Function gen circuit question - Page 1
Forum section: electronics / beginners

author: eventhorizon
author_rank: Contributor
timestamp: October 10, 2016, 05:07:12 pm
content: So I have been looking at this and got an lm324 and waiting on my caps to get delivered today so I can build this later and had a question about the schematic for this circuit. So it shows pins 2 and 3 connected to ground and pin 1. When first powered on pin 1 will not have any voltage on it so 0v there and 0v for the ground so as far as pins 2 and 3 will ever see is 0v and since they will try to keep the voltage on pins 2/3 the same it will never output anything (right?) as its drawn. So since I was confused I went to the datasheet and seen the square wave generator and its connected to v+ and the pin 3 so I trust the datasheet first but I am confused as to if this circuit has been created with assumptions or is in error.Original source:http://www.learningaboutelectronics.com/Articles/Function-generator-circuit.php

author: Dave
author_rank: Super Contributor
timestamp: October 10, 2016, 06:36:24 pm
content: You are assuming a perfect textbook style opamp. In the real world, you are always going to have some sort of offset voltage and bias current so the output will either jump to the positive or negative rail on startup. From then on, you can probably see why it is going to oscillate.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: October 10, 2016, 06:59:41 pm
content: The first circuit is an opamp based astable multivibrator. Your link does not explain it and you seem confused.Here is a more detailed explanation.http://www.electronics-tutorials.ws/opamp/op-amp-multivibrator.htmlHope it helps. (Even an ideal opamp will nicely oscillate).

author: eventhorizon
author_rank: Contributor
timestamp: October 10, 2016, 07:01:07 pm
content: Thank you that makes perfect since. Now I just have to wait for the delivery guy from amazon to get here with the caps and I can try it out.

author: eventhorizon
author_rank: Contributor
timestamp: October 10, 2016, 09:28:39 pm
content: So I got the caps in a little while ago and wired it up and well it didnt work I was getting 0v out then I redid it and again 0v and then I walked away and came back and seen that I had the chip backwards. We I fixed thatand then wired a 3rd time and this time I got 2.5 v out but not a square on my "toy" dso so could be the freq is too high for the dso or something is not wrong but I did notice that this lm324 is hot to the touch. Is this normal and expected?

author: rstofer
author_rank: Super Contributor
timestamp: October 10, 2016, 09:39:37 pm
content: That op amp should not get hot! Something is wrong. First guess: you fried the chip when you installed it backwards but it would be good to look at the circuit again. In fact, you can wire up one stage at a time and test the intermediate results.

author: eventhorizon
author_rank: Contributor
timestamp: October 10, 2016, 09:48:22 pm
content: Quote from: rstofer on October 10, 2016, 09:39:37 pmYep thats what I was doing is the first stage the square wave and thats when I noticed the heat and yanked the power. Well if I fried it, its not the first time and wont be the last but at least I order more than one chip when I get them.

author: eventhorizon
author_rank: Contributor
timestamp: October 11, 2016, 04:30:27 am
content: Well I have a few bits of good news. First I didn't kill the chip, turns out when I flipped the chip I never flipped the power and ground so once I did that no more heat. Second I found that unless I did the power into the pin 3 that it didn't do anything. But I did get square working.The best news so far is that triangle failed. So now I get to pull out a calculator, datasheet and google like crazy and truly learn what is happening in this circuit and troubleshoot the @#$% out of it.

author: zapta
author_rank: Super Contributor
timestamp: October 11, 2016, 05:23:49 am
content: The second amplifier works as an 'integrator', charging and discharging the capacitor with constant current based on the square wave input.