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!!!

 

270VDC to 48VDC via a drop resistor then to 24 through a linear regulator? At 1 amp you're using 24 watts at the load and blowing 246 watts through the resistor and regulator. Quick tip... rethink your design, that's less than 10% efficiency, and a thermal nightmare.

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Ever heared of ohm's law??

it's one of the basics of the electro technical trade

U=IxR

U= potential difference expresed in Volt

I= current expresed in Amperes

R= resistance expresed in Ohm

sucsess

Robert-Jan

and as sceadwian said it is a waste of a lot of energy

 

R=V/I so R= (270-48)/1 = 222 ohms; closest standard part is 220 ohms
P=V*I so P= (270-48) * 1 = 222 watts

One major problem with this idea is that when the current draw is below 180 mA, the voltage at the input to the LM317 will exceed its maximum rating. This will likely destroy the LM317.

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RadioRon

 

220 Watt ! That would be one big resistor, something like a skinny brick.

 

Ceramic wirewound at 225 watts:
http://www.vishay.com/docs/30208/hlnhl.pdf

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RadioRon

 

309 grams, almost a pound. Not as bad as I thought.

 

Nice to know that one can still get big resistors for those really big projects. That reminds me of the time back in technical school when we were counselled by the department head on which specialization (or major) we should take after completing our first year. He was addressing the entire class in support of choosing power engineering (ie. dams, generating stations, high voltage transmission lines, really big resistors etc.) over Control Electronics (as computer engineering was called then) and Telecommunications Engineering. His favorite argument was "why would you want to play with microamps and microvolts when you can play with kiloamps and kilovolts?". In the light of subsequent events, the answer is quite clear, but back then the microcomputer was just being invented and the personal computer was still many years away. Not to mention that I grew up in a region with a lot of talent and engineering firms specializing in hydro-electric generation. So the argument about Kilovolts was not entirely lost on us poor students.

The irony of it all is that the field of power engineering may well re-emerge now that there is a new focus on energy resources.

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RadioRon

 

When I was at college we spent a number of weeks in the heavy machines lab (until I complained, and it turned out they had 'messed up' and we shouldn't have been there).

One experiment we had to do was phase shift, using a resistor and capacitor - only thing was that the resistor and capacitor were three feet high in steel cabinets, mounted on wheels, and the signal source was 440V three phase mains from bare brass terminals on the wall!.

Bear in mind that some of the people on the course had already been there six years (it was my third - don't ask!) - I thought it was a little 'beneath us'. Anyway, no one else complained - then a few weeks later we were dropping magnets into a coil, and watching a galvanometer go 'wheee!!!'. So I was sat staring at it, and the lecturer came up and said "what are you doing", so I said "I'm not doing this load of sh*t, I did this back in school when I was 12 years old" - he took great offence to this, and stormed off to see the college principal. The outcome was that we should never have been in that room, or doing such simple experiments, so they sorted it all out and we moved elsewhere.

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PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

The one thing that sticks in my mind from the DC machines lab was a demo by a lecturer. He had an inductor that was about a foot diameter and 2 foot long connected to a power source via a knife switch that was also about a foot long. Open it slow and you got a small discharge across the opening. Open it as fast as you could and the two foot semi circular arc was just amazing.

Mike.

 

Where I was they had a sealed off locked room (Faraday cage) with massive walton cockcroft multipliers and jacobs ladders, spark gaps etc. I never saw it running, we were told it only ran very rarely, as it wiped out all TV and radio reception for the entire town!

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

I used to work at a company called Long & Crawford that made high voltage switchgear. In the test department they had a what we referred to as an impulse generator. It was an array of capacitors and sparkgaps arranged as a multiplier. This thing took up an area about 4x3x3 Meters and could generate a 500kV pulse at some silly current. It was used to simulate lightening strikes and was used almost daily. I suspect it was the same contraption you described above.

They were such great devices. Nowadays they would just simulate it on a computer. How boring.

Mike.
Edit, just had a google and found a picture of a switch I used to work on in my draughtsman years. A T4GF3 (I was sure it was T3GF3). 11kV 650A normal. 25kA fault current.