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  1. Audioguru

    Bench power supply upgrade

    A 20mA zener diode barely works at 5mA but a 5mA zener works fine at 20mA. R8 still wrongly connects to 0V instead of to the negative supply. this is from the TL081 datasheet:
  2. Audioguru

    Bench power supply upgrade

    I am back. Measure the voltage across the 5.1V zener diode because it is probably a Chinese one, not a European one. It might need 20mA or more for it to reach 5.1V.
  3. Audioguru

    How difficult to have a microphone made

    The audio spec's are missing since Amazon only cares about the size and weight.
  4. Audioguru

    How difficult to have a microphone made

    The link shows a "dynamic" mic but a smartphone has biasing for an electret mic. The biasing might prevent a dynamic mic from working properly.
  5. Audioguru

    Username change

    I had my email address from my old internet provider but I changed to a different internet provider so I needed to change my email address. I simply registered again at most other forums with a new user name.
  6. Audioguru

    How difficult to have a microphone made

    All people interviewed on TV have the microphone hidden far away so that all we hear is echo, echo, echo, echo, echo, echo, echo. My hearing aids automatically boost low level voices and reduce the level of loud sounds. They also beam into the direction of somebody talking anywhere around me so...
  7. Audioguru

    Adding capacitor to wireless power transfer circuit. Need guidance.

    A car battery is rated in hundreds of Cold Cranking Amps because cold oil is very thick which causes the starter motor to draw many more amps. A Super Capacitor is not rated in Amps, its is rated in capacitance that discharges quickly.
  8. Audioguru

    How change the Frequency of a 555 timer with another circuit ?

    I deleted my reply about the datasheet for a 555. The last circuit has a frequency determined by R1, C1 and the speed of the comparator.
  9. Audioguru

    How is current passing through the capacitor in the following schematic?

    The "cookbook" shows the circuit using a 9V supply and does not say that the capacitor driving the base of the transistor to almost -9V damages the transistor since its absolute maximum allowed reverse emitter-base voltage is only 6V.
  10. Audioguru

    Bench power supply upgrade

    In my fix of the old circuit, I used two power transistors at the output to share the heat. Some power supplies and linear amplifiers use many transistors to share the heat. The large 2SD1047 Oriental transistor can handle 100W when it has a "perfect" heatsink so that its case does not even get...
  11. Audioguru

    How is current passing through the capacitor in the following schematic?

    I already explained how the capacitor C1 is discharged: "Then when Q1 turns on, the left terminal of C1 goes to almost 0V and the charged voltage of the capacitor drives the right terminal to a negative voltage which turns off Q2 and C1 discharges into R1."
  12. Audioguru

    How is current passing through the capacitor in the following schematic?

    Current does not "pass through" a capacitor. Instead a capacitor charges or discharges. A transistor is not open or closed like a door. Instead a transistor is turned on or is turned off. If Q1 is turned off and Q2 is turned on then C1 is charged by r1 and the base-emitter diode of Q2. The left...
  13. Audioguru

    Bench power supply upgrade

    2.5A x 0.47 ohms (R6)= 1.175V. 1.175V/10k (R21)= 117.5uA. The voltage across R23 is 10.2V - 1.175V= 9.025V. Then R23 must be 9.025/117.5uA= 76.8k ohms, use 75k.
  14. Audioguru

    How change the Frequency of a 555 timer with another circuit ?

    This thread is about changing the frequency. The LM358 VCO does that. Now you want to change the pulse width to do PWM. Here is how a 555 can have an adjustable PWM:
  15. Audioguru

    Bench power supply upgrade

    The total of 200k for R23 plus the 10k of the current-setting pot R21 is 210k but only 10k of it goes to the current adjustment opamp. The reference of 10.2V/210k= 48.5uA. The 10k pot at max x 48.5uA= 0.485V. The current opamp output switches and regulates when it has 0.485V across the 0.47 ohms...
  16. Audioguru

    Bench power supply upgrade

    Opamp OP3 has a 5.1V zener diode and a gain of 2 so it produces the reference voltage of 10.2V that is fed to R23. If R23 is 200k then in series with the 10k pot of R21, the current is 10.2V/210k= 45.5uA which produces 0.485V across R21 and is fed to opamp OP2. OP2 also senses the voltage...
  17. Audioguru

    Bench power supply upgrade

    I do not see pic's of your case. I'm sorry I made a typo about R21. Texas Instruments invented the TL081 so I use their datasheet. ST Micro is a copycat and use 0 ohms for R8 which causes a huge change in the input offset adjustment which makes it difficult to set the offset trimmer R7. When the...
  18. Audioguru

    Bench power supply upgrade

    Which burns out first, a power transistor or a fuse? If the current is set to 3A and a load reduces the output voltage to a short or any low voltage then the output transistor will get very hot. 62k is 62 thousand ohms. 62K is 62 ohms, 10%. All resistors on the schematic are lower case k. With...
  19. Audioguru

    Bench power supply upgrade

    If the output is set to 5V at 3A then the output transistor will melt with 78W of heat. If the output becomes shorted at 3A then the output transistor will melt with 93W of heat. That is why my fix uses two output transistors to share the heat. 3A x the 0.47 ohms of R6= 1.41V that opamp OP2...
  20. Audioguru

    Bench power supply upgrade

    Voltage reference opamp OP3 has a 5.1V zener and a gain of 2 so its output is 10.2V. The voltage setting opamp OP1 has a voltage gain of (56k/27k) + 1= 3.07 and since you want the maximum output to be 24VDC then the gain should be 24V/10.2V= 2.35. Then change R9 to 36k then the max output will...
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