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quick electronics question

  • 09-09-2010 9:21pm
    #1
    deaglan169
    Registered Users, Registered Users 2 Posts: 1,045 ✭✭✭


    i have a kenwood radio that needs the bulb replaced in, its the standard "grain of wheat" type bulb, the original is 12v 50ma which i can get, but i can also get 12v 30ma, now im just wondering would there be much difference in brightness, i expect there to be but im thinking the 30ma should last longer and dissipate less heat


Comments

  • #2
    Taildragon
    Closed Accounts Posts: 100 ✭✭Taildragon


    It'll probably be less bright.

    Whenever I get these little bulbs go pop, I replace with a white LED. Sand the led "face" with some rough abrasive or a file to get diffuse light. Unless you overdrive the LED it'll never need to be replaced again. As it runs cooler, it's less likely to contribute heat to the VFO etc., so helping stability into the bargain.

    HTH


  • #3
    deaglan169
    Registered Users, Registered Users 2 Posts: 1,045 ✭✭✭deaglan169


    yeah was reading into that and think its best option alrite and the radio is fairly old so plenty of room to retrofit, think ill go with the led option and diffuse it with some fine sand paper


  • #4
    watty
    Registered Users, Registered Users 2 Posts: 32,417 ✭✭✭✭watty


    White LEDs are really violet or UV LED with phospors. The cheaper ones can be short lived. Real Yellow or Amber LED last 100x more.

    I've even used a shade of green in a Kenwood.


  • #5
    deaglan169
    Registered Users, Registered Users 2 Posts: 1,045 ✭✭✭deaglan169


    thats exactly what im looking watty, is green to replace the signal meter bulb in my tr-751 can you remember where you got yours??


  • #6
    watty
    Registered Users, Registered Users 2 Posts: 32,417 ✭✭✭✭watty


    It was a Green LED with 470 Ohms Resistor in series I think, I might have used two in series.

    assumming you have a 20mA LED and two in series, then
    2.2V x 2 = 4.4
    13.4V PSU - 4.4 = 9V
    9000/20 = 450 Ohms, 470 Ohms nearest higher preferred value.

    The little "wheat seed" bulbs in Kenwoods do go.


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  • #7
    deaglan169
    Registered Users, Registered Users 2 Posts: 1,045 ✭✭✭deaglan169


    cheers watty i should have all i need here at home so


  • #8
    Taildragon
    Closed Accounts Posts: 100 ✭✭Taildragon


    watty wrote: »
    White LEDs are really violet or UV LED with phospors. The cheaper ones can be short lived. Real Yellow or Amber LED last 100x more.

    A typical 50 cent 5mm white 1000mcd LED has an MTBF of 100,000 Hrs.

    That's over 34 years at 8 hours a day.


  • #9
    watty
    Registered Users, Registered Users 2 Posts: 32,417 ✭✭✭✭watty


    5c to 10c is a cheaper one.


  • #10
    Liameter
    Banned (with Prison Access) Posts: 898 ✭✭✭Liameter


    watty wrote: »
    White LEDs are really violet or UV LED with phospors. The cheaper ones can be short lived.

    Usually because people overdrive them. If you use the correct resistor value and limit the current to a couple of milliAmps, they'll probably outlast you.

    I think you can assume about 4 volts for a white LED. Calculate the 20mA resistance and try it. Note the brightness. Now double the resistance and note the brightness. There shouldn't be much difference. Try doubling it again (and again).


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