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Simple (I hope) safety question

Discussion in 'Electronic Basics' started by Robbs, Jul 26, 2003.

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  1. Robbs

    Robbs Guest

    I'm using a 9VDC/300mA block with the leads clipped off, hooked up to a
    breadboard for *basic* experimenting. It just occured to me that that might
    be hazardous, if by accident I grasped a lead in either hand. I think I
    remember reading that 300mA is above the 'let-go' threshold.

    I don't plan on doing any experimenting until I get this straight.


  2. Dan Fraser

    Dan Fraser Guest

    You will not pull 300mA through your body with only 9V or pressure
    (voltage) behind it. A 9V supply is quite save to experiment with. 30V
    and up is considered getting to dangerous levels.

    Dan Fraser

    From Costa Mesa in sunny California
    949-631-7535 Cell 714-420-7535

    Check out my electronic schematics site at:
    If you are into cars check out
  3. Gareth

    Gareth Guest

    The short answer is that this is safe, you don't worry about touching a
    9V battery do you?

    Long Answer:

    Have you come across Ohm's law yet? This is the relationship between
    the current (I) flowing though a resistor (R) and the voltage (V) across

    V = I*R

    or, rearrange for current you get:

    I= V/R

    Your power supply is rated at 300mA, that means it can supply 300mA
    without overheating, or the output voltage will be close to 9V when
    300mA is draw, but will drop below 9V if more current is draw. It does
    NOT mean that 300mA will be supplied into any load all the time since
    Ohm's law applies.

    For example, when you have nothing connected to the power supply, the
    resistance is (almost) infinite so no current at all flows. If you
    connected a 1K ohm resistor a current of 9/1000 = 9mA would flow.

    DO NOT TRY THIS you could damage your power supply, but if you connected
    a 1 Ohm resistor the current that should flow is 9/1 = 9A. However your
    power supply cannot supply this much current, so what may happen,
    (depending on the design of your power supply) is that the current would
    be limited to the maximum of 300mA so the output voltage would drop to V
    = 1*0.3 = 0.3 V, the other 8.7 v being dropped across the internal
    resistance of the power supply. This will mean that a lot of power is
    dissipated inside the power supply as heat, this is why it could be damaged.

    The resistance of a person will vary according to a number of factors but
    it is high enough that holding the wires of a 9V power supply is safe.

    The only thing I can think of that would give you a shock from 9V would
    be to put the wires in your mouth or on your tongue, that would
    certainly hurt, but I think you are very unlikely to do this by accident.

  4. John Larkin

    John Larkin Guest

    Your skin resistance will limit the current. Just grab the leads and
    try it! 9 volt batteries are harmless.

    48 volts AC or DC is considered the limit of safe low-voltage levels,
    according to UL and most building codes. New cars are planned to have
    42-volt DC systems some day in the near future.

  5. Ken

    Ken Guest

    Yes, 36V battery (3 x 12V).
  6. Don Bruder

    Don Bruder Guest

    Go ahead and do your thing. Ohm's law is on your side. Your skin
    resistance is so high (comparativly speaking) that short of attaching
    the wires to needles and jabbing them into yourself, you're not going to
    be able to get in trouble with a 9 volt power supply without making a
    pretty serious effort at doing so.

    All bets are, of course, off if you start messing with voltage
    doublers/triplers/etc, big capacitors and/or inductors, or similar "jack
    up the juice" circuitry... You can get yourself into big trouble in an
    even bigger hurry with circuits like that, even when they're powered by
    "just" a single AA or "coin"-type battery, let alone a mains-powered
    transformer/rectifier set.
  7. Rich Webb

    Rich Webb Guest

    Please note (for other readers -- I suspect that Don already knows this)
    that the key words above are SKIN RESISTANCE. Doing stupid battery
    tricks -- even with a 9V battery -- on cuts or broken skin can be
    a Bad Thing.
  8. Charles Jean

    Charles Jean Guest

    Knew a guy that used to check 9v batteries by sticking the electrodes
    on his tongue. Said he could "tell by the tickle" what the state of
    charge was. BTW, the old "tongue twister" is still with us.

    If God hadn't intended us to eat animals,
    He wouldn't have made them out of MEAT! - John Cleese
  9. Fred Abse

    Fred Abse Guest

    Goes back further than that:

    Flanders & Swann's "The Reluctant Cannibal"

    "If the Ju-Ju hadn't meant us to eat people, he wouldn't have made us of

    Stage show in the early 1950s, later an album.
  10. N. Thornton

    N. Thornton Guest

    No. There are always exceptions tho. If your hand gets cut up you can
    bypass skin resistance entirely. If you play with inductors you could
    get 100s of volts without even realising it. If you stick pins in your
    skin and conect up... :( But other than that I cant see a problem.

    Where people get into the water without realising it is usually
    a) when switching relays, which produce large voltage pulses when the
    coil is switched off
    b) testing transfomers with a multimeter, which again can produce some
    lethal voltages as the prods aer removed. Thats something often

    Bear in mind I havent sat here and figured out every possible way for
    you to die, so I haven't told you all of them. It is upto you to find
    out all the dangers, not me.

    I still do. I figure the tongue has a full scale deflection of around
    12v, beyond 9v it gets uncomfortable. Why get out a meter when you can
    just touch it? Far quicker.

    Regards, NT
  11. Terry

    Terry Guest

    Nothing new, to that I'm afraid. Back in the 1940/50s we used to
    check our two cell bicycle lamp batteries with two metal contacts
    on top, (3 volts) that way. If battery was OK then it was the
    bulb or the lamp switch.
    Not so much now but 20-30 years ago kid would hand you 'a
    transistor' i.e. a radio, saying "It doesn't work".
    First thing you'd do is pop out the battery and touch its
    terminals on your tongue. Oft times battery was dead and you
    handed it back saying "Get a new bttery and it'll probably work
    Cheers. Terry.
  12. Terry

    Terry Guest

    Your skin resistance, dry, is probably well above 100,000 ohms.
    Your voltage is 9 volts.
    Nine divided by the resistance = current. (That's Ohm's Law).
    So; 9/100,000 = approximately one tenth of one thousandth of one
    amp! Or 0.1 milliamp.
    Thai's hardly enough current to make even a mouse jump. And 9
    volts is way too low a voltage to burn or break through normal
    intact skin!
    Now if you were dealing with 9,000 volts; such as you can get
    inside a microwave oven high voltage circuit, or a TV or radio
    transmitter; well that's dangerous stuff. It can go right through
    skin! That's why power line crews wear those rubber gloves tested
    to at least 10,000 volts.
    Also if you were standing in water or on wet ground in bare feet
    and you got hold of even 90 volts .....and since most local house
    wiring is 115-120 volts; watch out, you may be dead!
  13. Wim Lewis

    Wim Lewis Guest

    I expect a normal 9v battery will supply more than 300mA if given a
    chance (just not for a long time). So the power block you're using should
    be no more dangerous than that. For the most part, 9v isn't enough to
    cause anything injurious to happen, because your body has too much
    resistance for very much current to flow.

    OTOH, you could hurt yourself if you tried. Get a low-resistance
    path into your body --- through wet skin, or a sharp wire poked through
    your skin --- and more current would flow. Do this near your heart and
    you could be in trouble. But really, I consider 9v to be safe for general
    experimenting and fiddling-around.
  14. anything below 55v is considered safe by the 16th edition british standards.
    you'll be fine...just don't put your tongue onnit won't cause you damage just
    bloody sting!!!

  15. Fred Abse

    Fred Abse Guest

    True, but then look at how they define "low voltage"
  16. hi,
    true that was off the top of my head, here's wot they say in regs, anything
    upto 50v ac or 120v ripple free dc between conductors or earth is 'extra low
    voltage'. this is also used in conjunction with selv (seperated extra low
    voltage) which is basically using an isolating transformer and no earth. more
    details can be gained from chapter 41 of the 16th edition/ bs 7671.

  17. Fred Abse

    Fred Abse Guest

    Ah, yes, but now tell us what "low voltage" is. I suspect it will
    surprise not a few.

    I had to cross swords with British regulations twenty-odd years ago, when
    it was the 14th edition. From memory, "low voltage" went up to 500V, but
    I may have remembered wrong. It was a lot higher than I'd expected,
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