Would This Lamp Be A Killer?

Discussion in 'General Electronics Chat' started by abrogard, Feb 4, 2019.

  1. abrogard

    Thread Starter Member

    Dec 20, 2018
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    My desklamp just stopped working so I got another of those Halogen globes for it but it still won't work.

    So I looked at it a bit closer and it seems to me that the arms that carry the globe mounting are the conductors that conduct the 12V electricity to it, too.

    And it runs at max 20W it says. So that's about 1.6A isn't it? And googling I learn you can get killed by as little as 0.1A, unlikely, but a lot more likely 1A.

    So is this thing for real or am I making it all up?

    Here's a picture of it, you can there's just a transformer inside and it wires to two terminals and they are the bottoms of the arms. There are no wires going up inside those arms. There couldn't be. Those arms are hinged and there's a pin right through them at the hinge points.

    I wanted more of them but couldn't find them in the shops any more. I wonder did they withdraw them quietly because of this danger?

    And that's my (novice) guess as the circuit. I'm not sure if it puts out 12V AC or 12V DC. Got no clue from the globe packet nor on the device.
    lampcircuit.jpg IMG_5483.JPG
     
  2. Yaakov

    Well-Known Member

    Jan 27, 2019
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    Ohm’s Law describes the relationship among current, voltage, and resistance. If the resistance is high, current will be low unless the voltage is also high. The resistance of the human body, with dryish, unbroken skin is quite high. 12V, particularly direct current, would not manage to produce enough current even to sense.

    Google Ohm’s Law for more, and “resistance of the human body” for some numbers to plug in.

    The formula is simple and useful, and there are many tutorials online.
     
    Last edited: Feb 4, 2019
    Tonyr1084 likes this.
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    As long as you don't climb out of the sea covered in salt water and rub the broken lamp wires on your chest, you should be fine. Even if you did that, you'd probably only feel a tingle and not even a real shock. I have $5 that says it won't kill you.

    Don't try that with voltages higher than 12V though.
     
  4. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    The really good news is that the 1.6 amps will flow through the lamp because it has a much lower resistance. So even if you did connect with those arm circuits not enough current would flow for you to even feel. It takes voltage to push the current through resistance, and the human hide has a lot more resistance than a 12 volt light bulb.
    So in summary, that lamp is not a hazard.
     
  5. abrogard

    Thread Starter Member

    Dec 20, 2018
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    I thought that might be it. They rely for safety on the natural resistance of the skin.

    And you'd reckon that's fine? Good enough?

    How about times we get zapped with 12V from a car battery?

    Not that I'm not fully aware that I gripped those arms many, many times, moving the thing when it was on, without even getting a tingle. Yes, true. I know. But.... Just saying...

    And so those arms are live. Then unknowingly putting a conductor across them you'd short out your transformer?
     
  6. abrogard

    Thread Starter Member

    Dec 20, 2018
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    $5 ? That's not much of a bet.... how confident are you...
     
  7. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    I've got $5000 that says it hasn't killed you yet.
     
  8. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    Have you got a meter? How about a harbor freight store?
     
  9. abrogard

    Thread Starter Member

    Dec 20, 2018
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    I'll go ask the wife... she knows everything...
     
  10. wayneh

    Expert

    Sep 9, 2010
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    You can grab the terminals of a car battery as firmly as you like and not feel a thing. The only way you could detect 12VDC is to put the leads onto your tongue or an open wound. I don’t recommend it. This lamp is probably AC, which is easier to detect, but is still no worry. I was just working on a project and was unable to feel 60VAC with dry fingers. I was not really trying to, but simply noticed no “bite” like you can expect at 120VAC.
     
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  11. abrogard

    Thread Starter Member

    Dec 20, 2018
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    I'll take your word for it. I've never had a shock from one, that's true. But I've been knocked over backwards by surprise and (psychological) shock when a lead from a battery terminal has arced to the chassis.

    I guess a dead short there. In those circumstances we can weld with a 12V car battery, can't we? I understand it all. Fair enough. But I'm still not game to trust my natural resistance to save me. I won't try it.

    Back to the lamp. So if it is as I thought it was then that makes troubleshooting it very easy. No volts on the arms and it's a busted transformer. The thing is just rubbish unless I find another transformer to put in it. And I can read no volts when I test.

    Bring me back to my question: so accidentally shorting across the arms can/will blow the transformer?

    and p.s. if that's all that drives these little halogen globes a bloke could be making his own desklamps all over the place. Plenty of 'wallwart' 12v supplies around.

    Or would LEDs be the way to go nowadays perhaps..
     
    Last edited: Feb 4, 2019
  12. wayneh

    Expert

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    Yeah, arcing is a whole different thing. Vaporized metal!

    If we assume the arms are the conductors, then you should see an AC voltage across them, as long as they’re not painted. There’s a chance it could be DC but I think that’s unlikely. Measuring the transformer directly would be better than trying to measure the arms.

    A prolonged short will usually burn up a transformer but a brief short shouldn’t.

    And yeah, I have a halogen bulb in my glassware cabinet powered by a 12V wallwart.
     
  13. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    If you want to build lamps then LED light is the way to go because of much less heat and MUCH longer life, if you stay within the device specified voltage and current limits. The hard part is that the LED elements must have DC power, meaning some sort of additional support electronics.
     
  14. abrogard

    Thread Starter Member

    Dec 20, 2018
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    I just googled a bit and I got that impression: LEDs are better than Halogen. And don't get any of the hassles about handling too hot globes, or destroying globes hy getting your grubby oily fingers on them.

    Well I'm thinking of all those 'wallwarts' I've collected over the years. Got a big box of them. Plenty of 12V 1 or 2A amongst them I'm guessing. And I just saw some on ebay for $4.

    I've got a couple of rolls of 12V LEDS, too. I can cut them into any lengths I like. I lit my shed with a couple of 5metre lengths of it.

    Might amuse my little self for a while building lamps or something with them. Try to 'repair' this thing by putting leds in it. :)
     
  15. Yaakov

    Well-Known Member

    Jan 27, 2019
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    An LED conversion sounds fun, but... you don't have "rolls of 12V LED" you have "12V rolls of LEDs", a critical distinction. The individual LEDs in those strips use much lower voltages. They are cunningly wired into series and parallel circuits to require 12V to operate them. There will be a specific lower limit to the number of LEDs you must use from the strip, and particular points at which they must be cut.

    Be careful or you will have a lot of light for a very short time followed by heat-induced LED smoke.
     
    Last edited: Feb 5, 2019
  16. Tonyr1084

    Distinguished Member

    Sep 24, 2015
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    Word of caution on wallwarts: The good ones are safe. But there are some cheap units that develop a differential voltage - for instance (and I'm not an expert on this) the difference between 120 VAC and 100 VAC is 12 volts. Then rectified to DC. There's a potential for contacting either of those lines and ground. Should that happen you can expose yourself to some dangerous voltages. And currents.

    I've been hit numerous times by 120 VAC (you think I'd learn). I've even taken it from palm to palm. And no - I don't have, nor am I a ghost writer. In every case where I've been hit I've been able to get away from the source simply by muscle contraction. NO - WOULDN'T RECOMMEND IT, but you can survive 120 VAC for the briefest instant. It's deadly when you have a grip on a power tool that is not properly grounded and defective in one hand and the other hand happens to grab hold of a grounded pipe. You can clench your grip on both with an irreleaseable grip, which WILL end your life.

    To be clear - electricity CAN be dangerous. Depending on the circumstances - you can experience discomfort or even a life ending event. But many of us have been stung and have lived to tell about it. Those who are not talking about their experiences getting stung are the ones who can't be here. Don't become one of them. Wallwarts CAN be dangerous too. Like I said - it all depends on the circumstances.

    The nice thing about a transformer is that it isolates you from main power.
     
  17. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    Indeed, Tony is correct. Thus it will be smart to check each wall wart for line leakage before using it. And also, most of the simpler ones have no regulation at all, so that the output voltage depends on both the output current drawn and the mains supply voltage. So you would probably need a series resistance to limit the LED current.
     
  18. Tonyr1084

    Distinguished Member

    Sep 24, 2015
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    A small 120 VAC to 12 VAC transformer with a half wave rectifier and one of those 12 volt LED modules you get as replacements for the halogen bulbs and you're good to go. I say half wave rectification because full wave can deliver more current. With LED you don't need much current. If you are able to notice flicker due to the half wave rectification then go full wave. Should eliminate the flicker.
     
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