Pls help to find resistor value.?

Question

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?

Thanks!!!

Answer 1

This is a very inefficient way to use power. You generate a huge amount of heat.

The dropping resistor will not reliably work. As the load on the regulator varies, the voltage drop across the resistor varies. When you start up, or have low load, the voltage will exceed the regulator capability and destroy it.

To use the 270V supply, you need IC's rated for 270V or more. While these exist, they are costly.

How is it that you are trying to use a 270V supply? Does it even have a 1A capability? What are you trying to power? This is rarely a real world need; sounds like some other approach altogether would  be appropriate.

Answer 2

you could use the resistor like you want but use a 48v zener diode after it just before the input to the regulator. This zener will hold the input to the regulator a a near steady 48v.

you stated the output is a max 24vdc and 1amp. this calculates out to 24 watts...right...P=IxE

now you have 48v input so we can assume a max draw of 24 watts or less. This should give you a input current max of around 1/2 amp in a perfect world....So lets give it 750milliamps max and we can calculate out the input resistor...

this gives us around a 60 ohm resistor.....

The wattage of the resistor is going to be large however to dissipate the heat. This resistor value is at "max" current draw so if your sure the output current is going to be less than the wattage of the resistor will go down. Try a metal oxide resistor, they can get very large wattage's out of a small resistor...there avail at any "Fry's" or electronics store...

If you don't have one handy try Digikey.com

They have free tech support and can help guide you with the proper size. That should answer you question....

make sure the zener can handle the input you are giving it also...if not it will make short work of it and its reallllllly hard to put that smoke back in.

Answer 3

Resistor would not work unless the load current is fixed at 1 amp, in which case you could use a 222 ohm 400 watt resistor.

A 200 ohm 400 watt resistor and a 48 volt 50 watt zener on a heat sink would work for varying loads, but you have a lot of hot components.

You could use a low power resistor like 20k ohms 5 watt and a 1 watt 48 volt zener to feed the base of a high power darlington transistor with the emitter feeding the 317, but the transistor will dissipate 220 watts, much too high. You would need 4 transistors in parallel and a large heat sink.

Much better would be a switching regulator, but it would have to be specially designed, you won't find one off the shelf.

Why do you have this requirement? where did you get 270 volts?

Why not just generate the 24 volts via a 28 volt transformer?