Shielded wire vs non shielded wire

Discussion in 'General Electronics Chat' started by LightMan, Mar 18, 2005.

  1. LightMan

    Thread Starter New Member

    Feb 14, 2005
    5
    0
    Howdy folks.

    I am very new to electronics. My purpose in life now seems to be to build a light box for my job. I repair musical instruments and we use lights to help us see leaks from under sax pads, piccolo pads etc.... The light boxes on the market are good, but they are really expensive and the replacement bulbs are too. I figured I should try to make a new one.

    I am constructing a light box that uses cold cathode light bulbs. I have the inverter, I have the light, I have a nice box to put it all in, now my only problem is with the wire that leads from the inverter to the bulb.

    I am using 24 AWG wire to connect to the bulb. It is about 2 feet long. It works fine and the light is bright, but when I grab the wires and put them together the bulb dims. So I figured I should try using shielded wire. Now the light doesn't even light up!!! Is this all some sort of shielding problem or am I losing my mind??!!??!??!!?? :blink:

    What is going on???? What can I try to make my light work????

    Any help would be greatly appreciated :^)

    thanks

    M
     
  2. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    What kind of voltage, currents and wattage we talking about and if you could post a schematic, a pic of the wiring, etc, I know one of us will be able to help you out.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Always wondered how cold cathode lamps worked. From your description of the problem, they must have a high frequency AC applied (it ionizes the gas inside so the bulb conducts). That's like the traditional test for RF leakage where you wave a fluorescent bulb over the open chassis to see if it lights up.

    The bulb dims because your body is acting like a capacitive load and drawing off some of the energy. The shielded wires do an even better job, so no power gets to the bulb. It may be that cold cathode lamps are not the thing for your application - the musical instruments are metal. and will also draw off RF energy.

    You may be stuck with going to something like a flexible light pipe. That's like a big optical fibre, so the heat from the lamp stays in the base, and the light can be snaked into tight places. I've seem 'em used as microscope illuminators, but don't know a source.
     
  4. LightMan

    Thread Starter New Member

    Feb 14, 2005
    5
    0

    Thanks for the info.

    The CCFL operates at about 750V. whether or not its AC or DC I don't really know. I'm pretty sure its AC. It works on very low amps though, about 5mA. I dont know the wattage, i think about 25, but I'm sure I'm wrong.....

    Do these flexible light pipes illuminate just at the tip or does the whole strand light up like EL wire. If so, is it bright??? Right now in the industry, we use standard T2 fluorescent lights. They are good but they get too hot. Thats why I'm trying CCFL.

    I'll try to post a pic of what I've got so far. It's starting to come together....... slowly.

    Thanks again

    M
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    I might have gotten carried away.. You need to insert a light source into the barrel of an instrument to check for the felt seal at a key. Something a bit less of a pain is a super bright led. They are low voltage and fairly low current (two volts and something on the order of 25 - 40 milliamps). Stick the led on a long length of wire and look for the eerie green/blue/white glow (your choice of color).

    Check out Hosfelt.com for lots of bright leds. They also have tons of wall transformers for the voltage source. Probably get set up for under $10.
     
  6. LightMan

    Thread Starter New Member

    Feb 14, 2005
    5
    0
    Thanks for the tip. I have looked into LED stuff. I need a light about 12 inches long. CCFL, I thought, was going to be easy. I suppose I could wire up a bunch of LED's in a row..... but we'll see.

    checking.........


    ok..... I checked into LED's. I am just starting to understand OHM's law and stuff. Its been a long time since high school.......

    I am looking at an LED that is rated at 3.6-4 VDC @ 20mA. It has the brightness I need. Now to do some calculations..... ok, here goes....

    So the resistance of each LED will be R=V/I

    R=200 ohms????

    If I need 5 LED's wired in parallel, do I use 4 VDC as my power source? If so does this work??? I'm sure I'm wrong though........

    Total Voltage will be 4 VDC, Total amps will be 100mA, and total resistance will be 40 ohms???? I hope I did this right.

    Then, if this is right, all I need to do is wire them up in parallel, get a 4VDC 100mA adapter and I'm off to the races right????

    If you could tell me if I am on the right track, I would be very appreciative.

    Thanks

    Sorry for the confusing n00b l4m3 5p0k3....... ;)

    M
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Leds don't really have a resistance. It takes a certain energy to make them conduct - the applied voltage. Once they start conducting, you have to limit the current with a resistor so they don't burn up.

    With your 5 leds in parallel, you need to give them 4 volts at 100 mills. Get a 12 volt wall transformer good for 200 mills so it doesn't get hot. You have 8 volts left after the drop over the leds. Divide the 8 volts by 100 mills (.1), and you have 80 ohms needed for the resistor. You should be able to find an 82 ohm resistor - that's a standard value. The power it will have to dissipate is 800 milliwatts. Get a 2 watt resistor and you're in business. One watt would do, but it would get hot.

    Poke holes in a craft stick or popsickle stick to mount the leds. Good luck!
     
  8. LightMan

    Thread Starter New Member

    Feb 14, 2005
    5
    0
    Thanks a lot. This helps me out a lot!! :D

    I'll let you know how it goes.

    M
     
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