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Posts posted by shekhar_dandya

  1. Hi Alun,
    I am trying to solve the problem as you told in the image.But by mistake I bought 100uF caps.could they help or should I strictly connect 100nF caps?
    Exactly what is the theory behind earthing the interwinding capacitance?how about using resistors instead of caps?do the caps form a certain kind of tuning circuit with the secondary windings?If so what effect does it have at 50 Hz?

  2. Hi kevin,
    Considering the thevnin equivalent with a voltage source in series with a source resistance and a series load resistance,my theory goes like this:
    If the load impedence is larger than the source impedence(open circuit)then power transffered to the load is zero.P=VI=0.
    If the load impedence is less than the source impedence(short circuit)then also power transffered to the load is zero p=VI=0.
    If the load impedence equals source impedence,voltage at the load is half the source voltage hence the power =1/2(VI) and this is the maximum power that can be transffered from source to load.If the load changes on either side,the power X'fered would drop.
    I may be wrong or too priliminary

  3. Hi All,
    A Happy 2005.
    I recently made a small adjustable power supply with a voltage range from 3V to 24V and a current range of 1A using the rectifier-filter-LM317 combination.
    I tested its outpput on a multimeter with its knob on"DC V" and it showed me the correct voltages.Then I turned the knob on "AC V" and found that it showed me an ac voltage of 56V! at the o/p of LM317.It also shows some 11V DC at the o/p of secondary (Knob kept on DC V) and a 71V AC at the filter(Knob kept on AC V).

    Should'nt the o/p of LM317 show 0V AC or atmost a few mV AC(The ripple)?
    Why am I getting a DC voltage at the o/p of secondary?despite the "AC V" shows the correct RMS value of 24V(RMS secondary).
    Due to lack of a CRO I cannot view the waveforms and have to rely on my DMM.is it faulty?It is not a true RMS meter.I guess it is directly coupled inside for ac voltages.

    I have used a capacitor of 3300uF and tests are carried out in no load condition.Is this making any difference?maybe the cap is not discharged?


  4. Hi Audioguru,
    Merry Christmas. :)

    I have put an emitter resistor to ground and also biased the base of
    x'tor from collector.This improoves dc stability.But for ac
    conditions,emitter is bypassed to ground by an emitter capacitor to
    enhance ac voltage gain.This would again make the gain formulae
    dependant on h parameters.

  5. Hi kevin,
    What is the difference between an open circuit and a 3 state?Suppose I have a memory device with tri state I/o buffers.If I tristate output buffers (which may be having valid voltage levels on their I/O pins corresponding to data from a previous read but not producing current)and enable input buffers for writing,then the data I put on the bus would cause contention resulting garbage written to the selected location.
    What is the voltage of a 'Z' state?Does it correspond to logic1/logic0 or any other?Why do they show a horizontal line at the center of the waveform if it can be any open circuit voltage?

  6. Hi All,
    Actually I was trying to make a small signal voltage amplifier from descrete transistors for audio frequency range.I used a small signal h-arameter equivalent circuit of the transistor.I decided to use a BC107 for it.Then I used the formulae from my electronics textbook to calculate the current and voltage gains.these were:

    I used a voltage divider bias to bias the Q point.(Vcc=10V,Vc=Vcc/2,Ic=5mA)
    Rb=R1 || R2 (R1,R2=Resistors of voltage divider)

    From the datasheet of BC107,I looked for the hFE v/s Ic curve to find that beta of X'tor was 250 @Ic=5mA.

    The 'electrical characterstics' table did not specify h parameters at these conditions.

    Now that the X'tor I use has a beta of 100.Also the Gains mentioned above depend on h parameters.Do these parameters differ from X'tor to X'tor? ie peice to peice? Can it be that the datasheets specify value of hie as 4Kohm but the X'tor piece you use has a hie about half of that?In that case above voltage gain would double.
    How does one calculate amplifier parameters(Ai,Av,Ri,Ro)to meet your specifications?Are there any other formulae to calculate these parameters to see the circuit meeting your design conditions?
    How much reliable it is to rely on the small signal equivalent model of the X'tor to design an Amplifier?How do you design one to meet the necessary conditions?

    Kevin, do you want to say that assume the test conditions specified in datasheets in your circuit and then calculate the remaining parameters?I find this inflexible.

    more help needed.


  7. Hi Everyone,

    The electrical characterstics of common components (like diodes,transistors,opamps,regulator chips etc etc...)
    are mentioned at perticular test conditions of voltages and currents.Our circuit may not have these test conditions.
    Then how do we decide the value of a perticular parameter based on conditions of our circuits?
    Is it that we can safely rely on values in "Typ" coloumn for our circuit or there is any other way of selecting.
    Help needed.


  8. I want to build a counter with 7490's and a BCD to 7seg decoder(common anode). I don't want to use a microcontroller.
    For control logic of switching transistors,I have used a 7490 and a 3:8 decoder which pulls base of PNP X'tors low sequentially.I wanted a logic that puts the correct BCD code onto the 7seg decoder when the correct LED digit is switched on.

  9. I wanted to know that if one uses an inductor in series with a switch
    (either a transistor or a mechanical switch)and a DC source without using a freewheeling diode,how will the inductor react when switch is opened?
    will it cause arching/sparking or melting of transistor?if so,why?
    what happens practically without a freewheeling diode?

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