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LM324 as buffer conundrum (single supply)
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VogonJelc:
Hello friends. This one is interesting. I decided to build dummy load to test dc power adapters. I decided to rip off Dave's design using lm324 and mosfets. I bought couple of lm324 (quad) since 741 I had were some 26+ years old and I figured they are either dead or soon to be. I use no2 op amp (pin 5 + pin 6 - and 7 output) as buffer and no 3 (pin 10+ pin 9- 8 output) to drive MOSFET . I tested setup with POT before hooking everything to arduino nano. I want to use 2 not used op amps as buffers for input Voltage measurement and to measure voltage across resistors connected to source of MOSFETS.
And the problem is LM324 is having huge problems when voltage on non inverting input is close or 0v when working with single supply voltage (in test case it was 6v but I will use 12v when I finish ironing "issues"). Everything works fine (except v-1.5v max output voltage but its ok Ill use 12V for supply). GOing into mV range gets strange first at about 300 mV output stops following input by few hundred mV. But as soon as I drop to 0V input I get 4.75 output voltage. Which kind of isn't good for buffer nor as output of voltage divider to measure input voltage. Anyone had similar problems and possibly suggestion on how to solve the problem or maybe replacement op amp that has same pinout - the problem of lm324.
I see that dave is using sam op amp on his dummy load but I also noticed on video he didn't start from 0 V but from 80mV.
There is no schematics attached because its literary the same as Daves quad op amp mosfet couple of resistors (6 x 3.3 ohm 5w resistors).
orbanp:
The LM324 is an early op-amp, and it could do funny things when you are getting close to the rail voltages.
Do study the datasheet!
To verify if that is the reason, do add some negative supply to the op-amp(s), and redesign your circuit with a more modern rail-to-rail op-amp, if proven so.
PS: Do include the schematics!
I (and probably the others) do not keep that at my fingertips!
Regards, Peter
VogonJelc:
Found the reason for it behaving weird. It seems TI is not what it was or my dealer sold me broken op amp. Op amp no 2 is "partially" dead (?) and no 3 is totally dead outputting 600mV no matter the input voltage is. I have to test it again but after replacing it with another chip everything works ok. But out of the 4 of the lm324 I have that are working there are 2 that behave same outputting 5v at no 3 op amp no matter what and 2 where op amp no 3 outputs from 2.2v to 5v.
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