LM741 testing

[RobH]

I have had cause to test a few of my LM741 op amps, and after googling this I found  that a voltage follower was one way to test them.

I put this together and proceeded the tests:
https://www.electrical4u.com/voltage-follower/

I found that I was getting varying results from the 5 LM741's I tested.
I got the exact same readings on pins 2 and 7, but each op amp gave a different reading.
EG
ON one LM741 I got +3.45V on both pins 2 and 7, and 0V on pin 6
On another I got 2.73V on both pins 2 and 7, then 0.6V on pin6

The lowest reading was 2.27V on another LM741 and the highest was 3.45V

With 10V input, it is obvious that I didn't get the same readings as is on the  site of the voltage follower.

Advice please
Thanks

[Ian.M]

Errrr........
You need to *SUPPLY* 10V to pin 7 (V+), as per datasheet!

[pqass]

Confirm that you've connected everything correctly. 
Pin 7 is the positive power pin (10V in your link).
Pin 6 is the output which should be connected to pin 2 (negative input).
Pin 4 is ground.
Leave the other pins (1, 5, 8 ) disconnected.

Feeding between 0V and positive power (via pot/divider) into pin 3 should give you the same voltage on pin 6 (or 2), EXCEPT when pin 3 is [approximately] below 2V or above positive power-2V.   The exception occurs because the 741 is an old op amp whose output doesn't work as expected near the rails.

[RobH]

@Ian.M,
sorry my bad, it was a typo!!

Thanks for the replies, and I have re tested with the +10V connected directly to pin 7 of the 741's

Results are now:
On 3 of them I get 0V on pin3 and 4.8V on pin6
On another I get 0.6V on pin3 and 0.6V on pin7
Again not the voltages I should get , so I suppose that they are faulty??

[Terry Bites]

This is the best 741 test set up I've seen

[Ian.M]

<pedant> No it isn't - those have too many pins to be 741 chips </pedant>

[magic]

Not sure what you mean by "get XXX voltage on pin 3".
You should apply some voltage between 2V and 8V to pin 3 and then see the same voltage appear on pins 6 and 2 (connected together), as shown in the text you linked.

And output on pin 7? That's the power supply.

[rstofer]

You can get documentation, including a datasheet here:
https://www.ti.com/product/LM741

Pin 1 - Input Offset (ignore)
Pin 2 - Inverting Input (-) (should be about 1V relative to Gnd)
Pin 3 - Non-inverting Input (+) (1V from voltage divider)
Pin 4 - Gnd or negative supply  --  I'm not sure this will work
Pin 5 - Input Offset (ignore)
Pin 6 - Output (should be about 1V relative to Gnd)
Pin 7 - Positive power supply (+10V)
Pin 8 - No Connection

The 1M/110k resistors should yield about 1V relative to Gnd for input to the non-inverting (+) pin (3).   If the voltage follower is working correctly, this will also appear on the output (pin 6).  The non-inverting input is the same as the output or 1V.  There will be no voltage difference across the pins, both will be 1V.

Watch Dave's video on Op Amps to understand why both input pins are at 1V relative to Gnd
https://youtu.be/7FYHt5XviKc

There are many other Voltage Follower videos on YouTube.

I'm not sure that a 741 will work within 1V of the lower supply (Gnd in this case).  Usually these op amps run with +-15V supplies.  Then there is no question that it will work.

FWIW, analog computers based on the 741 usually use +-15V supplies and signal swings of +-10V leaving plenty of room near the rails. 

Note that the 741 wants at least +-10V (or 20V across the device) according to the datasheet.  There are many other op amps that operate will far less supply voltage but the 741 isn't a low voltage device.  It would happily work at +- 18V so 4 each 9V batteries wired in series to produce 36V and center tapped to provide +-18V would be my first attempt if I didn't have a lab supply.

I have one of these dual output 15V supplies as well https://www.jameco.com/z/PD-2515-MEAN-WELL-Power-Supply-Dual-Output-Open-Frame-15V-1A-Negative-15V-1A-24W_2100857.html

Here's another way to go but the currents will be limited:
https://www.digikey.com/en/products/detail/murata-power-solutions-inc/NMH2415DC/1927087

These Power Bricks from Digilent are pretty handy.  They start with USB and can produce +-12V
https://digilent.com/shop/powerbricks-breadboardable-dual-output-usb-power-supplies/

[RobH]

@magic, oops it's another typo!!!!

Another retest and this time:
on 2 741's I get 4.8V on pin6, Pin3 and Pin2
On another 741 I get <1V on both pins 2 and 6, and 4.8V on pin3
And on another I get 1.5V on both pins 2 and 6, and 4.8V and on pin3.

Again it's not what I should be getting according to the schematic on that website
Really I just want to know if any of the 741's I have are faulty or not

Thanks

[RoGeorge]

There are DIL 14 pins 741, too.

Put a picture of the physical circuit.  Mark what you try to measure, and where.

Powering 741 at only 10V (as in +/-5V) might be too little for some brands of 741.  Better use something between +/-10V...+/-15V.  Same, 1V is too close to V-.  Use two equal resistors between 10k...100k as a voltage divider at first (instead of 1M and 110k).

[RobH]

I am using an Evantek DC power supply 0-30V and it is presently set at 10V

Now using x2 100K resistors as a voltage divider, I am now getting:

On 1 741 , 1.3V on pins 2 and 6 and <4.9V on pin3
On another , <1V on pins 2 and 6 with <1V on pin3
And on another 5V on Pins2 and 6 with 5V on pin 3
And a final one, 3.8V on pins 2 and 6 , and 5V on pin3

So there is no consistency there, and it looks like they are faulty.
Thanks

[pqass]

I am using an Evantek DC power supply 0-30V and it is presently set at 10V

Now using x2 100K resistors as a voltage divider, I am now getting:

On 1 741 , 1.3V on pins 2 and 6 and <4.9V on pin3
On another , <1V on pins 2 and 6 with <1V on pin3
And on another 5V on Pins2 and 6 with 5V on pin 3
And a final one, 3.8V on pins 2 and 6 , and 5V on pin3

So there is no consistency there, and it looks like they are faulty.
Thanks

Actually, the 3rd one's look right; 5V in (pin 3) should get 5V out (pins 2,6).

The 2nd one may be okay but you may have not supplied enough voltage on pin 3; should be about 2V or greater. You may have a better op amp than the typical 741; ie. can go closer to the lower rail. Could it be re-badged? Try 5V and see what the output is.  Nevermind, I see you're feeding all 741 pin 3 with the center of the divider (5V). So not seeing 5V on pin 3 means there's funny business going on inside the chip which is pulling it down.

Basically, the output should equal the input voltage for all input voltages between 2V and positive supply less 2V (conservative).  Try all your op amps with 5V as input. They should output 5V. If they don't they're bad.

See attached for my setup.  I'm using a 9V battery as a power source.
YELLOW=4Vp-p sine wave input to pin 3 from my function generator
GREEN=output pins 6,2

You will see that I vary the offset between 3V and 7V; peaks reach as low as 1V or as high as 9V.
But when the peaks go lower than 2V or higher than 8V, the output is clipped.

[RoGeorge]

The voltage is too low.
Repeat the test at 20V...30V.  It will work.

Attach a photo with the circuit where you measure, so others can double check.

[RobH]

Picture attached as requested.
There is a wire to the +Ve rail on the breadboard which is hidden by the PSU wire to pin7

[pqass]

Just to confirm...
is the red power rail (that which the 100K resistor is inserted in) also attached to your positive supply?
And the lower blue rail attached to the ground of the supply?

If so, then pin 3 should be 5V (given a 10V supply) and thus pin 6 (and 2) should be 5V also (measuring pin 3 to ground and pin 6 to ground with your multimeter, respectively).  If not, you have a bad op amp.

As I've demonstrated in my last post, the 741 can operate with a single 9V power supply.

[MikeK]

Image posted in #13 is bad practice, in my opinion.  Run the power supply input to the breadboard rails and then jumper the devices to the rails.  As mentioned, the image looks like the top resistor is connected to an empty rail.  Bypass caps can be put across the breadboard rails and/or at the chips as needed.

[RoGeorge]

From the picture the circuit looks unpowered.  In a breadboard, the inner holes are connected between them in columns, with vertical wires.  The most upper two rows of holes (and the most lower two rows) are connected in rows, with long horizontal wires.

What is on top of the IC?  Looks like scotch-tape, but whatever it is, better remove that.  It might mess your voltages.  Then, redo the wiring, modified as shown in the attached picture.



- note the red dot drawn on the IC.  Make sure the dent is in the left side.  From the original picture it is not visible where the dent is, but the dent in the capsule should be on the left side.  Sometimes the dent is slightly lower, near the pin.  That dent marks pin 1 of the IC.

- note the added blue wire, from pin 7 to V+, to power the opamp

- the yellow capacitor is needed only if you have very long wires to the power supply.  It's good practice to have it near the IC power pins.  Something like 100nF is OK, the value is not critical, can be 200nF, can be 10nF.  Sometimes you will see it in parallel with a bigger capacitor, like 10..100uF.

- set power supply at 20V.  If it has current settings, set current limit at 10mA at first (the 10mA setting is to limit the current in case anything goes wrong, the circuit will still draw its needed current as long as it is smaller than 10mA).  A 741 usually draws only 1.6mA ... 2mA with no load.


Now, turn on the power supply, and measure:

0.  First, check your instrument is set to measure Volts
1.  Measure between - (black wire) and + (red wire), and you should have the voltage you set from your power supply, 20 volts.
2.  Measure between - (black wire) and pin 3 (green wire).  You should have half of the supply voltage, 10 volts.
3.  Measure between - (black wire) and pin 6 (white wire).  You should have also 10V.

What did it measures for 1, 2, and 3?

[Zero999]

How about a Schmitt trigger oscillator? The 741 can probably drive a small piezo transducer directly, or use a lower frequency to blink an LED.

https://www.electronics-tutorials.ws/opamp/op-amp-multivibrator.html

[magic]

And what would it tell you about the chip?

You could have some Schmitt-triggered logic inverter with bizarre pinout here and this oscillator would still work

[Zero999]

And what would it tell you about the chip?

It will tell you whether it works or not.

[RobH]

@RoGeorge,

First, the white bit at the right end of the 741 is or was a white piece of paper as an identifier. As I am testing x4 741's, I didn't want to get mixed up etc.

The circuit is as per the picture, and the red and black wires to the left and bottom are from my meter.

On 2 of the 741's with 20V input, I get 9.97V on both pins 3 and 6.
On another 741 I get 1.9V on pin 3 and <1V on pin 6
On another 741 I get 10 V on pin 3 and 3.14V on pin 6

So 2 look ok while the 2 do not?

thanks

[magic]

It looks like you are doing it right.

The first two could be good units. Pull out the chip and see if the output of the divider is still 9.97V, chances are that the resistors simply aren't exactly equal so that's what you get. Also, set the DMM to lower range and measure difference between pin 3 and pin 2.

The rest doesn't look good. What's the history of those chips, where did you get them from and how were they abused?

[RobH]

@magic

Yes I still got 9.97V without the 741 inserted
These are the results , using a lower range on my DMM for all the 741's
On x2 ,I got 10V on pin 2 and 9.97V on pin 3
On another I  got 0.1V on pin 2 and 2.0V on pin 3
On the last one I got 3.1V on pin2 and 9.9V on pin 3

I bought the said chips form and ebay seller last year, and probably only used a couple of them at one time. I don't know about them being abused while I had them, but I can't promise

Thanks

[RobH]

And what would it tell you about the chip?

It will tell you whether it works or not.

Yes please if you will.

[magic]

They could be defective or simply fake (some other chips with LM741 printed on them). There is a lot of crap like that on eBay.

Can you produce a picture with clearly visible markings on those chips?

edit
There is another fun thing you could do with the first two chips which work correctly. Remove the resistors and connect pin 3 to ground. Leave pin 2 connected with pin 6. See what happens to the output:

1.5~2V - this is probably a true 741
less than 1V - this is probably LM358 with fake markings
almost 20V - this could be RC4558 with fake markings

These LM358/RC4558 fakes are fairly common. In China you can get those chips with anything printed on them: LM741, NE5532, TL072, OP07 - whatever you order, they will simply send one of those