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LED indicator Answered

how should I drive an LED from a 38 volts power supply? The led wizard says I need a 1800 ohm, 2 watt resistor, but it also says' it'll disipate excessive heat. I recently took apart a surge protector and found that they drive an LED from a 120volt source by having a diode, LED and what appears to be either a 1 or 2 watt resistor. I need help driving an LED from a 38 volts power supply just as an indicator.

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tech-king (author)2007-12-24

the power supply works by using a zener diode to filter of extra voltage. heres a hard to understand diagram of what happens( for a better one, check the pics) :

in->-----------------------resistor-----led---ground
I
I
zener diode rated to a turnover voltage of approx 100 volts
ground

basically, a zener diode is a diode that conducts in both directions after the current passes a specific turnover voltage. by wiring it like in the above diagram, approximately 10 volts pass into the led's resistor.
gmoon, they sell 10 watts at 2 dollars in Montreal.

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guyfrom7up (author)tech-king2007-12-24

are you taking an electronics course, and those your notes? What class are you taking, it looks like a good course

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Goodhart (author)guyfrom7up2007-12-24

Forrest Mims puts out a wide range of books on the subject of electronics engineering in an easy to understand format.

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tech-king (author)Goodhart2007-12-24

hey :-), the book i got this diagram out of is from forest mims III.

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Goodhart (author)tech-king2007-12-24

Ahh, I made a "good guess then" :-) I thought I recognized that from somewhere

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tech-king (author)Goodhart2007-12-24

this is from his manual exclusively for radio-shack, when the shack still sold electronic components, and at reasonable prices. this book is copy write 1983

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Goodhart (author)tech-king2007-12-24

Yeah, about the time I was gathering books of that sort from all over. I was about 25 - 26 at the time. I have a few of his Op Amp workbooks too.

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tech-king (author)guyfrom7up2007-12-24

no. this is from a handwritten electronics manuel from the 1990's. although i am taking an electronics course.

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NachoMahma (author)2007-12-23

. Use multiple resistors, such that each one handles only part of the load. Same amount of total heat, but you can spread it out.

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gmoon (author)NachoMahma2007-12-23

In parallel, of course. And you won't need to pay the extra $$ for 2 watt resistors, just use four 1/2 watt.

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guyfrom7up (author)gmoon2007-12-23

well, I'm ordering off of mouser anyway, I think I'll just get the 2 watt resistor, it's about the same price, but less complicated and less can go wrong. Thanks guys, I was just wondering if there would be a better way, but this'll work.

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NachoMahma (author)guyfrom7up2007-12-23

. Before you place the order, see if they have a lamp that runs at 36-48V. ;)

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guyfrom7up (author)NachoMahma2007-12-24

A lamp would be good, but I'd wrather use an LED because I would never have to change it, and it'll go with the rest of my power supply. Also the LED is suppose to tells me my power supply's status, like if any of the transformers break, etc. and if the LED breaks then I could be searching for a problem that doewsn't exist. 2 watt, 1800 ohm resistor- 0.13 cents, i think I can handle that

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gmoon (author)guyfrom7up2007-12-24

Cool. They sell 2 watters at the local parts shop for $2 per. It's ludicrous, but if you need one in a hurry....

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