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  1. audioguru2

    Reducing SIGNAL voltage to tiny servo from 5V to ~4V

    The servos in my RC airplanes are steady when they receive a continuous and steady PWM signal and the resistive strip is clean. One servo had a dirty resistive strip or intermittent wiper and it was jerking until I sprayed the resistive strip and its wiper with contact cleaner then moved it back...
  2. audioguru2

    Reducing SIGNAL voltage to tiny servo from 5V to ~4V

    An RC servo responds to pulse width, not voltage. Usually the pulses are about 4.8V and are at about 50Hz. A pulse width of 1.5ms sets the servo to its center position, 1.0ms sets it to one end and 2.0ms sets it to the other end. Your voltage divider is not needed and actually reduces the pulse...
  3. audioguru2

    0-30V Stabilized Power Supply

    A Jfet or Mosfet has a wide range of transconductance. Usually resistors are selected to make the voltage gain low for ANY device that is used. A bipolar transistor or an opamp has an accurate voltage gain determined by the values of two resistors.
  4. audioguru2

    0-30V Stabilized Power Supply

    The fixed and improved version of this project is supposed to have TWO 2N3055 output transistors to share the heat. The BD139 and the output transistors are emitter-followers. The small base current into the BD139 from U2 causes it to pass up to 86mA through its collector-emitter to the bases of...
  5. audioguru2

    12vdc to 12vAc

    A 555 oscillator driving complementary power transistors as emitter-followers that drive a power transformer connected in reverse. The maximum output power will be about 50W. The waveform will be a crude square-wave. The voltage will be too high when the load is low power and the voltage will...
  6. audioguru2

    I think this corrects polarity...

    You said that the main board works fine when powered from 12V. Then use 12V. But I think the main board will blow up when powered from 12V. We do not know how much voltage and current the little blown up board gave to the main board.
  7. audioguru2

    I think this corrects polarity...

    The little circuit board is probably a switched-mode step-down power supply. 12V input efficiently gives 2V or 3V to the LED and it is destroyed.
  8. audioguru2

    Regulators

    An LM338 has a maximum output current of more than 5A. Its output voltage is adjustable from 1.2V to 32V. An LM7812 or uA7812 has an output voltage of 12V and an output current more than 1A. Their circuit diagrams are in their datasheets.
  9. audioguru2

    12vdc to 12vAc

    It is easy to take a 12VDC power supply or battery, make an oscillator feeding Mosfets and have a square-wave output at 12VAC.
  10. audioguru2

    0-30V Stabilized Power Supply

    When the voltage setting pot, P1 is set to maximum then the input to U2 is 11.2V and the output of the project is 30.0V if it is not regulating the current. Then the (+) input of U3 is higher than its (-) input causing the output of U3 to be high at about +29V. Then Q3 is not turned on, the LED...
  11. audioguru2

    0-30V Stabilized Power Supply

    Guber, The (+) input of U3 has a voltage divider including P2 from the reference voltage of 11.2V. The (-) input of U3 has the voltage across R7 created by the output current. When the voltage on the (-) input exceeds the voltage on the (+) input then the output of U3 goes low enough to reduce...
  12. audioguru2

    0-30 Vdc Stabilized Power Supply

    This project has a current control pot, P2: 1) When it is set to minimum then the maximum current into a resistance or into a short is a few mA. 2) When it is set to maximum rotation then the maximum current is 3.0A. 3) When it is set to half rotation then the maximum current is 1.5A. 4) When it...
  13. audioguru2

    0-30 Vdc Stabilized Power Supply

    Ohm's Law usually controls the current. A low load resistance uses a high current. A high load resistance uses a low current. When you change the current yourself then you are also changing the voltage. This project regulates the current so you can set a certain maximum amount of current.
  14. audioguru2

    0-30V Stabilized Power Supply

    Yes you can short the output then set the maximum current to anything from a few mA to 3.0A. But most people calibrate 3.0A max then mark a dial around the knob on P2 with current settings.
  15. audioguru2

    0-30V Stabilized Power Supply

    The pot P2 does not limit the current. Opamp U3 regulates the current determined by the voltage set by pot P2 and the current sensed in R7. If Diode D9 is backwards or is burnt out then the current regulation will not work. The LED should light to warn that current regulation is reducing the...
  16. audioguru2

    0-30 Vdc Stabilized Power Supply

    A very large variable resistor and a power supply can increase or decrease the current. Usually people change the voltage, not the current because the voltage and the load resistance determine the current. Why do you want to change the current instead of changing the voltage?
  17. audioguru2

    0-30 Vdc Stabilized Power Supply

    Control potentiometer P2 increases or decreases the current in this circuit. Ohm's Law says that in a certain resistance, increasing the voltage across the resistor increases the current and decreasing the voltage across it decreases the current. That is how the current regulation in this...
  18. audioguru2

    0-30 Vdc Stabilized Power Supply

    This thread and a few other threads show the latest parts list and schematic. Some threads show a pcb layout. Here are the details again. Most of us learned electronics in high school and university. A good book (I got mine on the internet for free) is The Art Of Electronics.
  19. audioguru2

    0-30V Stabilized Power Supply

    The resistance between the collector and emitter of an unpowered transistor should be so high (tens of Megohms) that it might not be measurable. Your BD139 is shorted. It takes one second or less to blow up an opamp that is driving a short circuit.
  20. audioguru2

    0-30V Stabilized Power Supply

    If you connected the pins on the driver or output transistors backwards or if they are shorted then you will have your problems. The driver and output transistors are emitter-followers. Their emitter voltage should be 0.6V to 1V less than their base voltage. The (-) input and (+) input of a...
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