Simple (I hope) safety question

[Robbs]

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.

Thanks,

Robbs

 

[Dan Fraser]

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:
http://www.schematicsforfree.com
If you are into cars check out www.roadsters.com

 

[Gareth]

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.

Thanks,

Robbs

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
it.

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.

Gareth.

 

[John Larkin]

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.

Thanks,

Robbs

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.

John

 

[Ken]

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.

Yes, 36V battery (3 x 12V).

 

[Don Bruder]

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.

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.

 

[Rich Webb]

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.

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.

 

[Charles Jean]

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.

Thanks,

Robbs

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

 

[Fred Abse]

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

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
meat"

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

 

[N. Thornton]

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
overlooked.

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.

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.

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

 

[Terry]

Knew a guy that used to check 9v batteries by sticking the electrodes
on his tongue.

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
OK".
Cheers. Terry.

 

[Terry]

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.

Thanks,

Robbs

Robb.
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!

 

[Wim Lewis]

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 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.

 

[Electromechy1]

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!!!
regards

shaun

 

[Fred Abse]

anything below 55v is considered safe by the 16th edition british
standards.

True, but then look at how they define "low voltage"

 

[Electromechy1]

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.
regards

shaun

 

[Fred Abse]

anything upto 50v ac or 120v ripple free dc between conductors or earth
is 'extra low voltage'

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,
anyway.