Pls help to find resistor value.

Discussion in 'General Electronics Chat' started by wkyong, Jun 23, 2008.

  1. wkyong

    Thread Starter Active Member

    Apr 29, 2008
    I intend to drop DC voltage from 270Vdc to 48Vdc,and this 48Vdc acts as input for a LM317 regulator,output is 24Vdc & it may supply 1A.
    Pls help to calculate what value of the resistor in term of ohm &watt used?
    Or any other idea in this case?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    That is on the ragged edge of usefulness. That is dropping 222 volts @ 1 amp, for a dissipation of 222 watts. That is also 222 ohms. The fun part is that if the current drops much under 1 amp, the resistor will not drop as much current, and the input to output limit of the LM317 may be exceeded.

    Unless you have some good reason, I would recommend using a transformer and rectifier to produce around 28 VDC for the regulator. Much less waste heat.

    The resistor value may be hard to obtain. Use a larger value, perhaps 250 ohms, and 250 watts dissipation. The 48 VDC between the regulator and resistor will vary with the current out of the LM317.
  3. SgtWookie


    Jul 17, 2007
    I suggest that your best bet is "buck" DC to DC converter.

    If you attempted what you are considering, the resistor would waste a tremendous amount of power; 246 Watts, over 90% of the entire power consumed in the circuit.
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    Such a resistor would also be quite expensive to obtain, unless one were using heater elements. Transformers and DC-DC converters are much more economical choices.