resistor used to convert 220V DC to 12V DC

Discussion in 'General Electronics Chat' started by ruleworld, Oct 12, 2006.

  1. ruleworld

    Thread Starter Member

    Jun 9, 2006
    22
    1
    i made a pwm inverter which takes a little portion of the generated
    220V AC voltage as reference. the 220V AC is rectified with 1N4007
    bridge and given to the ic using a 220K Ohm resistor. is it safe to use
    a resistor this way to step down 220V DC? the ic operates at 12v dc.
     
  2. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    Circuit please.
    I assume the divided 220 volts is for reference for regulation purposes rather than supplying Vcc for the ic..??
    Again, circuit please.
    To use a resistor to drop a voltage for supply purposes is dodgy at the best of times for reasons of Safety, poor regulation, inefficiency, and the potential of smoke damage.
     
  3. ruleworld

    Thread Starter Member

    Jun 9, 2006
    22
    1
    yes u are right the voltage is for reference. i run the inverter without connecting that and it runs but no pwm control.
    the circuit is in a book, sry.
     
  4. ruleworld

    Thread Starter Member

    Jun 9, 2006
    22
    1
  5. Prodigal

    Member

    Oct 12, 2006
    10
    0
    Hi ruleworld,

    I can't quite see what the purpose of this feedback is, but from your diagram it seems although the said voltage is merely being used to drive an optocoupler in order to switch a lower voltage to be presented to the IC. The 220k resistor seems to be there to limit the current to 1mA, which would be enough to fully saturate the phototransistor.

    If this is the case then yes, this circuit should be perfectly safe as long as you build it correctly, observing proper safety precautions and keep your fingers out of the way.
     
  6. Prodigal

    Member

    Oct 12, 2006
    10
    0
    You probably worked it out for yourself, but I forgot to mention that you should keep in mind that the resistor needs to be able to dissipate 0.22W, so it's probably best to use one rated to 0.5W.
     
  7. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    2 of the half value (and 0.25W should do) is safer, as the voltage over each resistor will then be well within their 200V limit.
     
  8. ruleworld

    Thread Starter Member

    Jun 9, 2006
    22
    1
    thanks for the help to all of u. that worked.
     
  9. Prodigal

    Member

    Oct 12, 2006
    10
    0
    Sounds like a good plan, Sir. :)
     
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