Problem using Opamp as buffer with single supply

[savvej]

I want to use OP-amp(UA-741 from texas instruments) as a buffer to read voltage from a potential divider network(using a potentiometer).
I am using a supply of 5V (single supply).Now this is what happens:
Initially when the output of potentiometer is zero the opamp output reads 1.85 volts instead of zero volts.Then I increase the input,the output starts following input after the input crosses 1.85 volts(at around 1.89 volts).Why is it happening so?I want to use it a as buffer in the range of 0-5 volts and using 5v supply only.Please help..

 

[ARandomOWl]

Read the datasheet for the 741 op-amp. It is not a rail-to-rail output amplifier. The output will only swing within about 1.5V from each supply rail. Either use a larger range of supply voltage or use a rail-to-rail op-amp.

 

[savvej]

kk.so using 9V instead(single supply),the output should then follow the input?

 

[savvej]

will a dual power supply(+9v ,-9V )solve the problem?

 

[ericgibbs]

kk.so using 9V instead(single supply),the output should then follow the input?

hi,
You require two 9V batteries, one for +9V and one for the -9V supply to the 741, if you want the output to range from 0V to +5V

 

[savvej]

kk...can i use max 232 for providing supply to opamp?
I have done it that way before, you will around +/-9V

 

[SneaKSz]

Take a look at the amount of current the 741 draws, and if it's less then the current the max232 can give, the 741 will work properly.

kind regards

 

[ARandomOWl]

Can't you use a modern op-amp with a rail-to-rail output? That would be a much simpler solution.

 

[Ian Rogers]

Cant be much current from 10uf caps!! BUT if it works let us know (handy little tip)

 

[Gayan Soyza]

Use LM358 & supply it with a 9V single supply.No need dual supply.

Rail to rail op amps are harder to find.

 

[Nigel Goodwin]

Check my tutorials for the way I solved the problem - but for a start don't use a 40 year old antique opamp.

I didn't even consider 'rail to rail' opamps, simply because they aren't what they say, they are only 'near' to the rails - my solution gives from exactly 0V to whatever you want.

 

[savvej]

Check my tutorials for the way I solved the problem .

Could you tell the link?I couldnt find it on winpicprog.co.uk(visibly)

 

[be80be]

It's this page But it doesn't tell what op-amp he used

 

[colin55]

Why not use an emitter-follower transistor.

 

[Nigel Goodwin]

It's this page But it doesn't tell what op-amp he used

I used a TL072, it's labelled on the layout diagram.

But as there's no rail to rail requirement there are lot's of opamps that will do.

 

[be80be]

Sorry Nigel I see it I didn't look at that diagram. It's there

 

[Nigel Goodwin]

Sorry Nigel I see it I didn't look at that diagram. It's there

I had to look twice

 

[savvej]

Thanks @ Nigel and @be80be.
I used IC 555 along with diodes and capacitor to generate -ve supply though.
yet to try if max232 works.Hopefully it must.

@colin55 Your method is simplest .Will surely try it.And actually it serves my pupose directly as I need to clip of any negative voltage present in the input too.Though there will be a loss of 0.7 V across B-E.Thanks.BTW this is what I am exactly doing and needed a buffer to isolate the transformer output directly to uC and to supply greater current.I used a battery of 1.5V to level shift the sine wave which would'nt be needed in your case.Lets see how it works.The thread is here (though I removed buffer latter) :https://www.electro-tech-online.com...ormer-as-sine-wave-source.121343/#post1000410