transformer and rectifier...

Discussion in 'General Electronics Chat' started by hazim, Jun 8, 2009.

  1. hazim

    Thread Starter Active Member

    Jan 3, 2008
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    I connected a 220 to 6V transformer with a fullwave rectifier, and I put two 330μF capacitors.. I got about 8 to 9V DC at output... What does I have to do in order to have an output of 6V or if possible 5V without using voltage regulators such as LM7805/6 and LM7905/6...??? Thanks
     
  2. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    The trouble is that the AC will charge the capacitors to their peak voltage with no load on them, but there is only a tiny part of the AC cycle supplying power at the peak of the cycle.

    So while with no load, you'll get a peak voltage, a tiny load will immediately drop that voltage by a large amount, then as the load increases, more and more of the AC cycle is providing power, so the voltage seems to stabilize at a lower value.

    One way to get a more stable voltage without a regulator is to add a dummy load (a resistor) to pull the voltage down to where it is more stable. You'll waste some power in the dummy load, but then adding additional load won't vary the voltage nearly as much.
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,173
    397
    What is your average load current on DC side. Possibilities : add a choke between rectifier and capacitors, replace choke with resistor of proper ohmage & wattage, or add a resistor in parallel with load to bring output to desired level, or a combination of preceding.
     
  4. S_lannan

    Active Member

    Jun 20, 2007
    247
    2
    you could use series diodes. each dropping .6 or so of a volt until you have the voltage you need. Regulation is not good but it works. A zener diode shunt regulator will do the trick too.
    Having said that a standard regulator is the best way to go. You could use a switch mode regulator but it would be uneconomical.
     
  5. rvh002@gmail.com

    Active Member

    May 15, 2009
    118
    2
    No chance, you must use a regulator. Remember ac out when rectified produces 1.4+ times the ac voltage. This is fairly basic.
     
  6. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    I have tried to put a 2W resistor, 220ohm I think, voltage dropped a little.. and the output isn't stable... I can't use this voltage supply since it's not regulated as circuits I'm using require... I may use voltage regulators...
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    What do you want to power with 6V?
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    A 6VAC transformer has an output of 6VAC only when its input voltage is correct and when its load current is what it is rated for. It might have an output that is 10VAC when the load current is low.
    the 10VAC has a peak voltage that is 14.14VDC which is reduced to 12.7V by the full-wave bridge rectifier. Then the output voltage drops when the load current increases.

    If you want a regulated output voltage then of course you must use a voltage regulator.
     
  9. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    My senior project... there is many circuits and some 6v dc motors...etc. there is no constant current but I think the current will not exceeds 1A.
     
  10. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    for the capacitors between -6V and ground, the negative terminal of the capacitor is connected to -6 and the positive to 0, right?
     
  11. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    Correct, the negative lead of the cap goes wires to the most negative voltage.

    Lefty
     
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