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Posts posted by audioguru

  1. The datasheet of the ULN2003 shows that the saturation voltage loss is 1.4V. Then if the battery is 3.7v the motor gets only 2.3v but if the battery is 7.4V then the motor gets 6V.

    You did not say how much voltage and current the motor needs.

    You did not say what is the stalled current of the motor. Stalled is when something stops the motor from turning, which happens the moment it is powered. If the stalled current is higher than 1A (two ULN2003 outputs in parallel) then the ULN2003 will burn out.

  2. The very old circuit you posted works poorly. TR3 is its output transistor and it gets too hot. Add a second 2N3055 transistor parallel with TR3 to share the heat and add a 0.33 ohms/2W resistor in series with each emitter to the output. The transistors collectors can be connected directly together by the heatsink if the heatsink is insulated from any other part of the circuit. 

    If you want a 0V to -30V, 2mA to 3A power supply then make a second one of this project and use its +30V output as 0V and use its 0V output as 0V to -30V.

    A center-tapped transformer will produce an AC output, not DC and not variable.

  3. If the transformer is rated at 24VAC/2A then it provides a maximum power of 24V x 2A= 48VA.

    But if the output is 30V at 2.0A then the load uses 30V x 2A= 60VA plus heating power. The peak of 24VAC is 40V which feeds the bridge rectifier which charges the main filter capacitor to the peak voltage minus the rectifier forward voltage. So then the transformer must produce 40V at 2.0A= 80VA which is much more than the overloaded 48VA. 80VA/24V= 3.33A, not 2A.

    For a regulated output of 30.0VDC at 3A then I recommended using a transformer rated at 28VAC/4.3A.

    For an output current of 3A then you must look at how much heat is produced by the single output transistor which is why I recommended using two output transistors to share the heat.

    Edited with strike-throughs.

  4. Use a separate +5V supply to power the voltage meter and current meter.

    Connect the grounds of both meters to the (-) output of the power supply being measured.

    The current meter can measure the negative voltage across the 0.47 ohms R7 of the power supply being measured, then the current meter does not produce an additional voltage drop that ruins the good voltage regulation. R7 produces a voltage of 1.41V for 3A or 0.47V for 1A. You will need to make a voltage divider to reduce the 0.47V to 0.1V for the input of the current meter. Disconnect the (-) segment of the current meter display. 

  5. The 24VAC transformer probably produces 25VAC with a light load on the project. Then its rectified and filtered output is +34V and -5.6V which power the opamps. At times the voltages are higher. Your transformer will be overloaded if its power rating is 105VA or less (24V at 4.4A)..

    The old TL081 opamps have an absolute maximum allowed supply of a total of only 36V so they will not last long. Replace the opamps with TLE2141 opamps that have a maximum allowed supply of 44V.

    Many of the resistors and the driver transistor in the original project are overloaded. Upgrade them. The main filter capacitor C1 value is much too low, upgrade it.

  6. A frequency response spec must include how much the levels deviate. Plus and minus 30dB is horrible but 2dB is pretty good. If they do not spec the plus and minus dBs then assume it is horrible. 

    If you have background noise then you can turn down the volume. Using filters to cut frequencies you cannot hear will not reduce background noise.

    Instead you can cut all high audio frequencies to reduce hiss which will make words unintelligible and cut low audio frequencies so that guys voices sound like chipmunks.

    Audio opamps have low noise, "general purpose" opamps produce lots of noise. Your choice. 

  7. Most of the spec's for the headphones are absolutely useless. The frequency response spec's do not say plus and minus 30dB which is horrible or plus and minus 2dB which is pretty good. Who cares about the length of the cord and the weight?

    The important spec for the ear speakers is the impedance that you can measure with a multimeter but you didn't.

    If the circuit is modern then it uses tiny little parts that are difficult to solder and unsolder. 

  8. Can you provide a link to the ebay ad?

    Your schematic shows strings of 3 LEDs in series and a tiny resistor is also in series.

    Your pink and blue lines show that all the strings are in parallel.

    Since a white LED is about 3.2V to 3.6V  then the entire bunch of LEDs uses a 12V supply.


  9. You have 12V, not plus 12V and minus 12V which is 24V. What you label as -12V is actually 0V.

    An ordinary older type of Lithium Ion cell is 4.20V when fully charged so your charger should have an output up to 12.6V, not 12.0V.

    The charger circuit should have wires connected between the series batteries so that it makes it a balanced charger. 

  10. The circuit is the same as the old Greek kit here except a few part numbers are changed and the maximum output current is only 2A.

    It also has the problem of an overloaded transformer since with a 2ADC output the 25VAC input is 2.83A, not 2A.

    I ,saw some 741A opamps rated at 44V which might work but I do not think the maximum output voltage will be 30V, probably only 26VDC at 2A with lots of ripple. 

    I wonder which circuit came first? 

  11. I have some Li-PO batteries for my RC airplanes that had hundreds of flight and had storage for a few winters. I can charge one and use it and it produces a problem like you have but it charges normally. The battery is worn out and cannot produce normal power continuously for a normal duration.

    Measure the output voltage of the L298 that feeds the motor. If it drops lower and lower then also measure the input voltage of the L298 to see if it drops when the motor is slowing down.

    A Lithium rechargeable battery quickly becomes ruined if its voltage becomes less than about 3V per cell. my airplanes pulse the motor power as a warning to me that the voltage is becoming low then the motor stops but allows the steering servos to control the glide. 

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