Multisim and Ultiboard

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opamp help

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can someone help me for my project i was about to design an op amp comparator but i got stuck here finding the Vout of the circuit i need exactly 24v but the output i get is only 1.8v+ and thats stuck there i cant get even high there i keep changing the value of my resistor but still getting 1.8v please help me im just a student thank you 😞

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Hello,

 

first of all, you are measuring 1.8V only because you are probing the LED voltage which is color and current dependent, but it is a diode after all. You will get voltages close to this for a relatively wide range of currents provided by the opamp and limited by the series resistor. Move the DMM probe to the opamp output to see its correct voltage swing limits.

 

Second, you are switching between 12V and 12V again with the switch. Therefore the opamp will always be saturated HIGH. Try adding a second source with -24V to one of the voltage dividers so that the + input can change between 12 and -12V. Then the opamp will switch between HIGH and LOW saturation and will also switch the LED on and off accordingly.

 

Third, I did what I suggested above, but for some reason the LM358 does not switch to LOW, for negative input. The model might be broken. Try with an LM158 and the simulation will work. You should get around 8.5V and -10V voltage swing at that point, depending on the selected + or -12V input.

 

Fourth, the 12V input is outside the opamp input voltage range. You should check the datasheet for the acceptable input voltage range and then adjust the voltage division ratio to be in that range with your comparator input voltage.

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