led to ac or dc connection

Discussion in 'The Projects Forum' started by sangpo, Aug 27, 2013.

  1. sangpo

    Thread Starter Member

    Aug 17, 2013
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    1
    I have got a LED with wire connected to both terminals. There is no label on it. How do I know whether this LED could be used in AC or DC power.? Can I try this LED connecting with 240 AC volt with resistor in series?

    Thank you
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Only if you want to destroy the LED in short order.

    LEDs have very low maximum reverse breakdown voltages -- generally only 5V to 7V or so. The normal way to protect it when you have AC voltages is to put a rectifier diode in antiparallel with it (i.e., pointing the other way).

    It sounds like you are very, very new at playing with electronics. As such, I strongly recommend that you NOT do anything that brings you near 240V AC mains power -- you may quickly discover that you light up brighter than the LED!

    Instead, get a low voltage DC supply of some kind. A cheap wall wart or even just a 9V battery, for your electronic tinkering. That way you can make numerous mistakes and live to learn from them.
     
  3. sangpo

    Thread Starter Member

    Aug 17, 2013
    74
    1
    Thank you. Yes I am very new to electronic. It is very instereing subject. I have learn how convert ac to dc vlot using 4 diodes, with the help of u all. my rice cooker has light which indicate hot and cooking.( is this light not LED) This rice cooker operate with Ac volt of 240 volt only. I have open the rice cooker to see how that indcator light is connected and it with a resistor.
    I am really confuse. You said LED generally glow with 5-7VDC. Is the indicator light installed in Rice coooker is diffrent from LED I was talking about?

    Reverse breakdown voltages. What does this mean?

    Please please , kindly explan.
    Thank you la
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    We tend to think of diodes as having just two modes of operation - forward conduction and nonconducting. But if you reverse bias the diode with enough voltage it will breakdown and begin conducting current in the reverse direction. Some diodes, such as zener diodes, are specifically designed to operate primarily in this mode. But most diodes are not.

    Provided you limit the current adequately, any diode can survive operating in reverse breakdown without immediately destroying it. But the amount of current that can be allowed is generally less than when conducting in the forward direction, if for no other reason than that the power dissipation is generally considerably higher. LEDs can survive it, too, but are generally less tolerant because the nature of their construction -- particularly being compounds of multiple elements, such as gallium arsinide, compared to a single element, such as silicon -- make them susceptible to unwanted electrochemistry when a reverse current is passing through them.

    Put if you are making a cheap consumer product, then the life expectancy of the LED is of less concern than the additional cost of properly protecting it. So you use a big enough resistor to keep the current low enough to prevent rapid failure. You strike the balance between keeping the current high enough to produce an acceptably bright light output in the forward direction and low enough to keep customers from getting too upset when it eventually fails sooner than it would have had it been properly protected.
     
  5. sheldons

    Active Member

    Oct 26, 2011
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    the indicator light on your cooker is most probably a mains neon which is totally different to an led
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    Very good point.
     
  7. sangpo

    Thread Starter Member

    Aug 17, 2013
    74
    1
    Thanks all of you for your supports.
    sangpo
     
  8. wayneh

    Expert

    Sep 9, 2010
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    +1
    I find these quite often in inexpensive small electrics and appliances.

    Sangpo, this neon bulb could be replaced with an LED if you really want to but it would be simpler and safer for you to just obtain the proper bulb to make a direct replacement. And, LEDs connected to mains is a forbidden topic here due to safety concerns.
     
  9. sangpo

    Thread Starter Member

    Aug 17, 2013
    74
    1
    Thank you wayneh for your suggestion. Actually my rice cooker is OK. but to learn electronic I have open the cooker to see. How it is connected. What is diference between the neon bulb and LEDs? how do i recognise? I am very new to electronic.
    Thank you
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Google for images of both. The neon bulb will show two posts - the electrodes - a mm or two apart in a glass bulb. LEDs also have a distinctive appearance although their shapes vary. Once you see a few for yourself, you won't forget.
     
  11. sangpo

    Thread Starter Member

    Aug 17, 2013
    74
    1
    Thank you. Wayneh
     
  12. sangpo

    Thread Starter Member

    Aug 17, 2013
    74
    1
    As suggested by Wayneh. I have google to see neon bulb and LED. I have seen Neon bulb with resistor. Is it must that we have to connect resistor always?
     
  13. wayneh

    Expert

    Sep 9, 2010
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    Yes. Unlike an incandescent bulb, the resistor is needed to limit current. Resistance of the bulb actually decreases as it lights. An incandescent bulb filament naturally increases in resistance as it gets hot, and no external resistor is needed.
     
  14. sangpo

    Thread Starter Member

    Aug 17, 2013
    74
    1
    Ok. For neon bulb and LED, why it is necessary to limit current? Because I have tried connect one LED ihave to 9 DC volt. i have even connect to 12 vol dc. it glows well without demaging it. I kept glowing about 30 minutes. So with this expereince, i have in my mind it not really necssay to use resiissior in series.

    Please throw light on this
     
  15. wayneh

    Expert

    Sep 9, 2010
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    It's almost impossible that your LED was only an LED. It likely had a resistor built into it, to protect it. From you! ;) Post a picture if you can. The other possibility is that your power supply was unable to supply enough current to damage the LED.

    Many devices are able to control themselves and need no action from us to protect against over-current. A few do not, and that includes LEDs and neon bulbs. They'll pop like a flash bulb, or a fuse, if allowed to take all the current they can get.
     
  16. WBahn

    Moderator

    Mar 31, 2012
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    I recommend you get that notion out of your mind... quickly.

    Connect an unprotected LED across a car battery and you will immediately see the value of a current limiting resistor!

    If your supply has a high enough internal resistance, that can limit the current to a tolerable level. Lots of keyfob time devices rely on this.

    What kind of supply were you getting your 9V and 12V from? A battery? A wall-wart?
     
  17. sangpo

    Thread Starter Member

    Aug 17, 2013
    74
    1
    Thanks Wayneh and Wbahn. the LED Iam talking about is the one that I collected from my broken desk top computer.(which was meant for power on indicater. The DC volt is from the Plug pack. I mean the Power adapter which is adjustabe. from 1.5 vdc to 12vDC.
    Thank you.
     
  18. WBahn

    Moderator

    Mar 31, 2012
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    The real issues are (1) whether that LED has an internal current limiting resistor, and (2) how much current your power adaptor can deliver while maintaining it's output voltage.
     
  19. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It's against forum rules to discuss connecting LEDs directly to AC mains voltages. There needs to be transformer isolation between the AC mains and a LED, to be SAFE and comply with forum rules.
     
  20. wayneh

    Expert

    Sep 9, 2010
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    I took this as a question unrelated to any plan by the OP to actually alter his appliance by attaching an LED (or even a neon bulb) to the mains. He's trying to learn how things work. Note that his experiments have been with DC power supplies.
     
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