full wave bridge rectifier volts change question

Discussion in 'General Electronics Chat' started by minkey01, Nov 7, 2015.

  1. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Good morning. Could someone help explain what is going on here?

    I have a full wave bridge rectifier that changes volt amounts outputted when I hit a button in the circuit. Maybe I don't understand them properly. I thought their output stays at a fixed DC volt like a battery. Here is what I'm testing.

    Measuring volts with voltmeter at point 13 (+) and point 2 (-). Point 2 is ground. Point 13 is the + output of the full wave bridge rectifier coming from that transformer. I highlighted these points in yellow. Initial reading is around +30V. When I press PLAY (highlighted green button) the volts drop to about +27V. Any ideas? Please enlighten me.

    Thanks for the help!!

    recitifier volts question.jpg
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    It's perfectly normal to see the voltage of an unregulated power-supply fall when a load is connected, that's why voltage regulators were invented; to provide voltages that don't drop under load.
     
  3. minkey01

    Thread Starter Member

    Jul 23, 2014
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    also. 1 more question. do you always have to have a load on the full wave bridge rectifier? can a switch come off of it before the load therefore when the switch is off there will be no load?

    thanks.
     
  4. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    It doesn't need a load to be connected all the time, it's common to have a switch between this type of supply and the load.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Any type of rectifier-filter's output voltage will change with load.
    This is due to the resistance of the transformer and the diode's forward-drop dynamic resistance.
     
  6. minkey01

    Thread Starter Member

    Jul 23, 2014
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  7. minkey01

    Thread Starter Member

    Jul 23, 2014
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    another thought. if my load is never going to change, should I just add some resistor in series with my current load in order to get the 30V down to 27V?
     
  8. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Why do you need 27V?

    You can't really regulate your 30V down to 27V because, as you know, under load the unregulated voltage drops to 27V which would leave nothing for the regulator (even an LDO) to work with.

    If 30V is too high, you could add 4 silicon rectifier diodes in series to lose a few volts but the output voltage will still be load dependent.

    Perhaps you could add a switch-mode psu to provide the regulated 27VDC you think you need.
     
  9. minkey01

    Thread Starter Member

    Jul 23, 2014
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    Can't change the power supply. I have to use that transformer and rectifier setup.

    If you look at the original schematic, my new circuit is similar to theirs. I'm using that power and a bunch of switches, buttons, and relays. My circuit is just a little smaller. What I don't understand is how did they setup their total schematic using unregulated power? Wouldn't their volts be changing every time they throw a switch? I measured the old machine and somehow their volts aren't changing at the switches, but my new circuits are.

    That is why I wanted to take the 30V DC (that is straight from the rectifier with no load) and regulate it down to 27V and then hook everything up. This way the 27V will not change depending on what my switches and buttons are doing. Correct?

    Thanks!
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    Without seeing the original schematic, it is hard to tell from your description whether you believe in elves or factories.
     
  11. minkey01

    Thread Starter Member

    Jul 23, 2014
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    it's in the original post

    but never mind, i'll figure something out.
     
  12. minkey01

    Thread Starter Member

    Jul 23, 2014
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    let me get my full circuit together. i will start a new thread with a more clear description and question.

    i'll be back. :)
     
  13. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Minkey, what are you trying to achieve, do you need a fixed voltage of 24 to27v regulated, or lower, what do you need it for?
     
  14. Papabravo

    Expert

    Feb 24, 2006
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    No it isn't. You believe there is something that you did not understand or could not explain that should prevent the voltage from dropping when buttons were pushed.
     
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