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Kai Klaas
01/12/06 07:54
Modified:
  01/12/06 08:06

Read: 1551 times
Germany


 
#107140 - Output voltage
Responding to: Suresh R's previous message
Suresh said:
1) Why does a rectified o/p shows a high DC o/p compared to its AC i/p.
I used a bridge rectifier with 5000uFd filter capacitor to rectify 12 VAC. The filtered o/p shows 19 VDC. Can any one tell me why?

12VAC is the RMS value of output sine. The bridge rectifier allows you to achieve a much higher voltage though, because then the storage cap is charged to up the peak voltage of sine (which is SQRT(2)=1.414 times higher than RMS value) minus the voltage drop of two diodes.
So, you will idealy see a voltage at storage cap of about RMS_value x 1.414 - 1.5V = 12VAC x 1.414 - 1.5V = 15.5V.

But this simple formula is only valid for very strong transformers, which do not show relevant ohmic resistance of secondary winding, and if the nominal load is drawn from the transformer. But in reality the finite transformer impedance leads always to a lowering of output voltage of up to some volts under nominal load.

A special case are very tiny transfomers. These can show a very drastical difference between output voltage with and without load! I have seen transformers which produced up to 3 times more output voltage when running without load!! This mechanismn is very probably responsible for, that you see a voltage of 19V instead of "calculated" 15.5V.

How big should the storage cap be?
With a standard bridge rectifier the storage cap is filled every 10msec (50Hz mains). At worst case condition the storage cap must deliver current for 10msec. In this time period he loses voltage. How much?

C = Q / U = I x t / U

where C is capacitance of storage cap, I is the current drawn from it, U is the voltage it loses and t is the time period where the current is drawn.

This yields:

U = I x t / C

An example:
If C = 1000F, t = 10msec and I = 100mA, then U = 1V

In practice the storage cap must be choosen to be big enough to deliver current without huge voltage loss. On the hand, the storage cap should not be choosen too high though, because the higher this cap, the less time the transfomer has to give it back the charge and the higher and more narrow the charge peak current is. This can result in enormous interference.

So, a 5000F storage cap for a 100mA application is highly overdimensioned.

Kai

List of 56 messages in thread
TopicAuthorDate
Circuit Analysis      Suresh R      12/27/05 00:40      
   Analysis      Farshid Jafari Harandi      12/27/05 04:15      
   Dangerous!      Kai Klaas      12/27/05 07:09      
      bungling amateurs "bragging" on the web      Erik Malund      12/27/05 08:55      
         Not electronic, but an amusing example      Joseph Hebert      12/27/05 09:58      
            OLD idea ?      Steve M. Taylor      12/27/05 12:09      
               New and improved      Joseph Hebert      12/27/05 17:11      
                  Safety first      Steve M. Taylor      12/28/05 06:43      
         I think you meant...      Kalpak Dabir      12/27/05 10:01      
         deja vu all over again!      Andy Neil      12/27/05 18:23      
      resistor replaced by capacitor      Suresh R      12/28/05 01:11      
         Phase shift      Kai Klaas      12/28/05 06:30      
         dissapation      Terry Lingle      12/28/05 06:38      
            Dissipation      Steve M. Taylor      12/28/05 06:48      
               It's a real world after all      Terry Lingle      12/28/05 07:11      
                  What ? Make mistakes ?      Steve M. Taylor      12/28/05 07:33      
                     are you sure      Erik Malund      12/28/05 09:05      
                     or ..      Andy Curda      12/29/05 13:21      
            Non ideal cap performance      Kai Klaas      12/28/05 06:58      
   Diodes!      Andy Neil      12/27/05 07:14      
   Current Limiting Capacitor      Joseph Hebert      12/27/05 09:47      
      Capacitor Filters      Suresh R      01/12/06 01:15      
         Some basic theory needed      Kalpak Dabir      01/12/06 01:25      
         12 */~2 ~ 17 so I doubt you see 19V      Erik Malund      01/12/06 06:31      
         Output voltage      Kai Klaas      01/12/06 07:54      
            Regulation      Steve M. Taylor      01/12/06 11:20      
            current flow      Suresh R      01/13/06 00:43      
               What current?      Kai Klaas      01/13/06 06:10      
                  What iam trying to know is      Suresh R      01/15/06 21:29      
                     inrush current      Terry Lingle      01/15/06 23:20      
                     Naiive analysis      Steve M. Taylor      01/16/06 06:33      
                        Laplace Transformation      Kai Klaas      01/16/06 07:19      
                           Could be worse....      Steve M. Taylor      01/16/06 11:39      
                              Me too!      Kai Klaas      01/16/06 11:58      
                                 Minds are failing      Steve M. Taylor      01/16/06 12:28      
                     DC component      Kai Klaas      01/16/06 12:03      
                        Suresh, have as look      Erik Malund      01/16/06 12:18      
                           analog and digital ground      Suresh R      01/17/06 00:24      
                              discussed ad nauseam do a search      Erik Malund      01/17/06 06:56      
                              Use a splitted ground plane      Kai Klaas      01/17/06 07:32      
                                 did i got it right      Suresh R      01/18/06 00:49      
                                    Also datasheet of ADUC can help you      Kai Klaas      01/18/06 06:44      
                                       Placing Vias      Suresh R      01/24/06 00:18      
                                          Maybe..      Kai Klaas      01/24/06 06:08      
                                             Copper fills      Terry Lingle      01/24/06 08:25      
                                          I do not know about 2 layers      Erik Malund      01/24/06 09:09      
                                        Auto route      Suresh R      01/25/06 00:00      
                                          Solid ground plane has to be solid!      Kai Klaas      01/25/06 08:37      
                                             Thank you Kai      Suresh R      01/26/06 21:12      
                                                circuit for a despensing unit      Suresh R      01/28/06 02:58      
                                                   ICM7555 + 2M2 + 68      Kai Klaas      01/28/06 10:14      
                                                      why a totally unrelated post in the midd      Erik Malund      01/30/06 08:07      
                              Separate signal traces.      Joseph Hebert      01/17/06 10:01      
                                 What has God to do with this??      Kai Klaas      01/17/06 19:20      
               DC DC      Steve M. Taylor      01/13/06 06:19      
               DC DC      Steve M. Taylor      01/13/06 06:19      

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