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Kai Klaas
03/24/06 07:22
Modified:
  03/24/06 07:26

Read: 1642 times
Germany


 
#112978 - Constant current during inrush moment
Responding to: Suresh R's previous message
Suresh said:

Kai said,
When the lamp is switched-on, current through it is limited to about 0.2A and up to about 2.0W will drop across the transistor, but only for a very short time.
If the current through 3R0 resistor reachs about I = 0.6V / 3Ohm = 0.2A

from reading the above note, i tried to calculate how you have said that 2W would be dissipated in the transistor (BD135) and, the 3R resistor would have drop of .6V. BUT couldnt get clear with that. also with "what would be the voltage drop in the bulb in that circuit" since i think it would not drop 12V fully on it.

For the lamp shown above I have made the following inrush current test, remember?




So, this lamp shows an impedance of 12V / 0.18A = 66.7 Ohm, when being turned-on for a longer time, at least longer than the inrush phenomenon needs. At the inrush moment, on the other hand, flowing current is 8.8 times higher, as you can see from the scope plot. So, impedance then is 12V / (0.18A x 8.8) = 7.6 Ohm.

The above constant current driver limits the current through the lamp to about 0.2A at any instant, because about 0.6V drops across the 3R0 resistor. So, when 0.2A is flowing through the cold lamp, and its impedance is only 7.6 Ohm at that instant, then only 0.2A x 7.6 Ohm = 1.5V is dropping across the lamp. The rest, namely 12V - 1.5V - 0.6V = 9.9V is dropping across the collector emitter junction of BD135.
9.9V times 0.2A gives a power of 2W dropping across the BD135 during the inrush moment.

Kai

List of 57 messages in thread
TopicAuthorDate
inrush current + suitable voltage sour      Suresh R      03/04/06 10:14      
   Inrush current of lamp      Kai Klaas      03/04/06 20:47      
      Current inrush      Hal Albach      03/05/06 06:28      
         NTC?      Jan Waclawek      03/05/06 06:45      
            Soft-start      Kai Klaas      03/05/06 08:00      
               8 reed SW parallely + 21.12A / at 12VDC      Suresh R      03/06/06 05:29      
                  Car battery???      Anders Sandström      03/06/06 05:58      
                  inrush current      Terry Lingle      03/06/06 08:20      
                  totally irrelevant      Erik Malund      03/06/06 08:58      
                  No testing needed, standardly      Kai Klaas      03/06/06 19:46      
                     replies      Suresh R      03/07/06 02:05      
                        Reply to "replies"      Kai Klaas      03/07/06 07:29      
                           the difference between "cheap" and "inex      Erik Malund      03/07/06 07:44      
                           current limit resistor      Suresh R      03/07/06 09:17      
                              are you just grasping for straws?      Erik Malund      03/07/06 09:29      
            NTC, yes      Erik Malund      03/06/06 08:54      
               Filaments      Kai Klaas      03/06/06 18:38      
                  In the days of radios for AC and DC      Erik Malund      03/07/06 06:57      
                     Thanks      Kai Klaas      03/07/06 07:03      
            resistor parallel to Switch!      Suresh R      03/11/06 04:29      
               MOSFET, constant current source?      Kai Klaas      03/11/06 07:04      
                  not for my use      Suresh R      03/11/06 09:24      
                     Ok, but...      Kai Klaas      03/11/06 10:00      
                        but i think      Suresh R      03/11/06 10:25      
                           Relays      Kai Klaas      03/11/06 10:51      
                              yes ofcourse      Suresh R      03/11/06 11:46      
                                 here we go again        Erik Malund      03/13/06 06:57      
                                    Even more!!      Kai Klaas      03/13/06 07:14      
                                       werlcome to the club      Erik Malund      03/13/06 07:18      
                                       reply      Suresh R      03/13/06 10:49      
                                          why did you not say so?      Erik Malund      03/13/06 11:15      
                                          You made a fool of us!        Kai Klaas      03/13/06 18:24      
                                             reply      Suresh R      03/14/06 03:39      
                                                That's no fun!      Kai Klaas      03/14/06 07:06      
                                                   Suresh, I hope you had fun      Erik Malund      03/14/06 07:28      
                                                   reply      Suresh R      03/14/06 10:29      
                                                      yeah 6 days later.      Erik Malund      03/14/06 10:52      
                  Hello Kai,      Suresh R      03/16/06 22:03      
                     Look at here      Kai Klaas      03/17/06 10:56      
                        Thank you very much Kai.      Suresh R      03/18/06 07:40      
                           It just strikes me      Erik Malund      03/18/06 10:20      
                              yes you are right      Suresh R      03/18/06 12:32      
                                 Reed switch problems      Hal Albach      03/18/06 19:31      
                                    Current rating?      Kai Klaas      03/20/06 19:39      
                                       I recall long ago something about a "sti      Erik Malund      03/21/06 06:47      
                                       Reed problem      Hal Albach      03/21/06 07:11      
                                          Glueing together      Kai Klaas      03/21/06 07:35      
                                             Reed problems      Hal Albach      03/21/06 09:14      
                                                reed relayrs      Terry Lingle      03/21/06 14:01      
                           power dissipation calculations?      Suresh R      03/19/06 08:12      
                              bulb voltage drop      Terry Lingle      03/19/06 10:01      
                                 Thanks      Suresh R      03/20/06 02:12      
                                 (.2*bulb resistance      Suresh R      03/24/06 05:36      
                                    The bulb      Terry Lingle      03/24/06 07:06      
                              Constant current during inrush moment      Kai Klaas      03/24/06 07:22      
                                 Thanks a lot Kai      Suresh R      03/24/06 10:12      
         Choke would be huge      Kai Klaas      03/05/06 07:28      

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