matching.

Some people recommend tying unused opamp inputs together to a voltage near ground. I have never liked that because the output of the opamp is always biased ready to swing from rail to rail for a small bit of noise pickup, or change in offset voltage, or any other wacko reason. The internal switching noise can theoretically disrupt the other opamp or at least add a bit of hard-to-find random noise to it. I prefer to put the inputs at least 1V apart just so the opamp will sit saturated with no chance of noise propagating through it.

- - - Updated - - -

SunnySkyguy said:

LM358 allows differential input to both rails and senses near ground (V-)
Are pins 1& 2 connected elsewhere?
Are R2&R3 small Ω current sense power resistors or large and connected to a load?

There is a positive feedback loop from 2OUT(pin7) to 2IN+ (pin5) via diode D1,
such that if the output 2OUT goes high, it stays high at 12V.

1OUT (pin1) s not shown connected. (x) but is a bipolar logic level sensing the state of 2OUT (pin7) by comparing 2IN+(pin5) to Gnd at 1IN+(pin3) .

If the 2IN-(pin6) from (filtered) PWM signal drops below 2IN+(pin5) (~5V? minus load IR drop) then the Overvoltage indicator goes hi on 2OUT(pin7) = 12V otherwise it is -12V.

Not sure , but thats what I see.

Click to expand...

1) Not necessarily LM358. I just suggested that as an example part.
2) How could opamp sink the 5V current if R2 R3 are current sense? Dead opamp most likely if you tried, unless the opamp is short-circuit rated.

- - - Updated - - -

Revathysrini said:

Hi all,


Can anyone help me to find out the IC details from the attached circuit diagram..

Is it comparator???

Click to expand...

I forgot to mention- It's probably not a comparator. Most comparators of this (likely) pinout have an open collector output and there is no pull-up resistor shown. That's why I proposed it was an opamp instead.

Re: Reverse engineering

Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation. It often involves taking something (e.g., a mechanical device, electronic component, software program, or biological, chemical, or organic matter) apart and analyzing its workings in detail to be used in maintenance, or to try to make a new device or program that does the same thing without using or simply duplicating (without understanding) the original.

Re: Reverse engineering

alyna said:

Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation. It often involves taking something (e.g., a mechanical device, electronic component, software program, or biological, chemical, or organic matter) apart and analyzing its workings in detail to be used in maintenance, or to try to make a new device or program that does the same thing without using or simply duplicating (without understanding) the original.

Click to expand...

Maybe posters after this and based on that will have new look on this thematic.

[offtopic] Re: Reverse engineering

steveelliott said:

Jasonc2-
Some people recommend tying unused opamp inputs together to a voltage near ground. I have never liked that because the output of the opamp is always biased ready to swing from rail to rail for a small bit of noise pickup, or change in offset voltage, or any other wacko reason. The internal switching noise can theoretically disrupt the other opamp or at least add a bit of hard-to-find random noise to it. I prefer to put the inputs at least 1V apart just so the opamp will sit saturated with no chance of noise propagating through it.

Click to expand...

Cool thanks for explaining it. I found a great article on it, written by James Bryant from Analog Devices:

https://www.analog.com/static/imported-files/rarely_asked_questions/unused_op-Amp_article.html

The recommended solution there is to configure it as a follower and tie the unused output to the inverting input and the noninverting input to some point in the circuit with voltage between the rails.

Careful if you sing the song to not let it get stuck in your head though. ... :-(

This MAXIM article gives the same advice: **broken link removed**

J

Re: [offtopic] Re: Reverse engineering

jasonc2 said:

Cool thanks for explaining it. I found a great article on it, written by James Bryant from Analog Devices:

https://www.analog.com/static/imported-files/rarely_asked_questions/unused_op-Amp_article.html

The recommended solution there is to configure it as a follower and tie the unused output to the inverting input and the noninverting input to some point in the circuit with voltage between the rails.

Careful if you sing the song to not let it get stuck in your head though. ... :-(

This MAXIM article gives the same advice: **broken link removed**

J

Click to expand...

Yep, that's definitely a good way to do it. If you use a resistor from output to inv input and resistor from non-inv input to other voltage, you can remove resistors if you need to use opamp for something else.

Re: Reverse engineering

How could opamp sink the 5V current if R2 R3 are current sense? Dead opamp most likely if you tried, unless the opamp is short-circuit rated.

Click to expand...

Good point Steve E. .I was assuming there were some other connections to drain current the reason for the 2 R's was to get twice shunt resolution
but current drive not to backdrive the R's just clamp there.

Since the LM358 support shorts to ground,... I was thinking it was a clamped output to latch the fault state. I must be getting rusty in my retirement... in 76' I could debug a bass guitar synth. with 35 CMOS IC's with all the part numbers sanded off for IP security.

Still a few unknowns in input signal..

Maybe the D's are LED's ???