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resistor used as a fuse

Discussion in 'General Electronics Discussion' started by visitor, Jul 2, 2016.

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  1. visitor

    visitor

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    Jul 2, 2016
    Hi all, Ive bought a lot of led bulbs from China, and slowly getting some parts together to make them safer to use. So at first I was going to fit a fuse holder inside, but some bulbs sold in The UK had some 10 ohm 1/8W resistors in one of the bulbs a/c leads, so im thinking that would be a better was to add a little protection.

    These bulbs have a very simple design, with a 820nf(400v) capacitor dropping the voltage before the rectifier, then a 47uf smoothing capacitor (63v) on dc side

    So this was running those smd led's at full power, so I have replaced the 820nf with a 470nf and the 47uf with a 100v one.

    So now the output is slightly less, but still good

    So here I am undecided as to the value of the resistor inline with the 230v mains input, and its wattage rating. One the one hand I can calculate a value, but on the other hand I can see what already been fitted on the ones I bough in the UK, and I think those resistors was fitted to make them safer in the UK

    10 Ohm 1/8W metal film, with heat shrink on them

    I have to think about switch on current, and keep the value as low as possible so it never burns out unless there is something very wrong.

    So what you think ?

    I have had two bulbs go down, one went bang and blew the 47UF dc side capacitor, and another one of the led's went open, which then switches all led's off
     
  2. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    The LEDs are fuses in themselves.
    The 10R does nothing other than create a problem when it goes open.
    It will burn out when the electro fails.
     
  3. visitor

    visitor

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    Jul 2, 2016
    then why are they fitting them?

    Also when that cap went, there was a very big bang, im, sure if it had that 10 ohm inline it wouldn't have been so bad ?
     
  4. Colin Mitchell

    Colin Mitchell

    1,417
    312
    Aug 31, 2014
    They fit them to pass the "fused appliance" requirement, in some countries.
     
  5. visitor

    visitor

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    Jul 2, 2016
    Ah I see, oh well back to the 0.03A max needed, ill just work with that
     
  6. duke37

    duke37

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    769
    Jan 9, 2011
    The resistor in line with the mains input limits the surge current when the bulb is turned on and the high voltage capacitor has no charge on it. I see no need to change this
    The 400V capacitor controls current to the circuit. If the current is not used, the low voltage capacitor has to take it, so boom.
     
  7. visitor

    visitor

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    Jul 2, 2016
    Ok, well a lot of the bulbs do not have the resistor, and that is what i was asking about.

    I have contacted the supplier, an electronics company selling components, after I convinced them Im not a seller they were happy to tell me exactly why they fitted that 10 ohm resistor in theirs(not a normal resistor), I explained I wanted to reduce the current across those led's to make them last longer, and add some protection. ha ha ha they told me they have already done that on a lot they are selling, so im buying lots more from them with higher output(reduced) and even better they have some snd cree chips by the box :)

    thanks anyway, thought i might find someone with knowledge on this.. shame
     
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