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Replacing back-light incandescent lamps with LED

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

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  1. K5VV

    K5VV Ham Member QRZ Page

    The LED display on my old Kenwood TM-731A has four incandescent lamps back-lighting the LCD display. Two of them are in series, along with a 10 ohm resistor, and driven with 10.5V (7.8V when the dimmer is on). I realize the resistor will need to be much larger based on the forward voltage drop & current spec of the LEDs, and that the "dimmer" may turn the LEDs completely off.

    Question is..what color LEDs will work the best? What is this about FLAT or DIFFUSED LEDs?

    I am assuming bright white or soft white, but maybe it's simply a matter of preference?
  2. K4EEZ

    K4EEZ Ham Member QRZ Page

    Should a diffused or non-diffused LED be used when backlighting?

    One common mistake designers make is using a diffused LED for backlighting. A diffused LED distributes the light over a wider area, diminishing much of the LED's output. The goal is to have as much of the emitted light as possible directly reach the front panel. Therefore, non-diffused LEDs are more effective because their light is focused in a narrow beam. Waterclear or untinted styles are best to use because they do not have a diffusant that reduces light output.
  3. KA5IPF

    KA5IPF Ham Member QRZ Page

    Non-diffused LED's used in a circuit designed for bulbs causes a problem with points of light and not even lighting over the entire display. I still haven't found diffused LED's that spread the light evenly like a bulb.

    A from scratch design can overcome this but replacing bulbs with LED's it's harder to get the proper lighting.
    WA7PRC likes this.
  4. VE1IC

    VE1IC Ham Member QRZ Page

    Get yourself some NSI45020 20ma led drivers, makes conversions much easier.
  5. PD0AC

    PD0AC Ham Member QRZ Page

    For the best result you need flat top (wide angle) LEDs, color warm white. Put a 1 KiloOhm resistor in series. Normal LEDs will not light up your display evenly. The dimmer might be a problem; don't know how to fix that.

    Here is an eBay example: LINK

    These are 3mm LEDs, there are 5mm LEDs too.
  6. N7EKU

    N7EKU Ham Member QRZ Page


    If you have unobstructed access to the width of the display LCD, I might try SMD strip LED's. These are small SMD LED's spaced apart on a thin and narrow plastic strip. This should give you fairly wide distribution. Start with a 1k pot in series and use it to adjust the lighting to your preference, then you can measure the pot and replace it with a fixed resistor. The dim/bright voltage spread is not too huge so it may try setting the resistor value for a good compromise between the two settings.

    If the voltage difference between the dim/bright setting is too much, then you could add an additional small value resistor to the bright setting to bring it closer to the dim one.


  7. G3YRO

    G3YRO Ham Member QRZ Page

    Personally I suggest you stick with bulbs !!

    People seem to imagine that LEDs will last longer, but that simply is not the case, certainly the LEDs used for lighting.

    I have only just replaced some of the original bulbs in my car's tail-lights, that are 15 years old. Yet you see cars that are only a couple of years old where one or two of the LEDs in the display have failed - and they're VERY expensive to replace.

    Similarly, the backlights used in most LCD TVs last forever . . . yet the newer LED TVs (that simply use LEDs for backlighting, instead of cold cathode tubes) they fail all the time.

    I have just bought a faulty 47 inch LED TV cheap that is only 18 months old . . . there is no picture, because the LED backlighting has failed. (once it detects that 5 or more LEDs have failed, the circuitry turns the backlighting off within half a second). So I'm going to take the screen apart and replace them (very tricky job!)

    BUT . . . as far as I'm concerned . . . good old incandescent bulbs last FAR longer than LEDs! (and usually give more suitable light, as has been explained)
  8. AI2SS

    AI2SS Ham Member QRZ Page

    I wouldn't mind sticking with bulbs for my TS-940 LCD display, but they're no longer available. So I'll have to change over to LED I guess. Seems like a pain.
  9. K5VV

    K5VV Ham Member QRZ Page

  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    A flashlight works for me. In a pinch.
    K4EEZ likes this.

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