How can I reduce a 50V DC to 15-25V DC?

Hi.. 
so how can I reduce 50V DC to 12-25V DC?
The power source is a phone line and the load is a 5V regulator

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Thinking about it, if its "phone line" loads, its GOT to be very, very small. The best solution is a switcher of course, but the LR8 regulator will do it easily enough

Steve
ma7moodz (author)  steveastrouk5 years ago
Well Im using a LM7805 regulator.. but if I connect the regulator directly to the 50V power source isnt it going to generate a lot of heat and eventually burn the regulator?
The 7805 isn't the right regulator at all.

You need a high voltage regulator, such, as I say, the LR8.

You can mess around dropping some more input volts with a zener perhaps, but the '05 won't handle 50 V.

Steve
ma7moodz (author)  steveastrouk5 years ago
unfortunately it looks like my local radio shack doesnt have the LR8 :(
is there is any other way to reduce the voltage?
Like using resistors or something like that?
sorry but Im not really good at this..
How much current are you trying to draw ?
ma7moodz (author)  steveastrouk5 years ago
700mA to 1A
You'll have the phone company on your back, trying to take 50W from a phone line !!! I doubt the wire resistance will let you anyway.

Steve
ma7moodz (author)  steveastrouk5 years ago
well Im just working on my first little project and just test it on the phone line.. I wont be using it all the time
so what I get of what you're saying is I can reduce the voltage with resistors?
No, you need to use a switcher, so you don't take 50 W from the supply.
ma7moodz (author)  steveastrouk5 years ago
sorry for this mate but Im really a beginner at this.. I tried to do some research to find what is a switcher but I had no luck :(
so if you have a link to show it to me or explain it or at least give me a model number of something like that I will really appreciate it
Thanx
Here' s a real circuit that does what you need.
http://www.maxim-ic.com/app-notes/index.mvp/id/427
ma7moodz (author)  steveastrouk5 years ago
thank you
lemonie5 years ago
Don't do it. You screw with the phone-system, the phone company and it's customers get annoyed and you get trouble.

L

(it's a low-power system)
Fortunately, the phone line is fused, and if some idiot attempts to power the house with it, the line will fault, and the phone company sends some one out to look at it. You can light a couple of LEDs off a phone line before the load gets too big.
Why do they want to reduce it to 12-25V for a 5V output?
Anyway, USB-power might be too much.

L
rickharris5 years ago
I think it is illegal to tap the phone systems power in most parts of the world.
50V on a phone line? In the US the phone line only carries 12V.
AFAIK, the international standard (which the US set) is 48V.
ma7moodz (author)  mpilchfamily5 years ago
yeah its actually 45-50V where I live..