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  1. audioguru2

    0-30V Stabilized Power Supply

    U3 has a load of only 3mA through R20. There is no current in D9. Therefore U3 will not get warm. The current control normally reduces the output voltage when the output current exceeds its setting.
  2. audioguru2

    0-30V Stabilized Power Supply

    It is simple to make the modified 0v-30V/3A power supply produce up to 5A instead of 3A. There is a thread about it. It uses a transformer that has a 7A AC output, 3 output transistors to share the heat and the 0.47 ohm current-sensing resistor is changed to 0.27 ohms.
  3. audioguru2

    0-30V Stabilized Power Supply

    Make two completely separate projects (with separate transformers or separate transformer windings). One can be a positive supply when its negative output is common and the other can be a negative supply when its positive output is common. Connect the commons together and to earth if you want...
  4. audioguru2

    0-30 Vdc Stabilized Power Supply

    You are not bothering me. I wish that there is a place on this site to post the important modifications and I hope you understand that the original project will not make regulated 30VDC at 3A (it fails to meet its spec's) and that many of its parts were overloaded (then it has poor reliability).
  5. audioguru2

    0-30 Vdc Stabilized Power Supply

    I have looked it up on my hard drive and posted it on one of the many threads about the modified circuit hundreds of times. Here they are again:
  6. audioguru2

    0-30 Vdc Stabilized Power Supply

    Yes, the original project was missing calibration trimpots to reduce the maximum output voltage to 30.0V and the maximum output current to 3.0A. The modified circuit has the calibration trimpots added and uses a high current rectifier bridge module that can be bolted to a heatsink.
  7. audioguru2

    0-30 Vdc Stabilized Power Supply

    The input to the output amplifier is 0V to +11.2V from the voltage adjust pot. The output amplifier is simply opamp U2 with the driver transistor and output transistor as emitter-followers boosting the output current. The output amplifier has R11 (27k) and R12 (56k) for over-all negative...
  8. audioguru2

    c828 transistor

    You hyjacked this thread instead of starting your own thread. The 2SC828 is an old Japanese transistor. Any low voltage low current transistor will probably work fine like a 2N3904 or a 2N2222.
  9. audioguru2

    0-30V Stabilized Power Supply

    R15 wastes supply voltage. 1k is too high but 100 ohms is fine. When output transistors are connected in parallel so they share the heat then each one needs a 0.33 ohm emitter resistor so that their base-emitter voltage differences do not cause the one with the lowest Vbe to hog all the...
  10. audioguru2

    0-30V Stabilized Power Supply

    It is a combination of the original schematic and the latest schematic: 1) It uses Q2 to solve the "Opamp Phase Inversion" problem with the TL081 opamps used in the original project but the MC34071 opamps do not have that probem. 2) The value of R15 is much too high. 3) Its single 2N3055 output...
  11. audioguru2

    0-30V Stabilized Power Supply

    When the output current is 3A check that opamp U2 is not too hot.
  12. audioguru2

    0-30V Stabilized Power Supply

    If three 2N3055 output transistors share the 5A of current then each has 1.7A and its minimum current gain is 40. Then the current in the BD139 is a max of 125mA. Its max voltage is 36.2V so its max power dissipation is 125mA x 36.2V= 4.5W. The max allowed chip temperature of a BD139 is 150...
  13. audioguru2

    0-30V Stabilized Power Supply

    The very old 2N2219 driver transistor in the original circuit had a good high frequency response so its transient response was good. I suggested replacing it with a TIP31 transistor that can be cooled well but another member showed that its slow response almost causes oscillations and...
  14. audioguru2

    0-30 Vdc Stabilized Power Supply

    Here in Canada my inexpensive mains electricity is 120V. Sometimes it surges very high to 121V and sometimes it drops very low to 119V. It fails maybe every 2 or 3 years for a moment during a thunderstorm. There are many electronic parts distributors here that have ANYTHING and EVERYTHING at a...
  15. audioguru2

    0-30V Stabilized Power Supply

    With a 28VAC transformer the unregulated positive supply is +37.6VDC. When the output is shorted then the BD139 has a max current of 50mA and a voltage of about 36V so it dissipates 1.8W. It is in an old package so its ability to cool itself with a heatsink is poor.
  16. audioguru2

    0-30 Vdc Stabilized Power Supply

    Too much trouble. Use the proper 28VAC or 30VAC transformer and the proper opamps. I don't know why your input voltage changes, mine does not change. A TIP3055 at 3A has a maximum base-emitter voltage loss of 1.7V. A BD139 at 110mA has a maximum base-emitter voltage loss of 0.8V. The 1k series...
  17. audioguru2

    0-30 Vdc Stabilized Power Supply

    Since your voltages are very low, many resistor values will need to be re-calculated. You do not need to use U4 and D13 since without them the unregulated positive supply is only +26V which is fine for a TL071 or TL081 opamp. With U4 and D13 the maximum output voltage will be only about +14.4V...
  18. audioguru2

    Impedance relation between 2 Bjts

    There is no decoupling capacitor. Instead there are coupling capacitors. C3 couples the transistors together. C5 is the input coupling capacitor and C4 is the output coupling capacitor. Instead of wrongly guessing the impedances, calculate that the output impedance of the first transistor is...
  19. audioguru2

    0-30V Stabilized Power Supply

    If you don't use insulators for the transistors when you can insulate the heatsnk from the chassis and prevent somebody from shorting it to ground or getting a 40V shock. Thermal compound adds about 0.2 degrees C per Watt so the cooler is 0.7 degrees C per Watt. Maybe the ambient temperature is...
  20. audioguru2

    0-30V Stabilized Power Supply

    Your calculations are correct. When the output is set to 5a and is shorted then each 2N3055 output transistor must dissipate (190.5W)/3= 63.5W. A huge heatsink cannot provide enough cooling so a high velocity fan must be added.
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