11.5V AC Dangerous?

Discussion in 'General Electronics Chat' started by NZMikeV, Jan 3, 2011.

  1. NZMikeV

    Thread Starter New Member

    Jun 18, 2010
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    After reading through a few threads on dangerous voltages I am unable to come to any definite conclusion on this matter: I have a (plug-pack) transformer that outputs 2 x 11.5V AC 1.25A. The input is 230-240V AC @50Hz. The output from the transformer will be used for a +/-12V DC regulator project. The question I have is, how "safe" is AC at 11.5V RMS? (presumably 50Hz?).
    I can touch the terminals of my 12V SLAs and not even feel a tingle - but what if I accidentally came into contact with the output from the transformer? Could it be lethal or would I not even feel it? I realise that ANY combination of voltage/current has the potential to be fatal, this has made it all the more harder to be sure whether 11.5V RMS is "safe" to work with or not.
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    First of all, dont work on a live circuit.

    Second always use a transformer. IF you dont need a step up or step down, use an isolation transformer. It is simple 1 to 1.

    Voltage doesn't kill. Current kills.

    You will feel a shock or tingle at 12ish volts.
    A 9v battery to the tongue "tastes" weird to your electrochemical sensors..also known as tastebuds.

    A 12v car battery pumping 600A into you is dangerous,
    but a 12v camera shutter battery at 20ma wont do much of anything. It may BURN you if it shorted out, but electrocution hazard is near non-existent

    No voltage is "safe" to work with, per se.

    You should have 0v when working on the circuit.

    When it is completed, insulate! Use shrink tubing and proper connectors so wires and terminals are not with in reach.

    With lower voltages, you are at more of a risk of burns and fire than electrocution.

    You can, however burn a house down with a coin cell 3v battery and a paperclip.

    So, if you are not going to be safe, everything is dangerous.

    Better give up eating, if you are not going to chew. ;)
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    If the OP is talking about a true wall wart transformer it is as isolated as it gets, and is therefore safe. I would personally rate anything under 30V with low current safe, but I have been zinged with 12VDC under the right circumstances.
     
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    many years ago, and I'm surprised it hasn't happened on more than one occassion, I had connected an electrolytic cap up in reverse polarity. For what ever reasons I stood up to leave the room when kaboom. After jumping a couple of feet from the large firecracker, I marvelled at how the room was full of finely shredded paper. I believe the circuit was only 12vdc, and I shudder to think what could have happened if I was still over that circuit when she blew.
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,772
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    Any voltage under 21 volts is unable to maintain an 'arc'. So I use that as a guide when working on voltage enabled circuits.

    12 volt and 15 volt circuits are generally considered 'safe' to work on. ANY circuit(even single AA battery) that has a switching/step up/boost function should be considered a high voltage circuit.

    If you don't know what it does then you shouldn't be poking around in it.

    Simple rules for a simple man.
     
  6. NZMikeV

    Thread Starter New Member

    Jun 18, 2010
    22
    0
    Crikey! - That's a good one. Fortunately, I've not acheived anything that spectacular yet, but I've had smoke leak out of the occasional component. :D
     
  7. #12

    Expert

    Nov 30, 2010
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    Safety, safety, safety! There is no such thing as an old, stupid electrician.

    Practice safety constantly. I would have been dead several times if I didn't habitually keep my safeties in place. Circuit off, measure it with a meter, short it with a screwdriver just to be sure, dry plastic handles, rubber mats, one hand in your back pocket, don't lean against the sheet metal, all that stuff. Twelve volts or twenty only stings a little, but make safety a habit. It is very likely that you will graduate to repairing machines that work with power voltages. Start learning safety habits now.
     
  8. timrobbins

    Active Member

    Aug 29, 2009
    318
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    From a standards compliance perspective you have to get above about 32VAC to move in to 'non-safe' levels when just assessing the voltage level itself.

    With respect to safety risks, these are many and varied and dependant on the scenario you are in. It is always good advice to do a risk assessment with any kind of work. Examples above illustrate the varied risks from heat/ignition/flames, rapid energy discharge, reflex reaction from a tingle (eg. causing you to recoil and hit something or touch something else that is not guarded).

    Ciao, Tim
     
  9. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    There is no defined "safe" level of electrical exposure. Thus, one has to ask others, read, or learn through experience. Here's something to read -- note the comment about wet contact at the top left of page 12. Most of us know from experience that we can touch the terminals of a 12 VDC battery with dry fingers and not get a shock. The same is true for 12 VAC. I have occasionally gotten a tingle from 24 VAC when working on my sprinkler system wiring. I don't label those voltages as "safe", but I know that my dry skin resistance is usually too high for them to give me a sensible shock.

    I still remember the lesson my father learned when he was a kid (he told it to me when I was working on a car with a ring on my finger). He was working on a car and was wearing a ring. He got a bad burn on his ring finger when the ring shorted a battery and got very hot. So electrocution isn't the only hazard with low voltage electrical circuits.

    I like #12's recommendations -- make safety a habit. The right way to work on electrical circuits is to turn the power off, then verify that it's off. Probably the safest way to do this is to use a known, working digital multimeter. Test the multimeter's AC and DC measurements on known, working DC and AC sources, then test the circuit for both DC and AC, then test the multimeter again on those known, working sources to verify the multimeter didn't fail. Now I certainly don't do that when working on my home wiring, but I do check the multimeter first on a known working 120 VAC outlet, then the circuit that I'll be working on (i.e., I never check for DC voltages, but I would if it was a black box circuit). Since I have also found that occasionally we don't make good electrical contact with the DMM's probes (even though it sure looked like they did), I use my little non-contact AC voltage detector (a Gardner-Bender for $10 from Wal-Mart) to also check the wire. Last year this saved me from a shock -- the DMM indicated no voltage and I thought I had turned off the proper circuit breaker, but I checked anyway and my non-contact sensor said the line was powered. It was right and I confirmed it with the DMM when I checked it again. Those types of non-accidents don't make the record books, but I sure hope others take the lesson to heart.
     
  10. edgetrigger

    Member

    Dec 19, 2010
    133
    19
    with an inductor any voltage is a hazard, better use testers, multimeters and probes while dealing with any live circuit.
     
  11. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    Non-contact AC voltmeter:

    About 2 months ago, I was asked to remove the stub of a broken light bulb over a kitchen stove, little candela base. I sniffed it with the non-contact voltmeter. No AC. The shell of the socket was shielding the sniffer. I grabbed the guts with the long nose pliers and popped a fuse, but I was working one handed, wearing dry shoes, on a dry tile floor, and being careful not to lean against any sheet metal.

    Score: 1 for habitual safety
    0 for voltage sniffer
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    I forget the NEC code but in general anything at or under 24 VAC from a low current supply (isolated of course) is considered to be safe provided you don't do anything stupid like having wet hands or sticking the wires in your mouth. They of course expect you to use some common knowledge on this such as not mixing control wiring into a raceway that also carries regular line voltage wiring unless you follow the codes exactly.

    These codes were written for a reason and even if it's the previous edition you can often find copies for bargain prices off of eBay or better yet, most any large electrician's firm will gladly pass on their old copies for free now that the 2011 is out. Your local library is also a good source but it's a book to buy and hold onto. The standards we write over here are usually adapted by all countries so picking up a slightly outdated copy of the NEC 70 is highly recommended reading if you can find one at a good price or, as I mentioned, for free.

    It's pretty perfect as it stands thus the codes rarely change between the editions which come out every 3 years.
     
  13. tyblu

    Member

    Nov 29, 2010
    199
    16
    i've been zapped by 12V a few times working on automotive wiring... likely wet hands, laying in a puddle, clutching the frame (battery -ve) while twisting a live wire (batt +ve). Still alive! ;)
     
  14. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
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    Yea, wet hands can get you that from 12V but I'm an old nut that used to (and occasionally still does) test 9V batteries by sticking them on my tongue and I'd be willing to bet I'm not the only one that still does this on an occasional basis.
     
  15. retched

    AAC Fanatic!

    Dec 5, 2009
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    No.. You are not the only one. :rolleyes:
     
  16. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Years of practice and you can almost always identify the good from the weaker ones, can't you?

    Wouldn't suggest it for anyone but long before we had meters sitting easily at hand ....
     
  17. retched

    AAC Fanatic!

    Dec 5, 2009
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    You can DEFINITELY tell. There are a few "ways".

    The "strength" of the taste, and the "flavor" of the taste.

    It sounds weird, but it is almost like, different voltages produce different flavors. And the "stronger" the battery, the "more pronounced the flavor"

    We probably shouldn't be talking about this.

    It was safer years ago, then it is now.

    Battery production was not so "toxic" in the days of only alkaline.

    But now, there is likely lithium, cadmium, mercury, and hydride compounds just "on" the battery. Im not 100% sure, but Im pretty confident that they do not "wash" batteries before packaging.

    This means the production line has been "dusting" the batteries with the chemicals used for all the battery types produced on them.

    In other words, keep them out of your mouth.

    Let the pre-fessionals handle it ;)
     
  18. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Shoot, I used to strip wires with my teeth and often held lead based solder in my mouth so I had two free hands.

    Just tonight I I bought something I'm going to experiment with. I figured for $5 at the local Walgreens I couldn't turn it down.
    https://www.barkoff.tv/flare/next?tag=he|af&a_aid=4be47e4c6d099&a_bid=0e6a95a4&data1=899

    To find a 9V battery? I had a few, just took the one that had the best "taste" as you put it then measured it with a meter - 9.4V. There's probably a joke of a circuit inside, I would assume it is somewhat selective as to the frequency the internal mike receives then activates the large piezo it has on the front. My old Cocker Spaniel is about 80% deaf now anyhow, mostly selectively so, thus it will be interesting to see if she picks up on it. I live in an almost dead quite neighborhood where you can leave your doors unlocked all day if you want which is pretty rare these days.

    She won't bark at the mailman/woman but has an absolute fit if the UPS guy drives anywhere near or a casual jogger goes by. This was great when she was younger but now if left alone downstairs she tends to bark at anything nonstop on occasion. I go down there and can't for the life of me figure out why she's barking as she gave up on the squirrels and birds a long time ago. I used to be able to get her to chase the squirrels back up into the trees but over the past couple of years the critters will hit one tree and she'll go looking at another one. At least she still tries and its good exercise for her. Her nose still seems to be second to none as she can track anybody or anything foreign to our large front and back yards. If you've been out in your car she'll smell the heck out of it when you return so that just supports the fact that as you lose some senses your other ones sharpen up.

    We have one of those "electronic fences" buried around the perimeters of the yard but she learned her boundaries pretty quickly so it's rare we ever even even put the collar on her anymore. It first warns then if she gets closer shocks her on the neck area. I once made the mistake of carrying that thing with me to test the perimeter, never heard the warning noise but when I hit it my arm hurt for an hour.

    I'm more of a German Shepherd person along with having a few cats around but considering I'm taking advantage of living in my (here in a few days) 81 year old mom's house there are certain rules I have to go by and I can't argue with that. Free cable, phone, internet (which of course I set up) and tons of workspace become an easy exchange for chipping in on some of the expenses. Would love to have a dog of my own as well as at least one good cat but I can wait for that.

    Worst part? Try and teach an 80 year old how to use a laptop. At least I've got the only password into the administrator account on Win7 so she can't do many stupid things however it's been a rather challenging teaching project from day one.
     
    Last edited: Jan 5, 2011
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