Pls help to find resistor value.

Thread Starter


Joined 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?


Joined 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.


Joined 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.


Joined 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.