DC into a bridge rectifier

Discussion in 'General Electronics Chat' started by vonsworld, Jul 12, 2015.

  1. vonsworld

    Thread Starter Member

    Apr 27, 2011
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    Hello

    Please could someone answer a simply question about bridge rectifiers.

    I have a point switching circuit for a model railway and according to the data sheet it requires an input of 15v AC. However looking at the circuit diagram this AC is passed straight away into a bridge rectifier so the rest of the circuit basically runs on DC.

    I only have a 15v DC PSU available and so can I connect this instead, I am assuming that the DC will pass straight through the rectifier? Also does it matter how I connect the polarity of the DC PSU to the circuit, will the circuit side of the rectifier still output +ve and ground to the circuit as required?

    Is this all correct please?
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Yes, you can put DC into a bridge rectifier. You don't have to worry about polarity, the bridge will route the voltage for you. You will loose about 1.4 volts through the bridge.
     
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  3. paulktreg

    Distinguished Member

    Jun 2, 2008
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    Also bear in mind that 15VAC through a bridge rectifier will give approx 21VDC so 15VDC may not be enough?
     
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  4. vonsworld

    Thread Starter Member

    Apr 27, 2011
    24
    0

    Thanks for your fast reply, just out of interest, if you applied 15v AC to the rectifier would you get 15v DC at the other side without a voltage drop?
     
  5. paulktreg

    Distinguished Member

    Jun 2, 2008
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    No you'd get approx 15 x 1.414 = 21VDC.
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    3,846
    @vonworlds
    More correctly, you will get a signal that is a series of humps with PEAKs of 15 x 1.414 = 21 VDC and valleys going to zero.
    If you have capacitors on the DC side of the rectifier, the humps will be flattened to some degree. If current draw is small and capacitors are large, then you will have a very flat DC supply at 21 volts DC. Likely, with a capacitor, you will have something between the two extremes. With no capacitor, you will see the repeating camel humps (essentially the negative of the AC signal flipped up to the positive side of the voltage axis.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    The output voltage from the bridge with an AC input depends upon any filtering at the output as GopherT noted.
    Is there a capacitor at the output of the bridge?
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    And those pesky diode voltage drops...
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,570
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    You could remove the outboard bridge if possible.
    IOW you don't need the extra bridge if the PSU is already 15VDC.
    Max.
     
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