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  1. Audioguru

    Series Transistor Voltage Regulator

    No, I agreed that the internal resistance of this voltage regulator causes a voltage loss. "Therefore the voltage across he load will fall, not rise. Not true. The zener diode has some voltage and the emitter follower also has some voltage loss. When the input voltage rises then the poor...
  2. Audioguru

    Series Transistor Voltage Regulator

    Thank you for agreeing with me. Where are you applying 4.8V? If you apply 9.4V (double the zener diode voltage) to its series resistor and use its 20mA current rating then the resistor value is 4.7V/20mA= 235 ohms. The max dynamic impedance of a 1N750 is 15 ohms from one manufacturer so if the...
  3. Audioguru

    Chinese Hiland 0-30V/Audioguru PSU build (down the rabbit hole)

    Opamp U2 inverting input pin 2 is probably wrong at 0V because it is not being fed negative feedback through R12 and C6. In the complete circuit, U2 gets negative feedback from the output but for this test there is no output. Since the BD139 and output transistors are removed then R12 and C6...
  4. Audioguru

    Series Transistor Voltage Regulator

    But this simple circuit does not feed the zener diode from a constant current source, therefore the zener voltage changes with input voltage changes and with load current changes. Not with an emitter-follower. The emitter voltage gets dragged down when the load current increases because Vbe...
  5. Audioguru

    Need recommendation for rheostat or potentiometer

    I agree that PWM should be used to control the speed of the motors. A huge, heavy and expensive rheostat was used 100 years ago and worked poorly.
  6. Audioguru

    Fm transmitter data pins, what are they for?

    If it interferes with other people's FM reception then it is useless. It should be tuned to a clear channel that does not have a radio station.
  7. Audioguru

    Need recommendation for rheostat or potentiometer

    My electric model airplanes use 7.4V to 8.4V (2 Li-Po cells) brushless motors that last forever. The model airplane motors that use brushes need often replacement because the brushes wear out or the commutator paths wear out soon.
  8. Audioguru

    Series Transistor Voltage Regulator

    The datasheet for a zener diode shows its dynamic series impedance that lessens its voltage regulation. The dynamic impedance is the lowest for a zener voltage of about 6V that produces the best voltage regulation. Curves show the voltage rising a little when the zener diode current increases...
  9. Audioguru

    Series Transistor Voltage Regulator

    Why are you asking about the impossibility of something causing the output voltage to rise? Instead think about what happens when the input voltage rises. Then the zener diode current increases which increases the output voltage a little. I disagree that the output voltage is fixed because it...
  10. Audioguru

    Fm transmitter data pins, what are they for?

    The FM transmitter circuit is cheap: 1) It is missing a voltage regulator so its radio frequency slowly changes as the battery voltage runs down. 2) It is missing pre-emphasis that all FM radio stations have so its sounds heard on an FM radio will sound muffled like your stereo with its treble...
  11. Audioguru

    Audio tone generator

    Why were you looking at 300W Chinese amplifiers when your transducer has a maximum allowed power of only 25W? Why do you need a variac? It will melt or it will blow up the transducer because 25W into 4 ohms has a voltage that is only 10V RMS. Most bridged car amps produce 25 Whats, not 25 Watts...
  12. Audioguru

    Chinese Hiland 0-30V/Audioguru PSU build (down the rabbit hole)

    The collector of the BD139, pin 7 of U2 and C5 should always have +40V on them. I think the D11 shorted to +40V blew up things. With the BD139 removed and R12 and C6 connected to the output pin 6 of U2 only pin 7 should have a voltage (+40V) on it and also pin 3 is its input which should...
  13. Audioguru

    Chinese Hiland 0-30V/Audioguru PSU build (down the rabbit hole)

    D11 does not connect to +40V so how did it make the emitter pin of the BD139 at +40V? Maybe it had a solder whisker shorted to +40V. The collector of the BD139 is normally wired to +40V because it is an emitter follower. Opamp U2 will not be connected to the missing BD139 so with its output...
  14. Audioguru

    Design of a DC voltmeter circuit

    Does the teacher expect the student to ask on an online forum how to design a 48 years old circuit without giving any instructions about how it works?
  15. Audioguru

    Audio speakers

    15W into an 8 ohm speaker is a signal that is 11V RMS or 31.1V peak-to-peak. From 5V USB? No way. When an older amplifier supplies 15W to a speaker, the amplifier heats with about 14W so the power supply must produce 15W + 14W= 29W. USB can supply maybe 200mA at 5V which is only 1W. A 15W modern...
  16. Audioguru

    Design of a DC voltmeter circuit

    I made that circuit 48 years ago. Its "zero" drifted all over the place when the temperature changed.
  17. Audioguru

    Chinese Hiland 0-30V/Audioguru PSU build (down the rabbit hole)

    The 2N3055 transistors are disconnected from the output and from the emitter of the BD139 and the BD139 is removed. Then the BD139 emitter pcb pad has only R16 (1k) connected to the output that has the added 10k resistor to ground. Then the emitter pad of the BD139 should be 0V, not 40.2V...
  18. Audioguru

    Op-amp buffer

    The load for the opamp is not shown so we do not know why the opamp is needed. The input of the opamp has a high impedance lowpass filter and the output of the opamp has a lower impedance lowpass filter that feeds the unknown load.
  19. Audioguru

    Transistor Help

    If the transistor has the minimum current gain of 40 and you feed its base 1/40th the collector current then the maximum saturation voltage (collector to emitter) is 1V so at 500mA some S8050 transistors will heat with 0.5W and will smoke and burn since its maximum allowed dissipation is 0.3W...
  20. Audioguru

    Help with Mic activity LED

    An opamp output power is too low to drive a speaker, use a power amplifier instead. If the mic can hear the speaker then you will have acoustical feedback howling.
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