# help understanding LM324 IC

#### latitude

Joined Apr 24, 2013
3
i stuck on a homework question on lm324 ic in a Relay thermistor circuit {VElleman mk 138}

The use of LM324 IC and what are its advantages and disadvantages in a Relay thermistor circuit and how hysteresis works the circuit?
USING A .
all i know is that it is an op-amp and that out is going to greater than in put.

#### t06afre

Joined May 11, 2009
5,936
You would NOT have been given this assignment as your first learning about op-amps. And since we have no schematic. We are very much clueless. But I have good friend named Google. Maybe he can be your friend also
As general rule in the real world. You should not use an op-amp as comparator. It is a bad design idea

#### t06afre

Joined May 11, 2009
5,936
Tell us what you have done so far, and why and how you struggle. Then we can help you with a push in the correct direction. We will not just give you the answer.

#### latitude

Joined Apr 24, 2013
3
the teacher does not teach any theory on the chips or most electronics just gives assignments to build kit and in the wright up we have explain what each chip does or other parts do in the circuit. i kind off know what hysteresis but i do know any thing about LM324 IC i know it is 14 pin IC OP-AMP that gives you a big output than your input but does in circuit i dnot know or what its advantages and disadvantages

#### WBahn

Joined Mar 31, 2012
24,688
Have you read the LM324 data sheet?

How about explaining here what you think hysteresis is and it's relevance to this problem, both in terms of what good it is accomplishing and how it is accomplishing that.

How about explaining what the goal of the circuit is? If you were trying to sell this circuit to someone, what would you tell them it does for them?

How about walking through some of the portions of the circuit and discuss what they do. For instance, what is the purpose of IC1C and everything to the left of it? What is the role of R4? What is the purpose of R5/RV1/R6? What is the role of R7? What values can the output voltage of IC1D take on? On the right side of the circuit, what is the purpose of D1? Of LD1? of R3? How does the output voltage of IC1D relate to whether the relay is energized or not?

These are all things that should be addressed in your write up. Most of them only need a sentence or two. So take a shot at it here so that we can see what your level of understanding of the circuit is and then address any misconceptions you might have.

#### JoeJester

Joined Apr 26, 2005
4,077
here's the whole manual for the thermostat

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#### WBahn

Joined Mar 31, 2012
24,688
One approach to take with something like this is to assume the following scenario:

The company that sells this kit acquired it from another company that has gone out of business. They have hired you to validate the correctness of the kit and also to enhance it by providing a theory of operation section that they can add to the kit documentation. So you need to explain each portion of the circuit and evaluate the reasonableness of their component selections.

#### bountyhunter

Joined Sep 7, 2009
2,498
the teacher does not teach any theory on the chips or most electronics just gives assignments to build kit
Then you need to get in the teacher's face and politely request that he start doing his job. You won't learn anything accepting that kind of behavior from somebody whose official job title says he has to TEACH.

#### WBahn

Joined Mar 31, 2012
24,688
Then you need to get in the teacher's face and politely request that he start doing his job. You won't learn anything accepting that kind of behavior from somebody whose official job title says he has to TEACH.
I agree. Or go talk to the department head if you are uncomfortable talking to the teacher (but you really, really should talk to the teacher first -- they may not realize that they aren't being effective, you need to learn to exhibit professional courtesy by dealing with problems at the lowest level possible, and you need to develop the confidence to stand up for yourself).

Having said that, there is a place for this type of approach in which one of the objectives is for the student to research and find answers. But unless that is a major focus of the entire course, it should be limited to one or two homeworks or parts of one project. The fact is that you (directly or indirectly) are paying a lot of money to take that class -- $1000 to$2000 is the norm in the U.S. these days -- and you have a right to expect at least a serious attempt at quality instruction. Look at it this way: You entered into a contract with the school that basically says that the teacher possess knowledge that you do not and you are paying a fee for that teacher to make a good faith effort to transfer that knowledge to you. If they are doing nothing more but constantly saying, "Go figure it out on your own," then they are in breach of that contract.