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    Negative resistance experiment

    Rather think of negative resistance as a negative differential resistance that behaves opposite to ohms law in that region. Some high frequency diodes also have this property, with the tunnel diode the most well known. Uni-junction transistors also operate on this principle. Another...
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    0-30 Vdc Stabilized Power Supply

    The BD139 well heat-sinked will be a better choice with none of these problems. One can even look at a better output transistor with better gain to remove power issues in the BD139. It would be nice if we can get something with 2A capability and response of the 139
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    0-30 Vdc Stabilized Power Supply

    Increasing phase lead will improve phase margin but at the same time will increase overshoot and ringing badly on a load step. Increasing the value to 330pF shows a 2V overshoot with lots of ringing. At 560pF it is at the point of oscillation with 2.5V overshoot. The oscillation problem just...
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    0-30 Vdc Stabilized Power Supply

    I agree simulators and models  are not perfect but it shows a trend of what to expect. The models will tend to be much more accurate at fairly low frequencies than at the high end, where nonlinear models would be preferred. The problem might not be obvious as most members are using the cheap...
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    0-30 Vdc Stabilized Power Supply

    This is what worries me. The plots compares the phase margins for: 1) Error amp alone with feedback network, no driver no filter cap C7 2) Error amp, with FB network and driver, no filter cap 3) Error amp, with FB, driver and filter cap 4) Error amp, with FB, driver, 2N3055 and filter cap...
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    0-30 Vdc Stabilized Power Supply

    Much easier said than done.  :) The LF pole created by the slow TIP31A will cause many problems that won't be easily cured. As it stands now the design is unstable and may oscillate with certain loads. The problem is that the transistor creates a LF pole and the output capacitor another LF pole...
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    Crystal examiner

    Good find! Saves time doing calculations, but does not calculate Rs which is important.
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    Crystal examiner

    Using a series crystal on the output will act as a very narrow BPF at the crystal series frequency Fs.
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    0-30 Vdc Stabilized Power Supply

    After drawing the diagram for the PSU I could not resist doing some tests. I wanted to see the transient response during a step load change. As the design went through a couple modifications to increase output, it was a good idea to look into these changes and the effects on performance. I...
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    Crystal examiner

    It is difficult to comment without the whole picture. The missing parts are the crystal data. It is possible to use other ratios as long as you can get enough gain and enough negative resistance to satisfy oscillation. The article explains in detail most of the other circuit design values but...
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    0-30 Vdc Stabilized Power Supply

    This should help you, and others to find faults in this PSU. Voltages to expect with three different conditions. 1) PSU with almost no load (2.2k) set to 12V 2) PSU with 1A load (22 Ohm) set to 23V 3) PSU with current limit turned down to about 500mA, from condition (2). Current limit LED is...
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    0-30 Vdc Stabilized Power Supply

    Something strange in your circuit. Make sure you don't have any stupid mistakes, solder bridges, wiring errors etc.
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    Commodore 64 repairing gone forgotten?

    I look after myself!  :)
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    0-30 Vdc Stabilized Power Supply

    Can you adjust the output voltage with U3 removed?
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    0-30 Vdc Stabilized Power Supply

    Yes "-" U3 seems faulty. pin 6 should be positive as the non inverting input dominates.
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    0-30 Vdc Stabilized Power Supply

    Sorry, I meant voltage between GND-pin2 and GND-pin3
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    0-30 Vdc Stabilized Power Supply

    What is the voltage on U3 (pin 2,3)?
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    Commodore 64 repairing gone forgotten?

    I was very happy when my parents got me my first computer. It was a Sharp MZ-80A! Still have it. http://www.old-computers.com/museum/computer.asp?st=1&c=172 I would spend many hours on this thing, sometimes programming right through the night. I wrote my very first chess program on this...
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    LM8618

    May be a LM86-18N
  20. A

    Negative resistance experiment

    Here is a simple experiment to see the effect of negative restance from a reversed biased transistor. It will happen at a certain bias level. If you copy my circuit then the same values should work for you. We would expect the voltage to decrease when we add resistor R2 as we will be drawing...
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