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[SOLVED] Need help with OPAMP circuit pls.
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Chriss:
Hi!
I made this design to control a relay with a UA741 OPAMP.

The idea is when a condition is reached (temperature HIGH or LOW) on the NTC the output of the UA741
should power on or off the relay through a transistor.

Everything is fine, and it is working well as expected, only one thing I can't figure out how to solve that
or where I made my mistake in my design.

Problem:
When the relay goes from OFF to ON or from ON to OFF it shakes, actually, there is no clean ON or OFF click on the relay.
It makes some "trrrrr" sound.  :-DD
Btw. I interchanged the BD679 with BC639.
That BD was a bit overkilled would I say.

I tried to change the R5 from 0.5M to 2M etc. but no result.
I wish to have the schematic as simple as possible, so no some extra parts to add like some IC or so if it is
possible to make the clean ON-OFF output without.

I have to say, OPAMP's are not my shiny side of knowledge.  :-[

Any advice would be nice.

Thank you.
My best regards.
Zero999:

--- Quote from: Chriss on November 23, 2018, 01:52:24 pm ---Hi!
I made this design to control a relay with a UA741 OPAMP.

The idea is when a condition is reached (temperature HIGH or LOW) on the NTC the output of the UA741
should power on or off the relay through a transistor.

Everything is fine, and it is working well as expected, only one thing I can't figure out how to solve that
or where I made my mistake in my design.

Problem:
When the relay goes from OFF to ON or from ON to OFF it shakes, actually, there is no clean ON or OFF click on the relay.
It makes some "trrrrr" sound.  :-DD
Btw. I interchanged the BD679 with BC639.
That BD was a bit overkilled would I say.

I tried to change the R5 from 0.5M to 2M etc. but no result.
I wish to have the schematic as simple as possible, so no some extra parts to add like some IC or so if it is
possible to make the clean ON-OFF output without.

I have to say, OPAMP's are not my shiny side of knowledge.  :-[

Any advice would be nice.

Thank you.
My best regards.

--- End quote ---
The left hand side of R5 should go to pin 3, the non-inverting input, not +12V. It provides positive feedback, i.e. hysteresis, so the on voltage is slightly higher than the off voltage.

R4 and R6 aren't needed. The transistor is configured as an emitter follower, thus the base current is inherently limited.

Although it will work, the uA741 isn't the best IC for this. It's an op-amp, which is designed as a linear amplifier, not a comparator. I'd recommend the LM311 which has up to 50mA of output drive and will be able to drive that relay with no external transistor.
Chriss:
Wow!
Such a fast replay! Thank you mate!

So, here is the mod of my new schematic.
Is this like you suggested?
Just to check if I understand it correct.

Thank you very much.
Zero999:
Yes, that's good. It shouldn't oscillate.

It appears you're using the relay to switch mains. If so, make sure there's sufficient creepage and clearances between the mains and DC sides of the circuit to ensure protection against electric shock. Even if it works and doesn't shock you when you test it, a high voltage spike on the mains could be fatal, if the minimum creepage and clearances haven't been met.
Chriss:
Yes, you are right, I'm switching mains.
At the moment I was thinking about that what you wrote about the clearings of AC and DC side.
I didn't test it right now with a main connected, I'm playing now with the DC side to get the result I was expected.

I also was thinking to protect the circuit with some sort of painting so the painting would act as an insulator on the traces.

I don't sure does it really will do the job or should I make some other things.

The relay should drive an AC water pump which is Pmax=100w, most of the time it will work on Pmin=35w.

Do you have maybe some link with an explanation of a correct AC/DC separation etc.
or is there some rule of thumb maybe for such a circuit?

Thank you.
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