enter image description hereI made this circuit but no matter what the input Voltage is, the output stays high and the led lights. Is it because of the offset voltage ? And where to connect the offset nulls to make the output zero? It did not work when I connected the to ground enter image description here

  • \$\begingroup\$ What LED? Use the schematic editor to create a complete and accurate depiction of what you built, and also we need to know the input voltage range you applied. \$\endgroup\$ Feb 22 '19 at 9:47
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    \$\begingroup\$ What are "offset nulls" ? (I know what you mean but I want you to explain better what you want). There is no LED in the schematic. What is the circuit supposed to do? A 741 is an opamp not a comparator. Connect what to ground? Also go read: electronics.stackexchange.com/questions/304521/… \$\endgroup\$ Feb 22 '19 at 9:48
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    \$\begingroup\$ Gh, if you don't show us the whole circuit, or at least the while relevant circuit, we are forced to guess what it going on. The output is "fed" to an LED. That could mean many different things. Just show it with a diagram. \$\endgroup\$
    – Bort
    Feb 25 '19 at 13:50
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    \$\begingroup\$ Don't use a 741, that's about the worst choice you could make, especially for a comparator. \$\endgroup\$
    – Hearth
    Feb 25 '19 at 14:51
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    \$\begingroup\$ Your two schematics are not the same. The one you used in the simulation is extremely unlikely to work, the other one clearly does. \$\endgroup\$ Feb 25 '19 at 18:30

An ideal opamp-based comparator multiplies its input difference (Vdiff = Vi-pos – Vi-neg) by its open-loop gain (Ao).

But there's always a non-zero offset voltage (Vos) which should be added to Vdiff. So the actual comparison equation turns into Vout = Ao (Vdiff + Vos). This equation tels us that;

  • If Vdiff +Vos if greater than zero then Vout will hit the positive supply rail.

  • Likewise, if Vdiff +Vos is lesser than or equal to zero then Vout will hit the negative supply rail.

Thus, even if Vdiff is zero then the output will hit either negative or positive rail depending on the sign of Vos.

As for your question, since the output hits to the positive supply rail when Vi-neg = 6 VDC regardless the value of Vi-pos, the source of the problem shouldn't be the offset voltage.

You may try adding a potentiometer to zeroe the Vos like following anyway:

enter image description here

  • \$\begingroup\$ then what could be the problem ? and can't i realy use the 741 as a comparator ? \$\endgroup\$
    – Gh-B
    Feb 24 '19 at 9:16
  • \$\begingroup\$ The biggest problem is to use 741 even in 2019! Anyway, to have the comparator outputting high the positive input terminal should be greater than 6VDC. But you say no matter what the input Voltage is, the output stays high. So it's clear that the problem is not about Vos. I suggest you to use a different, modern-day opamp (e.g. LM358). \$\endgroup\$ Feb 24 '19 at 15:59
  • \$\begingroup\$ and if i use the lm358 the circuit will work ?? \$\endgroup\$
    – Gh-B
    Feb 25 '19 at 12:40
  • \$\begingroup\$ @Gh-B most likely, yes. Please note that you should never leave the positive input floating. \$\endgroup\$ Feb 25 '19 at 15:08
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    \$\begingroup\$ @Gh-B "floating" means "connected to nowhere". At least place a resistor (e.g. 10k) between positive input and the ground. This prevents the comparator to pick up (because of extremely high input impedance) noise from outside world. \$\endgroup\$ Feb 26 '19 at 15:17

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