Help with voltage threshold calculations

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Discussion Overview

The discussion revolves around calculating voltage thresholds in a circuit involving a Schmitt Trigger configuration. Participants are attempting to clarify the influence of various voltage sources and resistor configurations on the threshold voltages, with a focus on the correct application of circuit analysis principles.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the correctness of their calculation involving saturation voltage and resistor ratios.
  • Another participant points out that an additional voltage source (V4) significantly affects the threshold voltage calculation.
  • A participant expresses uncertainty about the appropriate formula to use, noting the absence of a third feedback resistor typically seen in Schmitt Trigger circuits.
  • There is a suggestion to analyze the voltage at the non-inverting input for different output conditions (Vout = +3V and Vout = -3V).
  • Participants engage in calculating the voltage (Vx) based on Ohm's law and the circuit configuration, with varying results and corrections regarding the influence of V4.
  • One participant proposes a calculation for Vx based on the current through resistors, leading to a discussion about the implications for threshold voltages.
  • Another participant confirms that the additional voltage source shifts the threshold voltage, indicating a dependency on the sign of V4.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the initial calculations, with multiple competing views on how to incorporate the voltage source V4 and its effect on the threshold voltages. Some calculations are corrected or challenged, but no definitive agreement is established on the overall approach.

Contextual Notes

Limitations include potential misunderstandings regarding the circuit configuration and the role of the additional voltage source. The discussion reflects varying assumptions about the circuit elements and their interactions.

BigMan52
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Thread moved from the technical forums, so no HH Template is shown.
Please can someone tell me if this calculation is correct? Please see attached question.
I have used the +/-3V as the saturation voltage and multiplied it by R2/R2+R3 to give me +/-1.5V.
 

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Is this a Homework ?
For the circuit in FIGURE 3(b) your answer is wrong.

Picture1.png


Notice that you have additional +3V voltage source (V4) in your circuit. And this voltage source will have big influence on threshold voltage.
 
Sorry yes, I think I may have posted it in the wrong place. First time using this site. I will have a rethink and post it back in the correct section.
Thank you very much for your help.
 
So now, can you show us the "right" answer? And my tip is: Find the voltage at non inverting input for two cases: one for Vout = +3V and Vout = -3V. And do not forget about V4.
 
Struggling to find the formula I should be using. I think this is a Schmitt Trigger and all calculations I have seen include a third feedback resistor which isn't here.
 
OK. But are you able to find the Vx voltage for this circuit?
1.png
 
Would it be:
Vx = R1/R1+R2 x Vout = 0.5 x 3 = 1.5V?
 
No, wrong again? Why do you keep ignoring the V4 source?
 
OK using basic ohms law:
I = 3/1000 = 3mA
So voltage dropped across the 1K resistor would be 9mV making Vx =2.991 V?
 
  • #10
OK, let us start again from the beginning.
The circuit look like this:
1.png


And notice that now V4 = 0V and R1 and R2 are connected in series.
The voltage at Va is equal to Vout = 3V, the Vb voltage is 0V because V4 = 0V.
So the voltage across (R1+R2) is Va - Vb = 3V - 0V = 3V and the current is I = 3V/(R1+R2) = 3V/2kΩ = 1.5mA therefore Vx = Vout - I*R1 = 3V - 1.5V = 1.5V
But in your circuit V4 is not 0V it is 3V so the situation looks like this (for Vout = +3V)
1.png


And when Vout = -3V
1b.png


Can you solve it for Vx ?

And I highly recommend you to read this by starting at page 3
http://www.ittc.ku.edu/~jstiles/312/handouts/312_Introduction_package.pdf
 

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  • #11
OK thank you very much.
So here goes:
Vx when Vout is -3V:
Vb =V4 = 3V
Voltage across R1+R2 = Va-Vb = -3-3 = -6V
Current I = -6/2000 = -3mA
Vx = Vout - (I x R1) = -3 - -3 = 0V

Does that mean the upper threshold voltage for this op amp is +3V and the lower threshold voltage is 0V?
 
  • #12
BigMan52 said:
Does that mean the upper threshold voltage for this op amp is +3V and the lower threshold voltage is 0V?
Yes exactly, this additional source (V4) will "shift" the threshold voltage up or down depend on V4 sign.
 
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  • #13
Brilliant, thank you very much for all your help!
 

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