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  1. crutschow

    AC audio signal superimposed on to DC question

    When you have an AC signal riding on a DC level, the value of the DC is the average value of the total. Thus if you have a 1Vac signal riding on a 2Vdc level, the value of the signal averaged over one cycle of the AC will be 2V. So if you measure the voltage through a high-pass filter (large...
  2. crutschow

    Dual Power supply switch with 5-10 sec delay

    Yes, this and a couple other electronic forums are a good place to learn the basics along with some tricks. For example, when I first started here I knew little about the 555, but there were many requests for timing circuits where the 555 was a good choice, so I learned more about it. Once you...
  3. crutschow

    Dual Power supply switch with 5-10 sec delay

    I agree. Below is the modified circuit:
  4. crutschow

    Ancient Adeptness of Hidden Powerworks

    The thermostat controls a relay to switch the motor power, so adding a transformer to the motor should have no effect on the rest of the circuit. You just wire the transformer/motor combination in place of the previous motor.
  5. crutschow

    Ancient Adeptness of Hidden Powerworks

    And you can minimize that if you switch power at input to the transformer instead of the motor.
  6. crutschow

    Dual Power supply switch with 5-10 sec delay

    True. But for this non-critical timing application I'm not that concerned about power bus noise. Typical power bus noise would likely only affect the timing by small fraction of a second. It likely will if I want to go the trouble of simulating noise on the power bus, but I'm not that...
  7. crutschow

    Dual Power supply switch with 5-10 sec delay

    Yes and No. Yes, the C1 connection is to have the circuit come up with A selected (555 OUT low) when the A power comes up. With C1 to ground, the OUT will be momentarily high upon power application. The circuit should, of course, have a decoupling capacitor from power to ground near the 555 to...
  8. crutschow

    Dual Power supply switch with 5-10 sec delay

    Yes. Your requirements are unique enough, that I doubt there's an off-the-shelf device to do that. Edit: You might be able to do something similar to my circuit if you can find a module that will both delay on and delay off at the same time (along with an added relay). The problem is, the...
  9. crutschow

    Ancient Adeptness of Hidden Powerworks

    If you don't have 115V available than the only thing I can think of is to use a 230V to 115V transformer of sufficient rating to power the motor. Likely won't be cheap. :eek: Edit: You might able to get by with one of the 220V to 115V travel converters (autotransformers). Likely with a small...
  10. crutschow

    Dual Power supply switch with 5-10 sec delay

    When you are retired, Saturday is generally no different than any other. ;)
  11. crutschow

    Transistor as a switch

    Of course not. My quibble it bringing the solid-state semiconductor theory and equations into a discussion with neophytes that may barely understand Ohm's law (and sometimes don't even know that). Just as we don't need to bring up the field theory of dielectrics when discussing electrical...
  12. crutschow

    Dual Power supply switch with 5-10 sec delay

    Okay, here's my take on using one 555 timer to give an overlap off delay when the supplies switch over (LTspice simulation below): Supply A is on continuously and supply B is shown turning on and off (green trace). The 555 is level triggered by the output from relay U2 (signal "T") which...
  13. crutschow

    Dual Power supply switch with 5-10 sec delay

    What are the relay coils voltage and current rating? That will connect B to the load immediately after A is disconnected, for the time it takes for RLY 2 to energize (likely several tens of milliseconds). Will that be a problem for this application? And how do you know that the load voltage...
  14. crutschow

    Generating 24VAC Output (2 Amp Capability) from 36-40VDC

    If you generate a square-wave. then it should be 24Vpk or 48vpp. This could be generated from a 24V supply with a bridge output as long as the output doesn't need to be grounded to the DC supply voltage.
  15. crutschow

    Generating 24VAC Output (2 Amp Capability) from 36-40VDC

    You mention sinewave but then talk about generating a square-wave. So does the output need to be a sinewave or not? If you need a sinewave, then one way would be to generate that with phase-shift oscillator, and use that to drive an audio amp. Does the load for the AC need to share a ground...
  16. crutschow

    Transistor as a switch

    Okay. But I still don't understand your statement: "Like the rule of thumb we use to sat a transistor Ib = Ic / 10, but this one does not always comply -" How does it not comply?
  17. crutschow

    Transistor as a switch

    Just trying to minimize confusion in the TS's understanding about the theoretical operation of a BJT versus the practical. Not sure why you say that(?). The graph does show saturation voltage is greater at higher currents when using a Ib = Ic / 10, but that's mainly due to the intrinsic...
  18. crutschow

    Transistor as a switch

    True, the theoretical physics of the BJT shows it as a voltage-controlled device. And I imagine some of the pedantic types gnash their teeth whenever anyone says they are current-controlled. But for analog designs, hand calculation of the bias points is generally more easily done if a black-box...
  19. crutschow

    Transistor as a switch

    If you understand what happens when you put 5V in the forward direction across a silicon junction diode, then you will know what happens when you apply 5V to the transistor base-emitter junction in the forward direction. :eek:
  20. crutschow

    Transistor as a switch

    The base-emitter junction looks like a forward biased diode, so it starts to conduct significant current when the base-emitter voltage reaches about 0.6V. At that point the transistor is conducting collector-emitter current with a Beta (current gain for base current to collector current) ranging...
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