Is this a good equivalent for te LM741 OpAmp?

[BGAmodz]

I have to build a circuit that requires an LM741 operational amplifier , its used in differencial mode .

Can i us CA3140 instead ?

The circuit is for a zener identifier :

 

[large_ghostman]

have you checked the datasheets to see how close they are? like max voltage and mA? especially on the inputs? there is a certain Canadian here that thinks the 741 is the devils spawn
is the pot going to br marked off with voltages?
just to be picky i would put R4 to the Anode not the cathode but probably dosnt matter

 

[audioguru]

have you checked the datasheets to see how close they are? like max voltage and mA? especially on the inputs?

The input current for ANY opamp is extremely low so it does not matter in this simple circuit.

there is a certain Canadian here that thinks the 741 is the devils spawn

Hee, hee.
The 741 opamp is one of the first opamps ever made 45 years ago before they knew how to make good opamps. Its spec's are pretty bad when compared with more modern opamps.

 

[large_ghostman]

The input current for ANY opamp is extremely low so it does not matter in this simple circuit.


Hee, hee.
The 741 opamp is one of the first opamps ever made 45 years ago before they knew how to make good opamps. Its spec's are pretty bad when compared with more modern opamps.

hmmm now where have i read that before lol

 

[BGAmodz]

I really appreciate your comments , i have just checked some websites , they say that it is the direct replacement for the 741 .
BTW wuld a non regulated 30 Vdc work on this circuit just as a regulated one ??
Also can i use a 50K potentiometer instead of the 47k one ??

 

[audioguru]

I really appreciate your comments , i have just checked some websites , they say that it is the direct replacement for the 741 .

The (also very old) CA3140 opamp is completely different to a 741 opamp. Its maximum allowed differential input voltage is only 8V but is the entire supply voltage for the 741. This circuit could have a differential input voltage much higher than only 8V.

BTW would a non regulated 30 Vdc work on this circuit just as a regulated one ??

Caution, non regulated 30V power supplies produce 30V only when they have their rated load. This circuit will be a very light load so the voltage might be high enough to destroy the opamp.

 

[BGAmodz]

Caution, non regulated 30V power supplies produce 30V only when they have their rated load. This circuit will be a very light load so the voltage might be high enough to destroy the opamp.

Actually i have have a rectified 24 V-AC going through 2 paralell 1000uF Capacitors , the DC measurement gives 30 Vdc .

 

[alec_t]

The spec says the absolute maximum supply voltage for the CA3140 is 36V so you'd be cutting it fine, especially if there were any glitches on the supply.

 

[BGAmodz]

The spec says the absolute maximum supply voltage for the CA3140 is 36V so you'd be cutting it fine, especially if there were any glitches on the supply.

Hey , thanks , i feel that th CA3140 is the right choice , now i only hav tow worries , is it ok to use a 50K trimmer instead of the suggested 47K , also the non regulated 30 volts that i got from a rectified filtered 24 V-AC supply .

 

[audioguru]

The CA3140 opamp IS THE WRONG CHOICE because its absolute maximum allowed differential input voltage is only 8V and the circuit can easily give much more differential voltage which will damage a CA3140 opamp.
The 741 and many more opamps allow the entire supply voltage to be the differential input voltage.

 

[large_ghostman]

i got a question about your circuit, i dont mean to be picky but lets see if i am reading this circuit correct, basically you start with a small voltage on the pot and slowly increase it until the zener conducts and lights the led because the inputs at that point are equal? if so then i assume you have some kind of scale around the pot to read off the voltage? in that case wouldnt you need a multi turn pot with one those micro scale things on like some analogue scopes have? otherwise wouldnt a normal pot be too crude? i mean how you going to tell if the zener is say any of the following
3.3V 1N4728A
3.6V 1N4729A
3.9V 1N4730A
4.3V 1N4731A
4.7V 1N4732A
what i am trying to say is surely a normal dosnt have enough resolution?? to say get exactly 3.3V or 3.6V so how you going to tell the difference as most of those in the list will look like they trigger at say 3V if you use a normal pot with a scale on? and if you just reading with a DMM then why use the circuit? i am not being funny just interested in how it works as a zener tester would be cool but i have some doubts on that circuit

 

[JimB]

Full marks to LG!

While everyone else was debating what type of op-amp to use, the real question should have been "Is this circuit any use?"

A much better idea would be to pass a known (constant) current through the zener and measure the zener voltage using a DVM.

JimB

 

[audioguru]

This project is almost useless when used to measure low voltage zener diodes.

Zener diodes are rated at one current. The current could be high or it could be low depending on the zener diode's part number.
Low voltage zener diodes have horrible voltage regulation that changes a lot with current changes (like a resistor) and it also changes with temperature changes.

Here is a graph of the voltage of some low voltage zener diodes when the current is changed. The graph is from Motorola.
Try to select a "3.3V, 3.6V or 3.9V" zener diode.

 

[large_ghostman]

This project is almost useless when used to measure low voltage zener diodes.

Zener diodes are rated at one current. The current could be high or it could be low depending on the zener diode's part number.
Low voltage zener diodes have horrible voltage regulation that changes a lot with current changes (like a resistor) and it also changes with temperature changes.

Here is a graph of the voltage of some low voltage zener diodes when the current is changed. The graph is from Motorola.
Try to select a "3.3V, 3.6V or 3.9V" zener diode.

funny you should mention that i have been looking at diodes for crude/ish temp sensor apart from a really scary equation with i forget the man name but boz someones constant all i can find is a rough guide of 2mV per C, what i was trying to find was a datasheet with a graph that showed forward voltage drop against temperature. i was mainly looking at signal diodes but maybe i should take more of a look at low voltage zeners with a constant current through them hmmm need to do some thinking!
thanks for that AG you have given me another path to look into

oh forgot to add thinking about it a zener tester wouldnt be much use for me as we have a dissecting microscope so in reality i would just read the part number on the diode but i do like the idea of building a gadget to test them

 

[audioguru]

i have been looking at diodes for crude/ish temp sensor

An ordinary signal diode or junction of a transistor is used as a temperature sensor.
Here is the datasheet with graphs of zener diodes:

 

[large_ghostman]

thanks for that they are better graphs than i found for 1N4148 i might try a zener around 6-7 volts see what its like, ive tried a signal diode but it hardly budged certainly not 2mV/C, i will need to try a few things out. i am using ds18s20 for main stuff but they are not that quick (750ms) between readings, the LM sensors are ok but for what they cost a diode looks like it would perform just as well and alot cheaper

 

[audioguru]

A 1N4148, 2N3904 and BC548 transistor have a temperature change of 2mV to 3mv per degree C. Their leads conduct heat and cold which might cause your "no change" problem.

 

[large_ghostman]

A 1N4148, 2N3904 and BC548 transistor have a temperature change of 2mV to 3mv per degree C. Their leads conduct heat and cold which might cause your "no change" problem.

yeah good point about the leads i need a test chamber (glass jar sealed) thanks for info

 

[BGAmodz]

After putting a zener diode under check , you will slowly vary the pot resistance until the LED shuts down , thats when you know the right zener voltage on the zener poles .
The led should be on when voltages are not equal .

 

[3v0]

There are some posts that I expect to end in a post or 3 because the answer should be simple. When they get longer then that I get curious. This is one of them.

LG one of the things you might run across in using diodes or transistors not intended for temperature measurement is that each unit might give you a different value for the same temperature. This is not a problem if you characterize each and use that information in your code or hardware. Shades of grey here.