AC Step Down Voltage form 230V to 18V

Discussion in 'General Electronics Chat' started by milind, Apr 27, 2006.

  1. milind

    Thread Starter New Member

    Apr 27, 2006
    Hi Everyone!
    Can I do like AC Step down voltage from 230V to 18V without Transformer.
    Actually I want to measre the AC Line Voltage with the help of Microcontroller's ADC so I want to Step Down the volatge.
  2. Papabravo


    Feb 24, 2006
    From that level it is very hard. The safest way to detect the AC voltage it is with a half wave rectifier and an optoisolator. The problem is making a voltage divider that produces enough current to turn on the LED of the opto without creating a boatload of heat in the divider.

    To actually measure the voltage, safely, without a transformer is going to be a challenge. I can think of ways to approach the problem, but I'm not sure any of them would actually work.

    What is magic about 18V? Don't microcontroller ADC's work from 0V to Vdd? Last microcontroller with an 18V supply was the RCA 1802, which went to microcontroller heaven some time ago.
  3. beenthere

    Retired Moderator

    Apr 20, 2004

    As pappabravoo says, there is no safe way to do this without a transformer. If you really have to, though, you could make a voltage divider with a 112K resistor attached to the line and an 18K resistor in series attahed to neutral. This will develop 18 volts across the 18K resistor. To save yourself timing problems, use a diode to half wave rectify the AC, and a capacitor to make smooth DC. Otherwise, you have to make a peak voltage detector to trigger your A to D to get sensible data.

    It's really not smart to fool around with line voltages without isolation.
  4. Papabravo


    Feb 24, 2006
    The peak power disipation in the 18K will be 112 mW so a quarter watt will do.
    The peak power in the 112K will be 701 mW. You need a 1 Watt minimum or 2 Watt just to be safe for this one. The current will be about 2.5 mA.

    Please be careful and don't let the magic smoke out of any of your components or yourself.
  5. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    Digital multimeters have the circuitry to do just that internally. If you can find the schematic for one of them you could tap of the low DC voltage and feed to your micro.
  6. SparkMan

    New Member

    Apr 27, 2006
    Absolutely correct. It would be pretty crazy to have no isolation. They never could stop talking about it in classes.

    Honestly, I think a transformer is the easiest approach. You don't need a lot of current with an A-D convertor, your only sampling the voltages to convert them to logic, so a 230V to 18V or 12V transformer with small current shouldnt be that bulky, and that's probably what I'd use.