op amp single supply

Thread Starter

tech1

Joined Oct 22, 2012
4
Op amps used : lm358
Image 'a' is inserted for reference.
In image 'b', a second op amp is introduced but with a single positive supply in contrast with the first dual supply op amp. The measurements taken with multimeter shown in image 'b' indicate that the voltage of the batteries seems to drop with time.
Image's 'c' measurements were taken after about an hour and they show that Vout has reached a negative voltage value. How is this possible with single positive supply voltage and why do the batteries seem to drop voltage?
Thanks for your time
 

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wayneh

Joined Sep 9, 2010
17,496
I don't know for sure what's going on, but your second op-amp won't like having a voltage on its input that is below its negative power rail voltage. I could speculate that's causing an unexpected current flow.
 

Audioguru

Joined Dec 20, 2007
11,248
Why don't most NOObs not read the datasheet?
The datasheet for the LM358 says that its maximum allowed input voltage is 0.3V more negative than its negative supply voltage. They say that if the voltage is more negative then input current will flow.
 

Thread Starter

tech1

Joined Oct 22, 2012
4
I forgot to mention that the voltage drop of the batteries was not battery drain, when op amp 2 was removed both batteries rose to their initial voltage values with only some millivolt permanent drain. The same thing happens when connecting a battery with e.g. a 220 Ohm resistor for some time. Thanks for checking the datasheet for the input current flow, should have noticed that myself, hasty post.
 
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