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Replacing a bathroom light

Discussion in 'DIY Disasters' started by shagster, 31 Mar 2018.

  1. shagster

    shagster

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    I've had one of these lights in my bathroom for a couple of years:


    It's a TLC Polo bulkhead light with a 4-pin 17w 2d fluorescent tube in it. It's not exactly brilliant in terms of aesthetics but it does the job.

    Finally last week it packed in. Assuming it was the lamp, I duly purchased a replacement:


    I fitted it and then decided that it was in fact probably the driver electrics that had packed in instead. Oops.

    Looking at the large box of electronics inside the light that seemed to now be proven non-functional, I started thinking (often a mistake that, at least where I'm concerned). I decided that since all the rest of the lights in my house were now LED, I might just as well investigate an LED replacement for this bulkhead light as well.

    The lowest power LED bulkhead I could see from TLC-direct was this one:


    That's a 12w LED, and it was on special offer. Of course, none of the mounting holes matched the old unit (what did I expect).

    There were a selection of 'knock-outs' for the screw holes in various positions, however they all turned out to be 'drill outs' as try as I might I couldn't get them to 'knock out' as I have on similar fittings in the past. These holes were skillfully positioned such that the chuck of the drill was sure to impact on the rubber seal that gives the fitting its IP54. So once I realised this, I decided to very carefully drill a 2mm hole one way (inside the unit, where the knock-out had a guiding reinforced surround, then widen to 5mm drilling from the top of the unit to avoid damaging anything when the drill bit lurched through. The kind of irritating stuff you end up doing when you DIY anything, all standard.

    But I digress.

    Once I got this thing mounted on the ceiling in place of the old one I finally switched it on.

    I guess that seeing the light from a standard Philips 15w bayonet LED must have failed to prepare me for what I now saw. This thing is bright. I mean, not nicely bright I mean so bright that I virtually need sunglasses for a bathroom visit. So bright that going back into the rest of the house requires some vision adjustment time before I can see properly!

    Thank goodness for the shaver socket light, it now sees a lot more use.

    I suppose this is all due to the fact that unlike with the 2d bulb all the LEDs are on a PCB plate that is pointing down. Nothing is going anywhere else than into the bathroom, which is small.

    Now the question is what wattage resistor I can put in series with the LEDs? The AC-DC converter helpfully says it outputs 22 -> 45 volts. WTF does that mean, I wonder? Or do I go for spraying something on the inside of the diffuser? I'm reluctant to modify a standard light fitting, even if there is bags of space inside the fitting and it's a simple circuit, and LEDs don't make much heat, and it's the LV part, blah blah blah. But then there's the time. I might just end up living with it.

    Damned nuisance.
     
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  3. flameport

    flameport

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    It means it is a constant current supply, so adding a resistor will not work - the supply will adjust the voltage to keep the current and LED brightness exactly the same.

    The only options are to add something to block some of the light, or buy a lower power fitting.
     
  4. endecotp

    endecotp

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    If all the LEDs are in series, consider shorting some of them out.

    (You’d want to measure the total voltage and current, short out one of two and measure again. The current should be the same as before and the voltage should have dropped in proportion.)
     
  5. shagster

    shagster

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    I may try this, sounds like a plan, thanks.
     
  6. shagster

    shagster

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    Unless I add it in parallel?
     
  7. endecotp

    endecotp

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    Don’t do that.

    It would kind of theoretically work if the LEDs themselves behaved like resistors. But they don’t; they don’t follow Ohm’s Law. So it won’t work.
     
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  9. John D v2.0

    John D v2.0

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    New LEDs not forced into the bc or es package can be so much better. I had a similar surprise, the existing retro fit were well under 100lm/w then I bought some decent make osram led tape and it is 148lm/w. Was a big shock when I turned it on!
     
  10. shagster

    shagster

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    I had another look at this tonight. It seems this is using the MT79338 chip for the AC-DC converter.


    The design used in the bulkhead light roughly follows the example circuit:


    And the data sheet says that the LED current will be governed by this equation:


    (I'll spare you the details as they are all in the data sheet). I've no idea about the number of turns (Np/Ns), but I could just measure the LED current to establish how the Rs resistor value will affect it. On the other hand, it seems I may have lucked out as they used two SMD resistors in parallel (3 and 3.3 ohms) to get Rs resulting in about 1.57 Ohms. So I could just take a chance and remove one of them giving me about half the LED current:


    (See the Rs1 Rs2: 3R30 and 3R00 respectively). The only thing I'm unsure about is the 4700 resistor between pins 1 and 2 (not shown in the example circuit). Guess I could run it on the bench for a while and check all is well before putting it back. Shorting out the LEDs themselves looks to be a lot more work but certainly doable. There are 70 of them in total, and losing an entire section would be a lot less work than spacing out the shorted ones in an even pattern, but it probably wouldn't look as nice.
     
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  11. endecotp

    endecotp

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    Yes that will work.

    Don’t worry about the resistor between pins 1 and 2.
     
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  12. shagster

    shagster

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    I took out Rs1 (3.3ohm), and that reduced the AC current draw from 59mA to 36mA. Ran it on the bench for about an hour and nothing remotely warm on the circuit board so refitted it all. It now seems about equivalent to the 2d I had before in terms of brightness.
     
  13. securespark

    securespark

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    Shagster

    I have exactly the same 2D light fittings in my bathroom, except they were 28W. The lamps kept failing, so I bought one of these to replace one of the Polo fittings so I could see what it was like.
    https://www.everything-led.co.uk/item-p-ilbhc007/25w-slimline-ceiling-wall-bulkhead/

    I like it, because it is uber-bright and I can see really well, but everyone else is like "woooaaahhh!!!"

    My lad came out after brushing his teeth and said, I think I got a tan in there...
     
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