Anybody For X Sub C?

Comments

• pjv Posts: 1,903

  2010-08-28 08:23 edited 2010-08-28 08:23

  Hi P.J.
  
  Discovered years ago that in the case of driving LED's, the parallel 1N400X diode is usually not needed as the LED will break over at some 10 ish volts when reverse biased. So a small cap is all you need, but at a high capable working voltage.
  
  I use a somewhat similar scheme to power an SX circuit straight off 120 V mains, but the your diode is replaced by a very robust "surge zener diode". So the negative cycle reverse charges the cap through the zener in the forward direction, and on the positive cycle the cap is dumped into the zener which limits the voltage, and that is tapped off with another diode followed by a cap giving me Vcc. Very simple... no dropping resistor with its heat generation.
  
  Caution of course on which is the Hot and Neutral lines!!
  
  Cheers,
  
  Peter (pjv)
• PJAllen Banned Posts: 5,065

  2010-08-28 08:38 edited 2010-08-28 08:38

  I reckoned that at first, too, but after a few croaked on me I added the diode to divert the negative alternation's current (LEDs make lousy rectifiers.)
• pjv Posts: 1,903

  2010-08-28 17:10 edited 2010-08-28 17:10

  Hi P.J.
  
  I must admit that after discovering the concept I have never used it in real life to drive LED's. I would agree that using the diode is a good idea as that is much more robust. Using the concept for driving (small) residential area lighting applications is intriguing.
  
  Cheers,
  
  Peter (pjv)
• mctrivia Posts: 3,772

  2010-08-28 17:46 edited 2010-08-28 17:46

  pjv wrote: »

  Hi P.J.
  
  Discovered years ago that in the case of driving LED's, the parallel 1N400X diode is usually not needed as the LED will break over at some 10 ish volts when reverse biased. So a small cap is all you need, but at a high capable working voltage.
  
  I use a somewhat similar scheme to power an SX circuit straight off 120 V mains, but the your diode is replaced by a very robust "surge zener diode". So the negative cycle reverse charges the cap through the zener in the forward direction, and on the positive cycle the cap is dumped into the zener which limits the voltage, and that is tapped off with another diode followed by a cap giving me Vcc. Very simple... no dropping resistor with its heat generation.
  
  Caution of course on which is the Hot and Neutral lines!!
  
  Cheers,
  
  Peter (pjv)

  
  PJV wouldn't the zener get really hot?
• pjv Posts: 1,903

  2010-08-28 22:41 edited 2010-08-28 22:41

  Actually no, the SX is running on its internal RC oscillator, and draws only a couple or so milliamps, so the capacitor is selected to give the right level of current at 60 Hz, and not a lot goes through the zener. But it IS a very robust surge protection zener.
  
  Cheers,
  
  Peter (pjv)
• PJAllen Banned Posts: 5,065

  2010-08-29 08:16 edited 2010-08-29 08:16

  Funny that you mention these "surge protection zeners". My experience with them has been their failing short. They knock fuses out that way and take systems down like that, it bugs me.