Search results

  1. L

    Transistors / hfe / Active region conflicts

    But again you misunderstand. I never claimed there was no connection. This is where the magic happens. That particular stance only goes so far. Transistors do not work just by themselves, they eventually end up being used in a circuit. And one objective of good circuit design is to make the...
  2. L

    Transistors / hfe / Active region conflicts

    Even in this example Vbe is some assumed constant value (that may drift over a small range), not an exact value of Vbe that sets the base current.
  3. L

    Transistors / hfe / Active region conflicts

    If my statement were taken in isolation then your objection could have merit. However, the assertion was in the context of base current being set by a current source (or a biasing network that approximates a current source). In that context, the exact value of Vbe is not relevant although it...
  4. L

    Transistors / hfe / Active region conflicts

    Exactly! No matter what the device does, nothing changes. It's as though the device were connected to nothing. Damnable Physics! One can do many things which are forbidden by common sense, but that leads to perdition. When the diode is driven by a current source the current is set by the...
  5. L

    Transistors / hfe / Active region conflicts

    So now let us return to your original statement: A current source has an infinite resistance; when anything is connected to a device through an infinite resistance, the device is unable to discern that anything has been connected. And this is quite true (from an external standpoint; while from...
  6. L

    Transistors / hfe / Active region conflicts

    Whenever you drive the base of a transistor with a voltage source, you are inserting an external resistor from the voltage source into the base circuit. The better the fidelity of the voltage source, the closer this external resistor (i.e., source internal resistance) approaches zero. As this...
  7. L

    Transistors / hfe / Active region conflicts

    Of course they are the same graph, but of different components. The first graph gives the characteristic curves of an idealized transistor whereas the second graph gives the characteristic curves of a real transistor, where the current gain h.FE varies with the collector-emitter voltage and...
  8. L

    Transistors / hfe / Active region conflicts

    Why does it seem the active region has a constant collector current for all base currents? What is it about the characteristic curves at university that is different from the ones used at BTEC? Please post the images.
  9. L

    74HC logic output signal

    The appearance of spikes at the clock frequency on an otherwise steady signal would indicate that the noise is on the power/ground lines. More/better decoupling is needed. Are your bypass capacitors the multi-layer ceramic RF type capacitors? Keep leads as short as possible and try to emulate...
  10. L

    help for circuit analysis

    Indeed you can write the node or loop equations and solve those, but that just changes an electronics problem into an algebra problem. However, by using Thevenin simplification and voltage division one can solve this circuit as an electronics problem instead. First identify the two major nodes...
  11. L

    1V audio burst every OTHER pushbutton

    This is a photo of the finished sensor head. Doing it again I would avoid using cross tubes for holding the LED and phototransistor since it is difficult to accurately drill large holes through tube. Would prefer to drill into a block of aluminum (½"x1"x1") or even dense hardwood. The slider...
  12. L

    Just how

    Yes, it was quite obvious. But at some time in the future you will have a project that is your own. Remember then to post the full design, and not just the finished photos.
  13. L

    Just how

    If in the future when you blog about one of your completed projects, remember just how worthless it is to post photos of circuit boards but include no information about the circuit design. A Google Search for "DC to AC inverter circuits" would be a good starting point.
  14. L

    DC pulse synchronization

    One generally refers to back-EMF in inductive circuits (as opposed to rotating machines) when a driver goes from a low impedance to a high impedance state. If the driver is always in a low impedance state, e.g. function generators, then the back EMF is quenched. So it seems this is a problem of...
  15. L

    1V audio burst every OTHER pushbutton

    Two time periods need adjustment, both relating to the velocity of descent of the damper. There is the speed limit pulse width which determines the slowest velocity to trigger the 10KHz burst, and the delay pulse width for the time between triggering the bottom sensor and the damper slamming...
  16. L

    1V audio burst every OTHER pushbutton

    Rather than attach smaller brass tubes to hold the LED/phototransistor I tried using a larger 11/32" tube and drilling a 9/32" hole to accept the 9/32" brass tube. Ordered some Vishay BPW77NA phototransistors, so when they get here I will cut the tubes to the proper length and prepare sleeves...
  17. L

    why is my op amp doing this?

    The issue is input bias current charging the capacitor and thereby changing the capacitor voltage. According to the spec sheets the typical input bias current for the LM324 is 40 nA whereas for the TL074 it is 65 pA. Both op amps have the same problem, but it is 615 times worse for the LM324.
  18. L

    1V audio burst every OTHER pushbutton

    The brass tube shown is not pre-made, but I wanted to see how difficult it would be. It took an hour in the garage shop to drill the holes and cut the slits. Essential to hold the tube tightly in a drill press vice; the tight fit wood dowel kept the tube from deforming. Used a centering drill...
  19. L

    1V audio burst every OTHER pushbutton

    Here is a suggested starting point for the optical sensor circuit. When light is blocked to the sensor the collector resistor raises the output HI. The sensor outputs are each fed to a Schmitt trigger to give a clean transition. The Upper Sensor transition is high-pass filtered to give ~10...
  20. L

    1V audio burst every OTHER pushbutton

    Here is an example of what I meant by using optical slits. This is a 9/32" OD brass tube with a 1/4" wood dowel. The slits are separated by 3/8" and measure 0.35mm wide. Opposite each slit is an opening for LED illumination. Still left to do is solder smaller brass tubes to hold the...
Top