Replacing back-light incandescent lamps with LED

Discussion in 'Radio Circuits, Repair & Performance' started by K5VV, Oct 31, 2015.

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  1. AI2SS

    AI2SS Ham Member QRZ Page

    That looks like it would work!! I'll call around tomorrow and see if any of the shacks near here have them. Thanks!

    Don, I don't like getting pinched ;)
  2. WA7PRC

    WA7PRC Ham Member QRZ Page

    The problem is the light source itself, not the driver. If you want a current regulator, a simple LM317L works, and its adjustable. It does require a single resistor.

    I agree with Clif KA5IPF -- a narrow dispersion angle (the angle of the "cone of light") of an LED often makes for a spotlight effect. You MAY be able to choose an LED with a wider dispersion angle, or just remove the lens. Since the light is spread over a larger area, it will appear dimmer. Tip: choose a higher output LED and/or more LEDs, and drive it/them with less current.
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    Those are also called "Grain Of Wheat" lamps, available from hobby suppliers. The approximately 5X inrush current is what kills incandescents. You can limit the current with a resistor but, to be effective, you may also be significantly limiting the running current. The fix is to use a current REGULATOR, such as the LM317 series. That's what I did with my olde Kenwood TS120S rig. I set the current to about 5% less than the normal running current. That was MANY years ago, and they're still working. Cost was about $1 for each lamp.
    AI2SS likes this.
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    One problem with a lot of bulbs, is they are rated at 12V, Not 13.8V.

    When ran at 13.8V, they do not last very long. SWR meters that you connect to your 13.8V radio supply prove that theory.

    A 12V regulator is cheap. Run them under 12V with a adjustable regulator and they last a long time.
    Last edited: Nov 6, 2015
    K4EEZ likes this.
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    Again, it's the ≈ 5X inrush (cold start) current that kills incandescent lamps. A voltage regulator won't limit the inrush current. A current regulator is the answer. There are lamps rated to run at 14V. That's initially what I used in my olde TS120S and TS130S rigs. But, they kept going open. I added LM317L regulators (datasheet) configured as current regulators and switched to 12V lamps (the '317 drops at least 1.25V):
    The LM317L can handle up to 100mA, while most GOW lamps pull much less current. Tip: for every 5% decrease in running current, expect 50% increase in lamp life. You won't notice a decrease in light output with a 5% decrease in current.
    K4EEZ likes this.
  6. G3YRO

    G3YRO Ham Member QRZ Page

    Actually that's not strictly true . . .

    All auto bulbs are marked 12V, but they run at 13.8V all the time . . . usually a bit higher.

    I really don't know why some of you think incandescent bulbs don't last long - I've just replaced one of the bulbs in my FT101E - that's for the first time in 37 years !
    WQ4G likes this.
  7. 2E0TZX

    2E0TZX Ham Member QRZ Page

    I do think that TVs are a dodgy example, like laptops they suffer from decreasing build quality with each generation.
    LED seems to be a favourite with manufacturers as it is easier/cheaper to make and implement.
    No mucking about with fragile tubes and high voltage inverters etc.

    Changing the CCF in a laptop panel is a pretty delicate task, never changed LED backlight yet.

    Bulbs v LED, apart from lighting quality, I should imagine that the longevity would be in the specifications of the devices.
  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    Your FT101E runs the 16V panel lamps at a significantly lower voltage (and current) from the 12.6V source. Additionally, it has a 5.6Ω/3W resistor (R71) in series with the paralleled PL1, PL2, and PL3. I'm surprised the lamps didn't last MORE than 37 years.

    I've owned my Heath SB-220 since 1975 and have never changed the panel lamps. They're 6.3V lamps, running on the 5V source for the 3-500Z tubes.
  9. G3YRO

    G3YRO Ham Member QRZ Page

    The point is that it's a MYTH that LEDs last longer than incandescent bulbs . . .

    Even in cars, where they are extremely stressed with constantly changing voltage, extreme temperatures and heavy vibration, they still often last 25 years!

    Personally, I also hate the look of a string of small LEDs for lights on cars, rather than a nice big red, orange or white panel. It's also less pleasing to the eye they way they instantly come on and off, rather than the inherently tapered on and off that you get with a bulb.
    WQ4G likes this.
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    That is like comparing them to flashlight bulbs.

    I guess a 1157 would last a long time as a meter back light.

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