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  1. R

    RLC series question

    Where is that formula you posted coming from? What you have to do is find the frequency where the inductive reactance and capacitive reactance equal each other. Can you do it? It's not hard. Ratch
  2. R

    RLC series question

    What's the problem? Your calculations for 100 kHz are correct. Why assume another frequency? Your calculations for 100 Hz are sort of correct, except you have the inductor and capacitor voltages interchanged. Perhaps you should show us your calculations. Ratch
  3. R

    Why the crazy price for a decade resistor/resistance box?

    Selecting a large resistance value does not justify the expectation that the total resistance will be within the tolerance of the lowest resistance value that could be selected (one ohm)? For instance 10% of 1 ohm is 0.1 ohm. For a 10 k resistor to be within that value, its tolerance would...
  4. R

    Help with phasors please

    Because you have probably handed in your homework, here is the phasor diagram. Notice that I = I1 + I2 . Ratch
  5. R

    Question about transistor

    Ah, my favorite subject. A BJT, FET, and thermionic vacuum tubes are all transconductance devices (voltage controls current). True, you can incorporate them into voltage amplifying, current amplifying, etc circuits, but by themselves, they are all transconductance devices. The base current of...
  6. R

    Decoupling capacitor

    Yes, microfarads. I never heard of anyone saying millifarads, although the prefix is correct. Ratch
  7. R

    Decoupling capacitor

    You need to learn about decoupling and bypassing. Ratch
  8. R

    Decoupling capacitor

    The usage determines what kind of cap it is. In your case, what are you bypassing? Caps that big are sure to be electrolytics. Ratch
  9. R

    Decoupling capacitor

    It doesn't work that way. A capacitance that small would never be able to sustain that much current for that length of time. The cap would de-energize in an instant. Ratch
  10. R

    Decoupling capacitor

    That is a question for your local parts supplier or a catalog, but not me, I would think the 12 volt system could keep the right kind of battery fully energized without over energizing it. Ratch
  11. R

    Decoupling capacitor

    OK, when the voltage is down to 9 V, and the current is 0.3 amps, the alarm looks like a 30 ohm resistor to the capacitor. We want the RC time constant to be about 5 times 0.1 sec, so that makes the capacitor value 16,667 mf. You may want to double that value because electrolytics are notorious...
  12. R

    Decoupling capacitor

    Let me get this correct. The system will operate at 12 volts until it switches to another voltage source. You do not want to go below 9 volts during the switchover. The time for the switchover will be 0.1 seconds, not 0.5 seconds. The current drain will still be 0.3 amps. Do I understand...
  13. R

    Decoupling capacitor

    Have you considered a battery? If you can get the right voltage and capacity, your problem will be solved. Otherwise, the use you propose for a capacitor is not decoupling. So you want to supply 300 ma for 0.5 sec? What is the nominal voltage and how low can it decrease and still be...
  14. R

    Question about transistor

    What type of transistor are you asking about? A FET or a BJT? Ratch
  15. R

    NPN transistor still working? Strange ..

    Those interested might want to read this thread, especially post #5. http://cr4.globalspec.com/thread/72501/Emitter-Base-Breakdown-Voltage-of-NPN-Used-as-Zener Ratch
  16. R

    NPN transistor still working? Strange ..

    It means its reverse breakdown voltage has been exceeded. You can study what that means by reading some of the rich variety of literature on semiconductors. Ratch
  17. R

    NPN transistor still working? Strange ..

    When no more current exists in the LED, of course. Due to the E-B junction no longer in its breakdown state. Ratch
  18. R

    NPN transistor still working? Strange ..

    When the E-B breaks down, current exists through the diode and it turns on. The timing depends on how fast the cap energizes and de-energizes. Ratch
  19. R

    NPN transistor still working? Strange ..

    The cap builds up voltage as it is energized. When it reaches the breakdown voltage of the E-B junction, the E-B junction shorts out, and another path for the current from the battery and the current from the de-energizing cap is available. When the current in the E-B path becomes too low to...
  20. R

    NPN transistor still working? Strange ..

    Let's start with the last question first. Current will exist where there is a voltage and a conduction path. If more than one path exists, more current will be present in the path with the least opposition. The opposition of the capacitor is the back-voltaqe that occurs when the capacitor...
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