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  1. audioguru2

    Driving a HDD motor

    Look in a catalog of millions of Mosfets to find some that work with only 4.8V on their gate. Most online catalogs like at Digikey allow you to select spec's.
  2. audioguru2

    Driving a HDD motor

    The BUZ11 is an ordinary Mosfet that needs 10V on its gate to fully turn on. Maybe a few in a bucket-full might turn on enough with only 4.8V. The 74AC14 has an output voltage only 4.8V. You wouldn't need to test a bunch of Mosfets for a low threshold voltage if you use Mosfets designed for...
  3. audioguru2

    High Power LED Mood Lamp

    Hi Lolong, I started this thread to hopefully prevent the dim problem you are having. I said: "The value of the base resistors for the transistors is much too high and stops the transistors from conducting anywhere near the high current rating of the expensive LED. Therefore the expensive LED...
  4. audioguru2

    T.V ( No display )

    My Sony TV went dead. People told me that the copyright is strong and I won't get a service manual. I entered the model number in Google and found the service manual download for free and forums where guys talk about fixing the same problem. I found places on the web selling cheap repair kits...
  5. audioguru2

    0-30 Vdc Stabilized Power Supply

    Hi Phaethom, I don't know your zener diodes. I think your project has something loose inside.
  6. audioguru2

    relay

    Simply look on the datasheet of the 8051 to see its max allowed output current and what voltage it drops down to (4V?). It is probably only 25mA and the voltage will be 4V. Then try to find a relay with a coil that operates at only 4V and 25mA or less. If you don't find an extremely sensistive...
  7. audioguru2

    0-30 Vdc Stabilized Power Supply

    Hi Phaethom, 1) The voltage of C1 is low because a 24VAC transformer is 25VAC without a load and its peak is 35.4V. the rectifiers drop it to about only 34VDC. 2) Pin 4 of U2 has a 5.6V zener diode. Maybe you used one that is made for a high current, so its voltage is low in this circuit. 3) Pin...
  8. audioguru2

    BJT biasing

    The emitter resistor provides the negative feedabck. The bypass capacitor across the emitter resistor shorts it at audio frequencies. The 100uF capacitor has a reactance of only 16 ohms at 100Hz. Very much lower than the 1k emitter resistor it is in parallel with.
  9. audioguru2

    0-30 Vdc Stabilized Power Supply

    Hi Phaethom, Welcome to our forum. ;D Did you make the original project or the modified one? Did you substitute any parts? 1) What is the DC volatge across C1? 2) what is the negative DC voltage at pin 4 of U2? 3) What is the positive DC voltage at pin 6 of U1?
  10. audioguru2

    relay

    The output of a PIC has a max current rating of 25mA and then its voltage would be about 4V not 5V. A relay needs 5V at about 40mA or more for its coil so a transistor must be added between the PIC and the relay.
  11. audioguru2

    Negative Reference Voltage

    Hi Nick, Welcome to our forum. ;D There are a few simple ways to make a negative reference voltage. An ICL7660 takes a positive voltage and makes a negative voltage. A few Cmos inverters or a 555 oscillator can have a rectifier and filter cap added to do it. I don't think DAC ICs require a...
  12. audioguru2

    a very simple FM Tx

    Hi Walid, I never have used an RF pi network. The output impedance of the Tuned RF Amplifier depends on its DC current and on its drive level if it operates in class-B or class-C. With a high DC current and high drive level then the resonance of the tuned circuit causes the output voltage to...
  13. audioguru2

    0-30 Vdc Stabilized Power Supply

    Hi Hanna, Your project looks nice. I hope nothing blows up because the voltage of your 36V transformer is much too high.
  14. audioguru2

    Telephone Ring Circuit

    Use Ohm's Law to measure current. A small value resistor in series with the phone won't affect the phone but it will have a DC voltage drop when the phone draws current when it is off-hook. An opamp can amplify the voltage across the resistor and the output of the opamp can drive a transistor...
  15. audioguru2

    transmission lines

    A 75 ohm source connected to a 75 ohm coaxial cable that is terminated by a 75 ohm resistor does not resonate, even though the transmission line has inductance and capacitance. The circuit is purely resistive. The source and cable's output will be messed up if the termination resistance doesn't...
  16. audioguru2

    BJT biasing

    The internal emitter resistance of a transistor depends on its collector-emitter current. It could be very high. A signal source usually has a lower impedance than a transistor circuit, then the input impedance of the transistor or its bias resistors don't matter. The current in the base bias...
  17. audioguru2

    a very simple FM Tx

    Hi Walid, I was talking about the parallel resonant LC at the collector of the RF amplifier transistor on my FM transmitter. Its impedance is infinity at its resonant frequency without a load. But it is a terrible match to a 50 ohm antenna. The article you are looking at shows a pi (CLC)...
  18. audioguru2

    Telephone Ring Circuit

    You connect a double-throw switch so that the telephone is connected to the other telephone with the switch thrown one way, or connected to the ring generator with it thrown the other way. To detect that the ringing phone has picked-up you must detect its DC current in the phone line.
  19. audioguru2

    transmission lines

    Coax cable is 50 ohms or 75 ohms. A short is zero ohms, a big difference.
  20. audioguru2

    BJT biasing

    The unbypassed emitter resistor provides negative feedback. With the emitter resistor bypassed with the 100uF capacitor C2, the transistor's internal Re is 0.026 divided by its emitter currentof 0.43mA= 61.1 ohms. Therefore its voltage gain is 10k/61.1= 164 and its distortion at a high output...
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