15000v LED

Discussion in 'General Electronics Chat' started by TexAvery, Jul 19, 2012.

  1. TexAvery

    Thread Starter Member

    Oct 7, 2009
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    I just discovered that LED's will operate on 2000~15000vac. 20ma.
     
  2. PackratKing

    Well-Known Member

    Jul 13, 2008
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    So won't those little neon lamps..........it makes ones' hair stand on end something wicked.

    Even @ only 20 Ma...........be dang careful with that stuff !!! I have a 25Kv Neon transformer, 30Ma if memory serves.........I haven't hooked it up in ages.. it puts up a 6 inch arc, and makes ozone like crazy.

    I had my finger too close to an electrode when I did mess with it, and listening to it ionize the air before the arc jumped, distracted me, and I got nailed. I ain't likely to fergit that any too soon.......:rolleyes:

    IT Freakin' HURT !!! :eek: I think the " one hand in the back pocket " rule is the only thing that saved my bacon !!!
     
  3. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    That's because LEDs are current devices, not voltage devices. As long as your power supply is providing the right current, or at least close to it, they will light. It's still not a good idea to connect it to such high voltage, though.
     
  4. DerStrom8

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    Feb 20, 2011
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    And I agree with PackRatKing. Neon sign transformers are extremely dangerous. 10mA is enough to kill in just the wrong condition. I highly recommend you have someone nearby at all times when you're working with this stuff, in case something goes wrong. They'll be able to call for help. Also, never have both hands near the circuit at once. If an arc jumps to one hand, and you're near a return path with your other one, the current will go straight through your heart and could cause severe injury, or death. Please be VERY careful when working with high voltage.
     
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  5. Papabravo

    Expert

    Feb 24, 2006
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    The neon tube acts as the load resistor and the LED drops its normal forward voltage. There is nothing that says that one end of the LED has to be at GND.
     
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  6. #12

    Expert

    Nov 30, 2010
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    Actually, what you have found is an LED that will work with 20 ma going through it. I believe the LED should have a diode across its terminals so the reverse voltage doesn't have to go through the LED, unless those neon sign transformers have internal rectifiers.
     
  7. DerStrom8

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    Neon sign transformers usually are not rectified--they put out high voltage AC.

    Even if a diode was placed across the LED to protect it from reverse voltage, it would not protect it from forward voltage. The LED is likely to fry no matter how it's connected. That's why we usually use batteries and a current limiting resistor to light them, NOT neon sign transformers. Just because it works doesn't mean it should be done. It's a stupid thing to do, if you ask me, and I recommend you not do it anymore. It can be dangerous.
     
  8. takao21203

    Distinguished Member

    Apr 28, 2012
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    The LEDs may experience breakdown, but because of the low current, they don't fry up.

    No guarantee how long they will survive. Usually exposing LEDs to high voltage will detoriate them rather quickly. You should for instance use two LEDs antiparallel.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The LEDs would be more likely to survive long-term if they were parallel, back-to-back.
    EDIT: Oops, I missed the previous post.
     
    Last edited: Jul 19, 2012
  10. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Take the neon lamp out of the circuit and see what happens. (actually, don't)

    I'm working on a project right now with an optocoupler connected to 1000V. 1.5V is dropped across the opto LED, and the other 998.5V is dropped across the current limiting resistor.

    Measure the voltage across one of your LEDs and see, it's probably the normal LED voltage. Current limiting resistor in this case is the neon lamp.

    Don't measure the voltage of the 15kv power supply though, you'll likely fry your meter.
     
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  11. #12

    Expert

    Nov 30, 2010
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    Why would you want to protect an LED from forward voltage?
    This is a current limited circuit. Dangerous to be sure, but it will work. The LED will light up and not fry as long as the current is limited and the reverse voltage is bypassed.
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    How many watts is your resistor rated for?
    I don't think neon lamps are good current limiters. Older neon sign transformers typically have built-in short-circuit current limiting.
     
  13. strantor

    AAC Fanatic!

    Oct 3, 2010
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    it's a 20MΩ 1W resistor. The opto is an Avago HCPL4731 with ultra low on-current.

    I know virtually nothing about neon transformers, but I assume, if the LEDs are working, the lamp must be working as a current limiter.
     
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  14. Ron H

    AAC Fanatic!

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    You will know as much as I do if you read this.:D
     
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  15. #12

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    Actually, no. Neon bulbs (and other discharge tubes) can be easily smoked with excess current.

    In a High Pressure Sodium fixture, an inductor is designed to limit the current by Xl=2PiFL.
    The transformer in this circuit must be doing some limiting, possibly by the resistance of the winding wire or by inductive reactance, or both.

    Edit: Magnetic shunt...over my head.
    The HPS fixture has an "ignitor" to fire kilovolt pulses into the discharge tube.
    Neon does not need an ignitor, just enough voltage to hit the breakover voltage.
     
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  16. strantor

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    Oct 3, 2010
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    So it's the transformer limiting the current. Cool. Learn something new every day.

    BTW congrats on reaching 3000 posts #12.
     
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  17. takao21203

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    Apr 28, 2012
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    All gas discharge devices work like that, they have a very high breakover voltage, but resistance after ignition will be low.

    Most high voltage transformers are current limited, on purpose, or by design.

    If you connect a neon bulb to a small inverter, it won't explode immediately...

    Generally LEDs + high voltage are bad, there can be static creeping over components, especially if AC, as well inrush surge can be much higher than expected.

    Devices for instance designed to use as transless supp. don't always work as intended- I have seen one myself which actually exploded and burned the PCB.

    I know it is not allowed to discuss, just saying, high voltage circuits don't always work as expected, and don't always stay within their limits. If you can think of the hissing noise maybe when you switch off a CRT...

    If you have any DC section + capacitors, even if only nF range, then your circuit will be VERY DANGEROUS. No matter if your transformer is current limited. 20 kV at 100mA can kill you.
     
  18. takao21203

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    Actually when I was young, I had one inverter PCB for high voltage, plus small rectifier and high voltage capacitor. For 20 or 30kV!

    One day I shortened the cap. with a screwdriver. It exploded with a loud band, and there was a considerable large arc inside it.

    If touched, it could have killed me I think. I was just lucky.
     
  19. SgtWookie

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    Jul 17, 2007
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  20. DerStrom8

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    Feb 20, 2011
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    I wasn't sure how to say it. I meant over-voltage in the forward direction. Over-forward-voltage? Forward-over-voltage? Anyway, you don't want too much going through it, because it will break down the insulation.

    Yes, the circuit will work, but like I just mentioned, it can cause the insulation to break down. I already mentioned that LEDs are current-controlled devices, so the voltage really isn't all that important, provided it's above the forward voltage of the LED. However, taking it to the extreme is not a very good idea.
     
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