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  1. L

    Quick op amp question?

    Suppose that your task was to design a curve tracer for an op-amp in a black box. How would you design it? {Oh, no pressure here, but your future employment at this firm depends upon how well you complete this task!}
  2. L

    Quick op amp question?

    Tried doing the substitutions but did not get anything like the NTU.EDU web page. Obviously, those guys are wrong.
  3. L

    Help on circuit

    If I were breadboarding something, this is what I would start with: Set the virtual ground at 200 mV so the capacitor will integrate a -200 mV signal (ground) through R1 at a current of 200 nA. Ramp output will start at +200 mV and rise at the rate of 1 V per 5 seconds. When power is...
  4. L

    Help on circuit

    For the PWM circuit I would investigate a setup with two integrators and a comparator. The first integrator is configured as a linear ramp oscillator in the 1 - 10 kilohertz range where the ramp ranges between 0.3 and 3.3 volts. The second integrator takes the PIR ON signal and creates a ramp...
  5. L

    Help on circuit

    One needs to narrow down the range of possibilities by focusing on actual design specifications. What is the intended time duration of "slowly power up to full brightness"? Can it power up in discrete steps, or must it transition smoothly to full brightness? What about power down? What kind...
  6. L

    A myth about Generators and backfeeding

    Then how do people with solar power systems sell their excess power back to the power company? In some U.S. states I believe state law requires the power company to accept solar/wind power from individuals and even sets a generous rate for reimbursement. How would a generator be different?
  7. L

    Op amp calculations for ramps!

    This is an algebra problem with two equations and five variables, so there is no single solution. See attached pdf for a method to produce a graph that will help find a solution. The graph below shows a plot of (R1/R2) vs (RF/Ri) along with a plot of (V1) vs (RF/Ri). The dotted lines show...
  8. L

    Op amp calculations for ramps!

    Make a graph of Vout versus Vin, with Vout on the Y-axis and Vin on the X-axis. Plot the two points (4,5) & (6,10). Note the slope of the line (which is the gain from input to output). Now extend that line to where Vin=0V. Do you get Vout=-5V? That is the offset when the input is zero. So...
  9. L

    How does gain apply in diff op amp?

    What it means is that the input terminals (+ & -) of the op-amp contribute no current at nodes Va & Vb. There are two ways to look at this circuit. The first is to see this as an asymmetric differential amplifier with ground referenced inputs where one channel has a gain of +0.9 and the other...
  10. L

    How does gain apply in diff op amp?

    It might help to perform a general analysis of this circuit. Write a node equation (the sum of all currents at a node must be zero) for the nodes Va and Vb. Then realize that for the op-amp to be in its linear operating region (the only region of interest here) both input terminals (+ & -)...
  11. L

    Best method to slow down a 12v motor

    If the motors have stator coils instead of permanent magnets, how would a series connection of the motors perform in the case that the stator and rotor are wired in series versus parallel? I was hoping the OP would have replied "No problem, they're PM motors." rather than being forced to reveal...
  12. L

    Best method to slow down a 12v motor

    If these are identical DC motors with permanent magnet stators and both motors are intended to run at half-speed at the same time, then consider series connection of the motors. Each blower fan should provide the same load, each motor should run at half speed, so the series voltage should...
  13. L

    why do we use average power for capacitor?

    The voltage vs time graph and the power vs time graph for the charging capacitor are both straight lines. Your last graph with the curved line claims to show power but the line shows energy (as you realized and should have corrected).
  14. L

    why do we use average power for capacitor?

    Instead of finding capacitor energy using an exponential discharge model, one can use a constant current charge model. See attachment. It is debatable that the mathematics is subject to the physics. The models one uses to explain the physics are definitely subject to the mathematics. If...
  15. L

    why do we use average power for capacitor?

    One method of finding the exact value of the energy stored in a capacitor involves integration of an exponential function. See attachment.
  16. L

    Calculation of phase error for GSM standard

    The purpose of the test is to verify that the transmitted phase error is within the limits set by the GSM standard. It would seem that a test set capable of recovering the ideal phase trajectory from the transmitted signal is more versatile and simpler to operate. In the case of a transmitter...
  17. L

    Motion Detection with Optical Absolute Encoder

    A 2-bit encoder should work just fine. First decide what is the range of angular position that needs to be encoded (something between 0 and 360 degrees). Then distribute the 2-bit Gray code over that arc on the encoder disk. For this application the encoded positions might not be evenly...
  18. L

    calculate effective resistance between node D & node E

    One must redraw the circuit so the solution becomes more intuitive. Notice there are three vertical branches in parallel. Move the R5,R6 branch with node 'd' to the far right. Focus on the R3 branch in parallel with the R4,R2 branch and reduce that to a single resistance which is in series...
  19. L

    Swamp cooler switch

    All the swamp coolers I am familiar with have a dual-speed motor, so the swamp cooler switch is multi-position with the following control states: OFF/LOW-VENT/HIGH-VENT/LOW-COOL/HIGH-COOL/PUMP-ONLY So is your fan single or dual speed?
  20. L

    Help with project

    If the power has been off for longer than 3 minutes, then the relay must turn ON as soon as the power returns. But if the power has been off for a very long time, then the supercapacitor will have discharged. How long will it take to charge the supercapacitor before the circuit will turn the...
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