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Posts posted by audioguru

  1. The 2SC5200 power transistors in a plastic case are much better than 2N3055 transistors in a TO-3 metal case. They should transfer more heat to the heatsink than the 2N3055 transistors.
    The heatsinks in your photos are tiny so no wonder the output transistors get hot. I do not know if you use thermal compound between the transistor and the heatsink to fill in the tiny imperfections and allow much better heat transfer.
    Our 5A version of this 30V power supply uses three 2N3055 output transistors in parallel, each with a 0.33 ohm emitter resistor.

    Each transistor is different. The Vbe of one transistor might be 0.9V and the Vbe of another might be 0.8V. If they are in parallel without emitter resistors then the one with the lowest Vbe conducts all of the current and burns out while the other transistor does nothing. The value of the emitter resistors (0.33 ohms works well) must be selected to balance worst-case unmatched transistors.

    Get rid of the 1.3W zener diodes because they might not even regulate with the low current given to them.
    I do not know why you are not using the TLE2141 opamps that we use. Their maximum supply voltage is 44V so the do not need your LM317 low voltage regulator.

  2. The progress is better now: Changed emitter resistors to 150mOhm each on Toshiba output transistors, then changed R7 to 2xparallel 330mOhm resistor = 165mOhm at R7 now.

    you reduced the values of the resistors so that the low supply voltage for your voltage amplifier opamp does not cause poor voltage regulation. then if the output transistors are a little different then one will conduct much more current than the other and get too hot. Also the input offset voltage for the current regulating opamp will affect its regulation.

    Current regulation works still and precision is still nice, At 3.4A i get now 60mV drop and voltage seems to be more stable.

    The regulation should be much better than a 60mV drop. Maybe your wiring has a resistance of 60mV/3.4A= 0.0017 ohms. Use thicker wires.

    If i go lower voltage then i get lower Ripple Voltage also

    Probably because the low supply voltage for the voltage amplifier opamp causes it to saturate when the output voltage and current are high.

    then the 2 output transistors heat crazy if i put for example 10V 2A load on PSU.

    It is simple to calculate the dissipation (heating power) in the output transistors then select a heatsink that will cope with it. The heatsink must not be inside an enclosure.
  3. It is very confusing when each new schematic has different parts designation numbers.

    Your new D10 has backwards polarity then the current regulation will not work.
    When the new U1 detects over-current then its output goes low enough to pull down the voltage feeding the voltage amplifier opamp so that the current in the load is reduced.

  4. But, the zeners... what about them ? Is there a point to get 0.5W zeners as showen on the schematic ? Or leave 1.3W different zeners as is?

    The BZX55 zener diodes have their voltage spec'd when their current is only 5mA. Then the 6.2V zener diode has a power of only 5mA x 6.2V= 31mW, not anywhere near its maximum of 500mW. The 3.3V zener diode has a power of 16.5mW which is also nowhere near 500mW.

    Why don't you look at the datasheets yourself? the BZX85C3V3 zener diode is 3.3V only when its current is 80mA! In this circuit the current is only about 7.8mA.

    But if i change that dual-opamp with NE5xxx one that is 40V max supply voltage... would that change regulation in a better way?

    The NE5xxx opamp has a slightly higher maximum output voltage than the TL081 but its 33V positive supply voltage from the LM317 is much too low so it will not make much difference in that circuit.
  5. Your project has poor voltage regulation when its voltage and current are high because the voltage regulating opamp has a low supply voltage of only 33V. Its maximum output is about 31.5V if you are lucky, the base-emitter voltage of the BD139 could be 1V and the Vbe of the output transistors and their emitter resistor voltage drops could be 2V.

    That is why we use opamps that have a maximum allowed supply of 44V so that they do not need a voltage regulator.

  6. The current regulator opamp is not oscillating. Its output is saturated as high as it can go which is about +28V because it is not regulating the current so of course it shows the 120Hz ripple from the unregulated +39V, which is reduced to +29V by the 10V zener diode.
    The moment the current regulator opamp begins to work then its output will have no ripple.

  7. Hello guys. Since i'm new to electronics, may I know what are the function of the op-amps in this circuit?

    We cannot teach the basics of opamps in one post but here goes:
    1) Everyone who modifies this circuit uses a different parts numbering system so I will describe the three opamps as the voltage reference circuit, the voltage amplifier and the current regulator.

    1) The voltage reference opamp provides a constant current to the 5.6V reference zener diode and has a gain of 2 times so the reference voltage is 11.2V.

    2) The voltage amplifier opamp drives the BD139 driver transistor which drives the two 2N3055 output transistors. Two resistors in the amplifier allow the amplifier to have a gain of 2.68 times so that the 11.2V reference is amplified up to 30.0V at a high current.

    3) The current regulator opamp compares the voltage produced by the load current in the 0.47 ohms current sensing resistor with the voiltage of the current setting potentiometer. If the sensed voltage is too high then this opamp reduces the voltage from the voltage setting potentiometer through a diode until the load current is the same as is set. If the output is shorted then the current regulator opamp causes the output voltage setting to drop to almost zero so the current is not higher than the setting of the current setting pot. 

  8. What about the sims C9 (orig C6)?  I can get the same results there as well.

    It slows down changes in the voltage regulation. Add a load then the voltage suddenly drops and when this capacitor charges then the voltage slowly comes up to normal. Disconnect a load and the voltage suddenly increases and when this capacitor charges then the voltage slowly comes down to normal.
    It prevents high frequency oscillation, maybe because the output transistors are very slow.

  9. I've been playing with the simulation and found that if I change the value of C6 in the sim schematic (C4 in the original schematic) to a larger value I can get the output to slowly ramp up.  I tried introducing the spike like I had it in the mosfet circuit but it doesn't really affect the output so it's as if the circuit doesn't suffer from the problem.  But, if we reduce the time the output can rise with a larger value here wouldn't the issue be solved for the most part?

    C6 in the latest SIM circuit prevents the current regulator from quickly cutting back or quickly allowing more current. Then if the output is suddenly shorted the current in the circuit will skyrocket until the slowly ramping opamp can catch up which is very bad.
    Maybe the current sensing resistor should be non-inductive (not wire-wound).
  10. Since you bought it from Amazon then maybe the IC is a fake or maybe the circuit is designed wrong. Please provide a link to it.
    The output pulses? When it is playing loudly? Does the heatsink get hot? What impedance are the speakers? What is the maximum continuous current from the 18V power supply?

    If two 8 ohm speakers are playing at 14W each with a little clipping distortion then the power supply must provide 18VDC at 2.5A (45W).

  11. Wouldn't elimination of the negative rail mean that we could get rid of the 10V zener as well?  Something like this?

    A 28V transformer might produce 29V when the project has no load. Then its 41.0V peak voltage is reduced to 39.6V by the rectifier bridge. It will be fine if the mains voltage is very stable.

  12. Yes,  I agree the proposed fix would likely give rid of a lots of parts and simplify the board. When you add the two addtional transistors, is Q3 still needed or will the lower transistor connect to the led instead?

    Good point. Eliminate Q3, turn the polarity of the LED around so its cathode is at the collector of the new lower NPN transistor and its current-limiting resistor is between the anode of the LED and the +27.6V.

    One thing I am curious about is how low the output will go with the recommended arrangement when the negative rail is deleted?
    While I didn't build the suggested fix, a while back I disconnected the negative rail and connected Pin 4 of U3 to ground, the minimum output voltage  nearly doubled.

    Another good point but there is no problem.
    If the output is shorted then we want the output voltage to go to 0.00V. The new upper NPN transistor has its emitter connected to the circuit ground. Its collector is connected to the input of the output amplifier that must be +1.41V higher (the voltage drop for 3A in the current sensing resistor) for an output voltage of 0.00V when the output is shorted and the current is set to 3.0A. So the transistor does not need to saturate.
    The output amplifier does not need a negative supply for its output voltage to be 0.00V because the output of the opamp drives the darlington-connected driver and output transistors so its voltage is two diode drops higher.

  13. Next question:  our DIYFAN option shows a 24V Zener and resistor across input +ve and -ve to provide a stable 24V supply voltage to pin 7 of U1.  Remove the connection of pin 7 to +ve. Since the +ve input voltage can drop at high loads, what do you think of this idea?

    Then you must replace the 10V zener diode with a resistor.
    I do not think U1 needs its power supply voltage regulated because a TLE2141 opamp has a minimum Supply-Voltage Rejection Ratio of better than 90dB so 2V of ripple causes an output of less than 62.5uV (typically less than 2uV) which is very close to zero.
  14. When the load current is higher than the current regulator setting and the voltage is set high, the "on" voltage spike is caused by the time it takes for the negative supply to reach a low enough voltage (-1V?) for the current regulating opamp to pull down the voltage feeding the voltage output amplifier.

    Here is my proposed fix: 


  15. I am not sure what you mean by 'electrical outlets that were wired backwards'.  In Australia mains outlets have three wires - live, neutral and ground/earth (for earth leakage protection).  Would you recommend that I simply do not connect the earth to the chassis.  The danger is that if there is a fault (fingers in the wrong place!)  on the mains side of the transformer then the full 240V could be fatal.

    In Canada we also have three wires, live, neutral and earth ground. I have seen electrical outlets wired wrong by drunk electricians or by homeowners who do not know how to do it correctly so the "earth" terminal is actually "live". Then if a person touched the metal chassis and an earth ground, the person would be electrocuted if the outlet was wired wrong and if the chassis was connected to the earth wire on the plug. If the chassis is not connected to the earth wire then there will not be a problem unless the transformer live wire or live wire in the cord shorted to the chassis.

  16. I have two questions:
    1) can the circuitry associated with RV1 be omitted?  What would be the consequence?

    RV1 nulls the input offset voltage of the voltage setting opamp U2 so that the output is exactly 0V when the voltage pot is set to zero. Without RV1 then the input offset voltage of the opamp (if there is any) will cause a small positive or negative voltage output instead of 0V.
    But some people say RV1 does not do anything.

    2) when mounting in a metal box with the chassis earthed to mains earth (green/yellow), should the -ve on the electronics also be connected to mains earth?

  17. Why do you use 30V instead of the collector voltage of approx 40V (with a 28V transformer) when computing the power?

    Did I calculate it wrong?
    28VAC has a peak voltage of 39.6V and the rectifier bridge drops 2V to 37.6V and the capacitor ripple drops it to maybe 35.6V. if the current sensing resistor is 0.47 ohms then it drops 1.4V to 34.2V. Then the 0.33 ohm emitter resistor drop 0.5V to 33.7V so the output transistors have a total dissipation of 101.1W. I DID calculate it wrong. Sorry. 

  18. I have a question. In your particular circuit the maximum voltage is 33V (and including the -ve rail it is 34.3 volts). So will a CA3140 opamp work well as the max voltage stays within its limits. Actually a MC34071 is not available in my locality.
    I will be eagerly awaiting your kind counsel.

    The MC34071 is no longer available in a through-hole package (it is only surface-mount now) so the TLE2141 opamp from Texas Instruments is used.
    I think a 24V transformer produces peak rectified voltages of 32V at 3A and ripple reduces the minimum voltage to about 30V. That is why the circuit shown with the 24VAC transformer is rated for an output voltage of only 25V. The original project also used a 24VAC transformer and also could not produce 30V at 3A.

    An old CA3140 opamp is very noisy and has a high maximum input offset voltage. Its supply voltage might be higher than its maximum allowed voltage of only 36V when there is low load current or no load. 
  19. Its datasheet says the relay is a Chinese "Songle" relay. What is that?? They cannot spell "single"?
    Its coil current is 120mA at 3V.

    I betcha the sensor cannot provide an output current as high as 120mA at 3V. Then the relay needs a transistor to drive it and the sensor drives the transistor.

    Instead of a transistor and a high current relay why don't you use a low input current darlington transistor to drive the sprinkler?

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