Voltage correction

Discussion in 'General Electronics Chat' started by thur10, Aug 26, 2014.

  1. thur10

    Thread Starter New Member

    Aug 25, 2014
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    I am a novice to practical electronics but here is my problem:
    I have a mini transformer 240V AC 50 Hz input - 9.5V DC 355mA output.
    I need to lower the output voltage to 3V DC maintaining the 355mA, how
    can I achieve this?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    An LM317 voltage regulator can accomplish that.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You only need 2.1vac for 3vdc, you could also look at taking a few secondary turns off, if practical.
    Max.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Do you intend to hack into the power supply, or do something external?

    Is it an electronic switch-mode power supply, or is it a real iron transformer? (You can tell by weight).
     
  5. thur10

    Thread Starter New Member

    Aug 25, 2014
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    Hi Mike, it's a real iron transformer; and I intend to do something external.
    Regards John (thur10)
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Using a linear IC chip regulator like a LM317 will do it, but the regulator will produce E*I = (9.5-3)*0.355 = 2.3W of heat. You will have to put the regulator on a small heat sink. Can you do that?

    Go to TI.com, download the data sheet for the LM317 in the TO220 package. The circuit is on the data sheet.

    Two resistors will program the '317 to output 3.00V.
     
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  7. thur10

    Thread Starter New Member

    Aug 25, 2014
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    Hi Mike, I checked TI.com and printed out the application information sheet.
    Although very explicit and easy to understand I am wondering if this is the path I should pursue, as all I am trying to do is to drive a 3V DC motor intermittently (approx. once every 60 seconds for a duration of about 10 seconds with ridiculously minimal mechanical load). Would a simple resistor not do the job as there is no electronic circuitry involved?
    Regards John (thur10)
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    With a very light mechanical load a resistor could do that job fine I think.

    How are you switching the motor on and off every 60 seconds?

    Maybe that system could also incorporate the feature to reduce motor voltage and speed.
     
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  9. wayneh

    Expert

    Sep 9, 2010
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    I agree. The LM317 regulator I mentioned can be thought of as a "smart" resistor. It's able to adjust the voltage even if the load is changing. If the load is nearly constant, and your's sounds like it might be, the LM317 acts basically like a fixed resistor and could be replaced by one. Heat dissipation is still an issue, as mentioned already.
     
  10. thur10

    Thread Starter New Member

    Aug 25, 2014
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    The switching is achieved by the means of a magnet acting on a miniature reed switch, the lot is incorporated on a disc and mounted on the second axis of a 1.5V clock movement.
     
  11. thur10

    Thread Starter New Member

    Aug 25, 2014
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    The switching is achieved by the means of a magnet acting on a miniature reed switch, the lot is incorporated on a disc and mounted on the second axis of a 1.5V clock movement.
     
  12. ISB123

    Well-Known Member

    May 21, 2014
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    He could use zener diode to drop some load?
     
  13. crutschow

    Expert

    Mar 14, 2008
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    The miniature reed switch can't switch 355mA. You need to add a transistor switch, if that is what you are attempting.
     
  14. crutschow

    Expert

    Mar 14, 2008
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    But the zener would have to dissipate 2.3W so it would need to be a 5W zener on a heat-sink.
     
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