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  1. AnalogKid

    Capacitor / filtering for PSU

    It is common to see 10 to 100 ohm resistors in series with the two power supply connections, with the decoupling caps on the chip side. 100 ohms seems large to me, but something in the 10 to 47 ohm range should help kill the high frequency spike noise. ak
  2. AnalogKid

    555 PWM or is it?

    When I'm bored, I answer questions on engineering fora. ak
  3. AnalogKid

    555 PWM or is it?

    ??? I'm not gunning anyone. I always try to go beyond the minimum tech answer and provide some context to help make the answer more clear or more useful. Pointing out that the answer is thousands of years old, isn't gunning, but I can see where it might be taken that way. Never my intent...
  4. AnalogKid

    555 PWM or is it?

    I do. There is no standards organization or regulating body for modulation schemes. Like the Smith Chart and S-parameters, these are industry standard terms, developed and refined over decades, and taught in curricula as a part of the baseline information necessary to function in the field of...
  5. AnalogKid

    555 PWM or is it?

    Pretty much everyone. https://en.wikipedia.org/wiki/Pulse-width_modulation ak
  6. AnalogKid

    EEG circuit - need help please!

    Start at the front, with the 620. It has no inputs. What do you expect to see at its output, and what do you actually see there? ak
  7. AnalogKid

    Speaker output relay

    My guess is that the internal oscillator ha an undervoltage lockout to protect the output transistors from operating in the linear active region due to insufficient gate drive. When Vcc is high enough, the oscillator kicks in the output DC jumps to Vcc/2. ak
  8. AnalogKid

    Circuit Modifications for reducing current

    Nope. An external shunt would be in parallel with the meter's internal shunt, and thus would *increase* the current to the fan while making the meter reading completely incorrect. That way lies folly. ak
  9. AnalogKid

    Speaker output relay

    Add a 1 K resistor across each output, from the 1000 uF negative side to GND. This provides a current path to charge up the output capacitor while the speakers are disconnected. Connect each side of a DPST or DPDT relay between the negative side of the output capacitors to the speaker. To...
  10. AnalogKid

    Circuit Modifications for reducing current

    6 years and 20,000 responses, and that's the best you can come up with? Granted the question was *slightly* poorly worded, but do you really think that was an informative response? ak
  11. AnalogKid

    Is it possible to use optocoupler 6N137 as an solid state relay ?

    Technically, yes. Practically, no. This is a specialized part for high speed data transfer. As such its output can handle very little power, and requires a separate power supply to function. ak
  12. AnalogKid

    Apple vs the FBI

    ???? = emotion Shoot people in the back of the head without a judgement, trial, or arrest, because of rumor, unsubstantiated accusation, or their political opinion. ak
  13. AnalogKid

    'Chasing' LEDs

    A 15-stage light sequencer in a car? ak
  14. AnalogKid

    Apple vs the FBI

    A terrorist walks into a bar... But seriously, folks...a terrorist walks into Kroger and buys bread. Can (not should) Kroger be prosecuted for supporting terrorism? And - a known terrorist walks into Kroger and buys bread. Can (not should) Kroger be prosecuted for supporting terrorism? ak
  15. AnalogKid

    Apple vs the FBI

    Make that ***convicted*** criminals. Note that in the US that is not a tautology. ak
  16. AnalogKid

    Transistor hfe

    What he said.
  17. AnalogKid

    Transistor hfe

    Because the point is NOT a point. As the voltage across the base-emitter junctions increases, the transistors turn on over a range of values anywhere from 0.3 V for a single small transistor to 1.5 V for a giant power darlington. AND, that value increases as the collector current increases...
  18. AnalogKid

    Transistor hfe

    In post #26 I spelled out my reasoning. Based on the datasheet for this and other similar power darlingtons, my estimate of the combined Vbe at 5 A Ic is 2.0 V. 5 A per boost transistor was an early design goal. So 2.0 V across Vbe and 0.5 V across the ballast resistor. ak
  19. AnalogKid

    Transistor hfe

    True and true. It's resistor physics: E = I x R 1 V = 0.005 x 200 2.5 V = 0.1 x 25 Again, it is your call. *Decide* the 7812 current level at which you want the boost transistors to start to conduct. Note that this is not an instant thing; the transistors will assume more and more current...
  20. AnalogKid

    Transistor hfe

    True, but don't care. The current boosting external transistors do not have to be active in the circuit all the time, just when *you* want to take some of the thermal and current load off of the 7812. When that happens is a design tradeoff, not some hard and fast design rule. That is why I...
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