Higher Physics

Electricity and Electronics - Activity 21

The inverting amplifier used to control heavier loads

Aim - To show how the Op-Amp can be used in a circuit to switch on a high current device.     Instructions
Click on the right side of the potentiometer to increase the input voltage V1.
Click on the left side of the potentiometer to decrease the input voltage V1.
  • Gradually increase the input voltage  V1. At some point the relay contacts should close and the motor will work.
  • Note the current value A1,  A2   and A3.  Comment on the size of these currents.  
  • State the gain of this circuit.
  • At what value of  V1  does the transistor switch on?
  • What is the value of the output voltage of the Op-Amp at this value of V1?  
  • The output voltage of the Op-Amp is also the input voltage of the transistor.  At what voltage does the n-p-n transistor switch on?
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