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  1. CircutScoper

    Transistor as a switch

    Actually, Dana, doesn't your simulation show Ic maximizing at saturation (Ic ~ 500mA and Vc ~ 0V) and then decreasing as Ib continues to rise past saturation, thus confirming my answer of "no?" That is to say, dIc/dib goes to zero at saturation, and then reverses sign as Ib continues to rise.
  2. CircutScoper

    Transistor as a switch

    No. In fact, that might be the best practical definition of saturation -- when AC beta (d_Ic/d_Ib) drops to zero.
  3. CircutScoper

    12 volt delay timer

    57.295779513082320876798154814105... degrees?
  4. CircutScoper

    BJT AC Analysis - feedback pair equivalent circuit

    Is it just me, or are other folks also looking at that goofy bias arrangement and going EEE-YUCK! :(:eek: If built with real transistors with real-world beta variation, you wouldn't be able to predict quiescent Vc2 within an order of magnitude!
  5. CircutScoper

    BJT AC Analysis - feedback pair equivalent circuit

    If Ic1 = beta1 x Ib1, then Ie1 = (beta1 + 1) x Ib1. Perhaps the +1 term was omitted because beta1 >> 1, making Ie = Ic a reasonably accurate approximation. Meanwhile the direction of Ie is up because the Ie vs Ib relationship is non-inverting.
  6. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    You did notice my subsequent silly-simple plan B. Correct? https://www.electronicspoint.com/forums/attachments/temp-gif.55073/ You could almost put that sucker -- with its total of 3 components -- together with wire nuts!
  7. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    Good enough! :D
  8. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    All true. But don't forget that the OP's spec' wasn't exactly for timekeeping accuracy. It was ~+/-50% A good rule to remembed (and live by): Better is the enemy of good enough.
  9. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    Point taken. But until RLY1 drops out, the cap' is still connected to the load, which should (?) provide enough current to pull RLY2 in. Then the cap' is at least connected to ground through RLY1's coil. Given that must-release is only 1.2V, it's reasonable (?) to expect that the typical...
  10. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    Here's another idea. If constructing an electronic version of the circuit is too much of a pain, this stone-axe version might (emphasis on MIGHT) work adequately. The -ND numbers are DigiKey stock numbers. Total cost (sans shipping) is about $12. The (stupid-simple) operating principle is...
  11. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    If you do, and just in case it wasn't obvious, you should go with crutschow's circuit. It's far superior to my pathetic attempt.
  12. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    Plus, the 555, unlike most FFs, has an output that can handle bi-directional load currents up to 200mA while including inductive kick-back protection. I'm not sure that last feature is mentioned in any of the datasheets, either.
  13. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    Thank you. In a different thread you said that an unusual 555 application circuit shown in a datasheet was wrong. That particular case was correct, but we've just demonstrated that the reflexive connection to ground of every CV capacitor, that's shown in every 555 datasheet, sometimes IS wrong...
  14. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    C2 tied to V+ would make Vthreshold immune to supply noise. By contrast, C2 tied to ground is worse than no C2 at all.
  15. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    Actually, that's not entirely true. For best noise rejection, ideally you want the trigger points to track the timing capacitor. Since the timing capacitor is usually referenced to ground, the CV bypass capacitor usually should be too. But in this case, with the timing cap' tied to V+, doing...
  16. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    Question, Maestro. Why did you return the C1 timing cap to +12 instead of ground? In a non-critical timing app' like this, it won't make any practical difference, but more generally I think it would tend to maximize the negative effect of jitter, noise, and ripple on the supply upon timing...
  17. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    Speaking of "other days," I imagine a simpler time when grandpaw would have just replaced RLY2's coil with a bi-metallic strip and 5-10 second thermal timeconstant heater connected to B, and thus dispensed with all this silly silicon nonsense.
  18. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    My compliments to the chef, chief! Meanwhile, I gotta' say it -- what a weird way to spend a Saturday.
  19. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    If the inductive kick of the relay tries to drag the output pin negative, the 555's internal output darlington will turn on and safely absorb it. No external protection is necessary. Meanwhile, good point about wondering if the output will drop low enough (i.e., < 4V) during the switch-over to...
  20. CircutScoper

    Dual Power supply switch with 5-10 sec delay

    I think the circuit sketched below will do what you need. 1. When supply B is off, both relays are de-energized, connecting the load to supply A. 2. When B comes on, RLY 1 is energized, causing it to switch contacts. This disconnects A from the load and causes a brief (a millisecond or two)...
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