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Oliver Sedlacek
02/29/12 00:54
Read: 526 times

#186272 - Some basics
Responding to: Chico Magalh„es's previous message
Using two polarised capacitors back to back to make a non-polarised capacitor of half the value is an old tecnique, but not one I like much personally. Quite why you want the first DC blocking cap is a mystery as firstly mains doesn't have any DC, and secondly a bridge rectifier doesn't mind some DC on the input. It does add about 640 ohms in series resistance, which is probably it's purpose.

The reservoir capacitor on the output of the bridge can be polarised, as the bridge generates a positive and negative output. The reservoir capacitor needs a 400V rating, and I would use just a single polarised capacitor.

As you have calculated, 72 LEDs drop about 273V. Assuming a maximum AC RMS input voltage of 264V, the reservoir may get up to 373V, so you are looking to drop 100V through the series resistance. Calculating the LED current is quite tricky for the circuit shown, but it looks like it's about 100mA if you include the input capacitor. This means that each 470 ohm dropper resistor dissipates nearly 5W. If you manage to adjust the components to give 350mA, then the dropper resistor will dissipate nearly 50W!

Note that a mains transients will rip through the series capacitor and might blow all your LEDs.

By the time you've added tarnsient suppression, heatsinks and isolation for a control transistor, it might be cheaper to use a proper isolated PSU.

List of 27 messages in thread
AC Powered LEDs + strobe      Chico Magalh„es      02/28/12 22:58      
   Isolated Switch      Jim Granville      02/28/12 23:39      
      Switching      Chico Magalh„es      03/15/12 00:41      
   Some basics      Oliver Sedlacek      02/29/12 00:54      
      Not a good design - and expensive        Per Westermark      02/29/12 03:00      
         Answers + need some explantion      Chico Magalh„es      02/29/12 23:55      
      Cap voltage      Chico Magalh„es      03/01/12 00:03      
         More than 280V      Oliver Sedlacek      03/02/12 04:30      
            Measurement      Chico Magalh„es      03/15/12 00:45      
               Measurement correct      Erik Malund      03/15/12 07:54      
   two caps polarity reversed on DC!! ?      Erik Malund      02/29/12 07:13      
      More common long time ago      Per Westermark      02/29/12 08:33      
         I was around then      Erik Malund      02/29/12 09:21      
            Some polarized capacitors survives AC but not reversed DC      Per Westermark      02/29/12 10:01      
               POOOOOOOOOF      Erik Malund      02/29/12 10:27      
                  Yes, very likely a failure      Per Westermark      02/29/12 11:10      
                     that need bold face      Erik Malund      02/29/12 11:44      
            Back to back connection...      Kai Klaas      03/02/12 17:05      
   And another thing...      Andy Neil      02/29/12 15:30      
      Pseudo-CC with resistor requires much % V over resistor      Per Westermark      02/29/12 16:52      
      chip      Chico Magalh„es      03/01/12 00:11      
         one of these      Erik Malund      03/01/12 08:00      
         Chips      Andy Neil      03/01/12 14:33      
   PC PSU      Chico Magalh„es      03/15/12 00:47      
      Definitely better with lower voltage design      Per Westermark      03/15/12 01:11      
         PSU      Chico Magalh„es      03/15/12 18:37      
            Some bricks can inform about rating      Per Westermark      03/16/12 02:21      

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