Transformers are strange and mysterious. Maybe somebody can drop some science on me.

Knowing ohm's law, I need to get my head together on this whole transformer thing. I know how transformers work, so you don't have to deal with that. I'm concerned with transformers as related to ohm's law. I have various questions, but where to start? Let me just start with one set of questions first.

If I have a transformer that's 120v in and 12v out, then I put a 3 amp load on the 12 volt side, it seems that I would have 36 watts and 4 ohms of resistance.

But wait a second - 120 volts is coming in to power things. Do I calculate my amps (3) based on 120 volts? If so, that would mean that I'm working with 360 watts, and 40 ohms of resistance. Is my amp load different on the 120 volt side, or does something in my resistance and my amperage change somehow between the 120v and the 12v sides? Does the 120v side "know" or "feel" the amperage load of the 12v side?

How do I deal with the discrepancy between the amperage on the output side of the transformer at 12 volts, and the input voltage, which changes all the numbers?

I understand that the resistance doesn't change, so this messes with my mind.

This is my first set of questions.

Upcoming: questions related to variac-controlled isolation transformer calculations regarding ohm's law.

Thanks for your help!

Watts in equals watts out. The amperage changes to make that work. If you get 36 watts out, you had to put 36 watts in. 36 watts on the 12V side is 3 amps; 36 watts on the 120V side is 0.3 amps.

Keep primary and secondary calcs separate and it will make sense. Power will remain the same. W.

watts in = watts out X power factor. that is listed on the transformer. usually around 98 to 99 percent.

So...the watts in equals the watts out (thanks ghost & bobH)...the resistance of the wire is NOT the same, but increases with an increase in "heat" (thanks to gfretwell).

Wait a second...I thought the resistance remains the same, but now I'm hearing that the resistance changes with the heat? Does that mean that the resistance increases with an increase in the voltage, or the heat itself?

spark master, your question will not get answered any faster because you post several new questions.

the resistance of a transformer before you apply power will be near zero.

when the transformer is powered the magnignetic fields cause impeadance. due to the magnetic flux created by the windings.

impedance works the same as resistance when doing calculations.

try asking another specific question. we may be able to help, or you may need to study this in a school.

Okay, White, I feel you. I feel your pain. I'm white too.

Here's a specific question. If I'm cutting foam on my hot wire cutter using 5 volts AC (and 3 amps) that's coming out of my 12 volt transformer which is controlled by a 120 volt variac set at 50 volts AC output, what exactly makes that different or safer than controlling my voltage with a variac alone set at 5 volts?

I guess the physical separation of the primary and the secondary doesn't mean there won't be some big amp load if somebody inadvertently contacts the cutting wire.

They say to use a transformer after the variac because it's safer than going off the variac's single wire toroid style transformer for some reason. I believe I can change the size of the fuse in the variac to a smaller amperage rating, so I wonder what it is that makes it safer to run the 2nd transformer after the variac.

And White, read this: "...magnignetic fields cause impeadance..." Where did you learn that? Be nice.

Thanks.

fraid i have to agree with jwhite, sounds like you don't know how a transformer works. Mebbe some reading on that first at the local library would help.

Resistance varies with temperature, yes. Every material has a thermal co-efficient, changes it's dimensions as it heats, also changes the excitation energy available to the atoms hence changing the resistance, usually in a relatively linear way.

Magnetic field cause impedance, particularly if there is a variation in the field (changing current flow or core variation)based on lenz's laws of magnetics. This is derived from the inductance just as a resistive element has impedance derived from resistance.

Capacitors exhibit impedance properties also while i'm at it, charge on the plate will eventually reach saturation in a dc situation causing an open circuit and rising impedance based on capacitance.

Having said all that, i did work on a site wher half a dozen engineers took three days to work out why one of their number flew across a subyard after removing a 9VDC powered loop test unit from the secondary of an 3.3kV to 415 trannie.

Spock

I know how transformers work, one coil around an iron core creates a magnetic field in the iron core and the other coil is electrically energized by the magnetized iron core, and the ratio of the number of coils determines if the voltage is stepping up or down.

I've got some devices with super small trannies,like smaller than an inch wide, small wire, and I wonder what their ratings might be.

Thanks.

spark master, if you know so much about transformers, then why are you asking questions?

I have tried to answer you... do you want help or not?

Spark,
The Load impedance of a transformer's secondary is reflected back to the primary by the square of the turns ratio. Let's take an example of a 12V, 10A transformer. It should have a 10:1 turns ratio if the primary is 120V. To get a 10A current flow in the secondary, the load resistamce/reactance has to be 1.2 ohms. 1.2 ohms load impedance X 10^2 = 120 ohms reflected back to the primary. 120 V/120 ohms = 1A. The numbers work out and all is good in the neighborhood.

Experience tells us that if you have your TX plugged in, unloaded, and only get 12 volts out, it probably is not considered a 12 volt TX. The voltage out should be what it puts out at rated load.
Joe

[This message has been edited by JoeTestingEngr (edited 12-21-2005).]

lets not forget that spark master already knows about transfromers,

and that he assumes ones name has bearing on thier heritage.

Guys, let's please leave personal barbs out.

Bill

VA in = VA out. But please note that there are ironlosses ( eddycurrents and hysteresis losses in the iron core )and copperlosses from the windings to be added to the primary power absorbed from the mains. the copperlosses increase with the load I²R, the ironlosses remain almost constant

Transformers are normally rated in VA's. At UPF power factor you can call it Watts but generally its lagging at around 0.95 PF hence VA's are used.

Winding ratios are:
Up*Ns = Us*Np where:
U is voltage, N is winding turns, p is primary winding, s is secondary winding.

Current ratio's Ip*Up = Is*Us. In your case
Ip*120 = 3*12 » Ip = 36/120 = 0.3 Amps.

Personally I don't like to rely on the resistance values measured across the primary or secondary windings because of the different gauges of wires used and the transformer is an AC device. It's Ok to identify a primary or secondary winding.

By unknown transformers, to test I usually put a 200 Watts lightbulb in series during testing which will absorb the fault current if it was accidently the wrong primary voltage. In NZ we have 230 V. and there are a lot of trannys out here with 2 * 115 volts primary windings.

I hope this explanation may help out a little.

The transformer makes this infinitely safer, not because of current or voltage limiting, but because it provides electrical isolation.

A Variac (a variable autotransformer) has only a single winding, and provides no DC isolation between input and output. Shock hazard.

Hey Joe,

Thanks for your response. The TX in question might have been 13.8 volts or something unloaded, I was just rounding it off to 12 for illustration purposes.

RODALCO, thanks too. It just seems like if the NEC tells us that, for example, 12 gauge wire is for 20 amp circuits, something should tell us the amp rating of an unmarked transformer, going by the wire thickness. How will a transformer fail anyway? I don't know, but would assume that it would be the wire burning in half and/or shorting out due to over-amping. JWhite? Any opinions or advice from the master?

SMF

Hey Larry,

Define "electrical isolation" if I can still get grounded and severely amp-loaded. If I'm getting electrocuted, how is electrical isolation helping me? How exactly is my entire electrocution experience getting all warm and fuzzy and comfortable with this 2nd tranny?

SMF

SMF,

Again, let's please leave the personal barbs out.

My apologies to all for anything I said that might have offended. Thanks to webmaster for running a tight ship to prevent things like that.

I love electrical work, it's all so fascinating. The more I learn, the more I'm blown away by things electrical. Nobody knows it all, right? Some people are experts in some areas, some are master electricians who may have only dabbled in some aspect of electrical once in their career, while by chance leaning toward certain aspects of electrical work, becoming highly experienced in those areas.

People who are highly educated in one particular area might pass through here and be able to really change my life, turning me in the right direction quickly so I can finish this project sooner and do it first class.

I've got almost 4 years of experience in commercial electrical, and I work on other projects on the side, like building a hot wire cutter or a gyroscope, building world-record-breaking machinery, signs, building networks, fixing all kinds of electrical and electronic devices, and right now I'm thirsting for certain answers to my questions about transformers.

I tend to look at the resistance of a nichrome wire, for example, at 1.5 ohms; then figure how many amps it would draw at 12 volts, which would seem to be 8 amps. Now I'm told that the resistance changes, so I don't know what to think exactly.

I have a device I bought at a garage sale called a Fido-Shock, it's a device that's wired up to an electric fence to keep an animal in the yard. The Fido-shock has a transformer that steps 120 volts up to 700 volts or something like that, but I guess that increase in voltage reduces the amperage. Somehow it's supposed to be safe because of the reduction in amperage. Can somebody tell me how a step-up transformer, going to 700 volts, can be safe enough to use on Fido because of reduced amperage, while a step-down transformer, dropping 120 to 12 volts, can be safer because of reduced voltage (while the amps can go super high)? What's more dangerous? What would cause more damage to somebody who became part of the ground path? Would it simply be a matter of VA, regardless of the voltage?

SMF

Sparkmaster,

Please do not take this as a personal barb: You _don't_ know how transformers work.

You know the basic concept, but you don't know the details. As an analogy, you know that a car works because gasoline gets used inside it, but you don't know the details of how an engine works, and you are asking why cars go at particular speeds.

But that is okay! This is a place for learning. Just please be open to changing your basic understanding of how transformers work, as you better understand the details. Also please understand that most if not all science and science _teaching_ is a continuous train of more refined ideas, with each new idea expanding and _correcting_ the previous concept. Very often we use one of the earlier understandings of physics because it is accurate enough for the application, even though we _know_ that there is a better approximation that we could use.

Newton described a few laws of motion, eg. F=M*A, and the laws of Newtonian mechanics are still taught in school. Newtonian mechanics is still used for designing machines and building buildings and pretty much all of our lives. But we _know_ that Newtonian mechanics is wrong. Einstein's 'Relativity' theories provide more accurate laws of motion, a better picture of the universe. And Einsteinian mechanics is also taught in college physics. But for most of every day life, Newtonian mechanics gives answers that are almost equal to Einsteinian mechanics, with the 'error' being smaller than the measurement errors in the tools that we use. In other words, for most of everyday life, you can't tell the two apart.

I bring up that little example because when people describe complex beasties like transformers, they _knowingly_ use mathematical descriptions that have known errors, and only bother with more complex mathematical descriptions when the magnitude of the errors actually impacts the operation of the devices that they are building. Usually most of the details of a transformer are simply ignored because those details are down in the noise of the production process.

Okay. Someone said 'back to the books'. Here is a lovely little 'book' free on the web: http://www.ibiblio.org/obp/electricCircuits/AC/index.html

This text will take you through the concepts of AC reactance, mutual inductance, transformer operation, 'reflected impedance', and other concepts. For a deeper understanding, you should also look at the separate section on DC circuits, to get the concepts of galvanic isolation, etc.

In a nutshell:

1) Whenever an electric current moves through a wire, you will get a magnetic field.

2) Whenever you have a wire in a _changing_ magnetic field, you will get a voltage induced in the wire.

3) The wire interacts with its _own_ magnetic field. In other words, when you apply a voltage to a wire, current will start to flow, creating a magnetic field. But this means that the magnetic field must be _changing_ since we changed from no magnetic field to having one.

4) The magnetic field created by a wire will always act _against_ any change in the current flowing in the wire. It won't be able to _prevent_ the current flow, but it presents a drag to the change in current flow. Think 'mechanical inertia'; when you push on an object, it pushes back even as it starts to move.

5) If you coil a wire up and add an iron core, you amplify the magnetic field, and thus amplify the voltage produced. You increase the electromagnetic inertia of the system. By coiling up a wire, you have produced an inductor or 'choke'.

6) When you apply AC voltage to an inductor, an interesting thing happens. The applied voltage is constantly changing. The current flowing is constantly changing. The magnetic field is constantly changing. You will find that the current that actually does flow is that amount which will produce a magnetic field which will balance the applied voltage. Increase the voltage, and more current flows, so that you get more magnetic flux, and thus more induced 'coil produced counter' voltage.

7) In a transformer, you have _two_ coils interacting with the same magnetic field. The changing magnetic field in the core induces voltage is _both_ coils. With no load connected to the secondary coil, no current flows in the secondary coil. The changing magnetic field induces voltage in the primary coil, which acts to balance out the applied voltage. A bit of current flows in the primary coil, just enough to produce the magnetic field.

8) When you connect a load to the secondary coil, current flows in the secondary coil. This current also interacts with the magnetic field...by _weakening_ it. But now we have a weaker magnetic field, so the counter voltage induced in the primary coil is reduced. Which means that more current will flow in the primary, bringing up the magnetic field strength, so that the voltage induced in the primary coil balances the voltage applied to the primary coil. What this means is that you don't simply have the primary coil 'producing' a magnetic field and casting it out on the waves; instead the primary and the secondary are mutually interacting with the magnetic field. When you draw current from the secondary coil, you are changing how the magnetic field responds to the current flow in the primary, and causing more current to flow in the primary. The primary circuit sees what is happening on the secondary circuit, by the _mutual_ induction of the transformer.

9) You can think of each turn of each coil in the transformer as separately interacting with the magnetic field, and that any turns of wire electrically connected together will have their interactions added up. If the primary and secondary coils have different numbers of turns, then they will have different voltages and currents needed to be balanced with the magnetic field, but they will _both_ be balanced with the magnetic field.

10) Because the primary coil 'sees' the load on the secondary coil, you can come up with equations for the how loads on the secondary side 'appear' on the primary side. You will see the term 'reflected impedance' used to describe this. In your hot wire cutter example, you might have 6A flowing in the secondary, with 12 turns in the secondary, for a total of 72 ampere-turns. The primary will have 120 turns, and must supply the 72 ampere-turns, so you would have 0.6A flowing in the primary. 12V, 6A, Ohm's law: 2 ohms. On the primary side: 120V, 0.6A, and thus 200 ohms. The impedance 'seen' on the primary side of the circuit is (turns ratio)^2 times the impedance connected to the secondary side.

I hope that this gets you started. I strongly suggest that you read through the texts mentioned above, because this will provide you with essential background that we would simply be repeating here...see how large the above 'nutshell' has become. Feel free to ask questions about the points that you don't understand, but please do the reading. Transformers are complex beasties, and can't be dumbed down if you still want answers to your questions

-Jon

It's hard to top a response like winnie's.

Just one other comment on the size of wire to it's current carrying capacity. Think of it in terms of not how much current a wire can carry, but how hot it gets while carrying it.

You'll notice in the ampacity spec pages that there are various ampacities listed for different types of insulated wires, yet they have the same guage. It is a question of how hot that wire will get before its particular type of insulation melts. So while THHN insulated 12 AWG wire is limited to 20 amps, this same conductor can carry significantly more than that if it is asbestos covered.

When you get to your transformer, gauging the wire will not tell you how thick the insulating varnish is, or how well air can circulate around them. It's all about heat disipation.

Large power bank oil filled transformers often have several ratings, depending on how they are cooled. For instance, you might see a transformer rated at 12/16/20 MVA OA/FA/FA. This would translate to 12 MVA in open air (OA), 16 MVA with one bank of fans running (forced air or FA), or 20 MVA with both banks of fans running (FA?FA).

How will a transformer fail anyway? I don't know, but would assume that it would be the wire burning in half and/or shorting out due to over-amping. SMF

-.-.-.-.-.-.-.-.-.-.-

Faillures in dry transformers:

Insulation breakdown: Primary to secondary windings, this is one of the main reasons that the sec. winding needs to be earthed at a low voltage TX. to avoid getting a higher mains voltage on the sec. (safe) side.

Shorted turns: Depending on how well the windings touch each other eg. low resistance contact between turns means that the secondary becomes a single turn transformer and the winding will burn out very quickly and a lot of damage will be done to the remainder of the windings and a rewind or write off is necessary.

Winding open circuit: Could be at a tapping or shorted turn gone o.c.


Faillures in oil filled distribution transformers.

Same as for dry TX's and a few more below

In oil filled TX's there are acidity levels in old oil , ingress of moisture, deteriation of paper insulation to name a few.

In New Zealand we usually take oil samples twice a year from substation TX's to determine the condition of the oil and TX.
Carbonlevels indicate some form of arcing at the windings or terminations. Acidity can cause leakage current to earth. lots of varnish can block radiator pipes and reduce circulation hence overheating etc.

I draw a line here now because I think the subject pertains more to dry TX's.

Merry X mas and a great 2006. Raymond

Thanks, Rodalco...

As for winnie, jwhite and WFO, maybe you guys can answer the question I posed to Larry above. I read what you suggested, winnie, and didn't find the answer. Thanks for passing along all that book knowledge that didn't really answer the question. I did learn some things from that, so thank you!

It appears that if I get shocked on the secondary side, it would be the same VA as if I got shocked on the primary side...which appears that it would make no difference. People say to me, "it's the amps that kill you, not the volts." I reply, "OK then, how about if I grab ahold of 480 volts at 1 amp?" They reply, "Well...at higher voltage, the volts will kill you." Either the volts will get me, or the amps will get me, but in my example above, the VA is the same. What's the difference in how badly I get shocked on the secondary vs. the primary, and why?

Either you know the answer or you don't. Please don't act wise and tell me "go read the manual"...it's not in there.

SMF

It's not the amps that kill you, it's the milliamps.

And volts drive milliamps--Ohm's Law. 12 VAC doesn't typically drive enough current to kill someone. Although it can, under the right (wrong) conditions. 120 VAC often can drive enough current to kill. 480 VAC is a lot more likely to kill than 120.

I use the term "VAC" above because frequency is related to lethality. DC is much safer than AC. It turns out that Mr. Westinghouse's choice of 60 Hz puts our AC power system pretty close to the peak of the lethality curve.

The VA rating of a transformer is pretty much irrelevant here--I don't know what the power is that's needed to kill, but it's much less than the VA rating of most transformers.


You're barking up the wrong tree trying to understand transformers though Ohm's Law--they don't work that way. Resistance is just a parasitic that's incidental to the operation of the transformer; if you could make the resistance zero, the transformer would work better. Transformers work by magnetic reactance and by magnetic coupling.


Seriously, if you really want to understand this stuff, I'd suggest a couple semesters of Differential Equations, a semester each of Complex Variables and Linear Algebra, and then take a few courses in Circuit Analysis and Engineering Electromagnetics. That's what the rest of us have to do to really understand what's going on. When you can understand it as a system of interacting, time-varying tensor fields in a complex Hilbert space, a transformer is no longer such a "mind-boggling, mysterious beast."

[This message has been edited by SolarPowered (edited 12-23-2005).]

Quote:
"People say to me, "it's the amps that kill you, not the volts."

True.
But you're not going to get the amps without the volts to "push" it.

If you've ever grabbed the spark plug of your lawnmower while it's running, you know that you have just taken several thousand volts and you're still kicking (and cursing). But the current is so small that no damage is done (unless you're 80 yrs. old with a pacemaker).

Now go put your thumb and little finger across the terminals of your car battery. You're well aware that there is the potential for hundreds of amps there, but not enough volts to push it through your epidermis.

How much current is available from a source depends on the source and its design. The condenser (capacitor) in your car can knock the snot out of you with no real damage. Get across a 100 Kvar capacitor on a distribution line and they'll be lighting cigarettes off your ass for days.

Story time.
My boss and I were going to have to cross an electric fence and were looking for the best place to get across. I wondered out loud if the fence was even hot. He said there was an easy way to test it. Get a long piece of grass, hold it at one end with the other end touching the wire. Slowly run it closer until you feel a tingle.
So Ol' WFO (also known as Mr. Gullible) started sliding the grass closer and closer to the wire. Just about when I thought I was close enough to have felt something, my boss reached over and touched my earlobe.

If you've never gotten several thousand volts through your earlobe, you just aren't living right. It's, well......breathtaking!

Moral of story. Electricity (and Bosses) are unforgiving and shouldn't be experimented with without a good solid basis in the fundamentals.

Thanks, WFO...

I prefer to watch somebody pull the spark plug cap off a CR250 laid over on the throttle side WFO, just getting into the rev limiter.

As for your boss, if you weren't getting energized with the grass touching the wire, how did him touching your ear change anything? Was he energized, touching the wire? At first I thought he only startled you, then I started thinking you actually took the voltage. Pls. explain!

If you ride CR's, I guess you know where my name comes from.

I wish I could explain what happened. It DID knock the fool out of me and DIDN'T BOTHER HIM!!!

I have no explanation. I've even posted this story on forums before hoping someone else would come up with an answer. The ONLY thing he touched was me....nothing else.

I can't explain it. I don't understand it. But one thing it does say is that just about the time you think you've figured everything out about electricity, it jumps up and gets you.

So please be careful with your experiments. We can't afford to lose any dirt bike riders (even though you should be on a KX or YZ).

How interesting!

I rode the predecessor to the KX250, the Kawasaki F-11M Prototype (same type Brad Lackey rode in FIM races for Kaw), and I've raced YZ125s and 250s, Hondas, even other brands you might not want to hear about.

Talk with you soon.

Sparkmaster,

By 'the question you asked Larry', I presume you mean

Electrical isolation in this case means that the secondary of the transformer is not 'galvanically' connected to the primary. In other words, there is no flow of electrical current between primary and secondary. Energy flows between the two, by the mutual magnetic coupling, but there is no electrically conductive path from primary to secondary.

This makes the secondary 'ungrounded'. If you were to ground either of the secondary terminals, there might be slight capacitive charging current, but not much current flow. It is only with _two_ ground faults that you would get significant current flow.

Remember that electricity does _not_ 'seek ground', and simply touching an energized conductor is not sufficient for a shock. Electricity 'seeks' to close the circuit, and find a path back to the source.

The power distribution systems that we generally use are _intentionally_ grounded and bonded, meaning that one of the transformer terminals is connected to ground. Because of this, any contact between one of the 'hot' terminals and ground, either directly, or through a load (such as a person) will result in current flow. But with an _ungrounded_ secondary, grounding a lead will not cause significant current flow, because there is no complete circuit back to the source.

Your variac is not isolated. This means that there is a shock risk between the output and _any_ grounded metal. But the step down transformer is isolated, meaning that the only shock risk would require that you contact both output terminals at the same time.

On top of this, the step down transformer means that the maximum output voltage will be perhaps 14V; 5V out of 14V is probably safer and more easily controlled than 5V out of 140V.

WFO,

My guess is that your boss did feel the shock, but was probably ready for it. The current was quite low, intentionally limited by the design of the fence charger. You didn't feel the current passing through you, you felt the spark right at your earlobe.

Your boss could have also done something as easy as holding a bit of metal to your earlobe, say a key or a ring. If he held the metal tightly, he wouldn't feel the arc, but you sure would. I do this when I am getting out of my car on a cold, dry day; I use my key to discharge myself to the car frame.

-Jon

Long story short,

The Secondary side of the Isolation Transformer being used (between the VARIAC and the load item) is not Grounded - therefore it offers an isolation, so there is a reduced hazard for a shock to ground.

Getting caught between the two output leads of the Secondary still gives the "Barbequed Effect" to whom ever is unlucky enough to be the Conductor.

In reality, there will still be a potential to ground through the isolation transformer (with an ungrounded secondary side).
The potential ("Voltage") is a result of Capacitive Coupling, and will vary with distance.

Also, if the Primary Winding faults into the Secondary Winding - either through a loss of smoke (extended overload scenario), or flash-over scenario, the resultant connection becomes an Autotransformer - which both eliminates the Ground Isolation safety thing, and increases the Voltage on the Secondary side.

Do a search on this site for threads regarding operational characteristics of Transformers, Inductors and AC Generators.
We have had many such in-depth talks regarding the basics through the extremes of these beasts.

Do searches in the General area, the NEC area and the Technical Reference area.

One very important point of safety regarding Transformers:

Don't drop them on your foot! That really hurts!

Good luck!

Scott35

Oh, just noticed that Winnie posted the same thing, before I did!

Sorry for the redundancy!

BTW - Jon (Winny);

I really liked your Transformer explanation text!!!

That is an excellent description - may I use it in the future?

When I try to explain how they work, the words coming out are just too dang confusing to the listening party(s) - resulting in the "Deer Caught In Headlights" stare.
(words coming out resemble that of the words found in Electrical Engineering Handbooks ["Standard Handbook for Electrical Engineers - version 14" is a good example]).

Scott35

Quote:
"My guess is that your boss did feel the shock, but was probably ready for it. The current was quite low, intentionally limited by the design of the fence charger. You didn't feel the current passing through you, you felt the spark right at your earlobe."

I know I'm getting off topic here, but getting shocked was "kinda" a topic.

I think you're right. the more I think about it, I think I was "charging"....as if I was one plate of a capacitor. When we touched, I charged him.

....either that, or ambiguously mitigating radio waves refracting through a plasma polarity regurgitator.

Winnie, Scott and WFO, Thanks a lot for your contributions to this discussion.

One statement that I use alot is, "All the world's a voltage divider." When you start thinking of the human body playing a part in a voltage divider, it should probably be considered as a highly variable resistance. One day I had my Fluke 87 out and asked everyone to test for resistance by pinching the probes, one in each hand. Most results were pretty close but for one guy, whom we'll call Dave "the reptile". It was surprising to see his skin resistance more than twice as great as the highest of the rest of us. It would not be unreasonable to assume that Dave might have better resistance to electrical shock considering our observations that day. But you really need to think of humans like the reverse of the nichrome that you were trying to figure out. The nichrome's resistance per given length increases as you flow more current and heat it. Once you start getting shocked, bad things start happening that will tend to lower the body resistance, such as ionizing body fluids and increased sweating.
As far as the shock sensation is concerned, I could see more sensitive parts of the body being able sense a current flow that other areas wouldn't.
Joe

[This message has been edited by JoeTestingEngr (edited 12-23-2005).]

Sparky, I'd like to explain something not always understood, and so far not answered:

A transformer (or any power source) with a current capacity of, say, 10 amps does not force 10 amps through the load no matter what.

The voltage and the load's impedance control the possible current, to the limit iof the source. Ohm's Law comes into play: 1 volt can push 1 amp thru 1 ohm.

E = I x R, I = E/R, and R = E/I

Just like an ausio amplifier capable of, say, 100 watts, does not automatically damage speakers with a lesser rating; you have to over-drive them with voltage.

Thus, a 12v source with 10 amps capacity will flow the same current through 12 ohms as will a 12v source with 1 amp capacity: 1 amp.

I would say that Larry is right, but yet he isn't. If his 12V sources are a 12V, 10A regulated supply & a 12V, 1A regulated supply, then, of course, he is right. If his sources are 2, 12V transformers, 1 a 10A, the other a 1A unit, the 10A transformer will flow more current through the load. This is naturally the case because we know that the output voltage of the 10A TX will be more than that of the 1A TX when they are both loaded at 1 Amp.

When we design linear supplies we consider how they can handle their maximum load conditions. But we also have to consider them under no or minimal load conditions, watching for excessive voltages on the input filter.
Joe

Spark Master,
Don't think for a second that you are alone in not understanding AC and Transformer Theory.
When I first got started as an Apprentice Electrician, it all seemed like Chinese to me.
The more I thought about it, the more it made sense.
Mind you, I didn't have ECN back then either.
You guys that knock a fella like SMF have forgotten what it is like to get back to the basics.
The basics are what matter to our trade.
Yet those that would knock a guy down for not knowing about theory, can't use simple tools like a Megger or explain why it should be used.
Just my 2c worth.

I like that. "All the world's a voltage divider."

I'd also like to point out that the exchange between Joe and Larry is a perfect example of the 'refinement of approximations' that I mentioned before my transformer lecture

Larry said 'If you have 2 12V transformers, one rated at 1A and the other rated at 10A, then they will both push the same current through the load.' Which is a fine way of saying that if you apply 12V to a load, the load will carry a particular current.

Then Joe came in with a refinement: the output voltage of the transformer is _not_ constant, and will change with the connected load. It is quite likely that the output voltage of the 1A transformer will fall more than the output voltage of the 10A transformer, thus making it quite likely that the 10A transformer will put a bit more current through the load.

We could keep going further, for example looking at how transformer heating messes with output voltage, and how load characteristics change over time, at each stage developing a more complex model of the world which more closely approximates how the world works. You will find this over and over in science texts; where a good approximation is taught as 'this is how the world works', and then a couple of paragraphs later you get 'what we said previously was not quite accurate, here is the real way that things work'.

Joe, there is a story (urban legend? I have no proof, here is a Darwin Awards reference http://www.darwinawards.com/darwin/darwin1999-50.html ) of someone electrocuting themselves with a 9V source using a multimeter to measure body resistance.

Scott, you are welcome to use my text.

-Jon

Thanks, guys!

Joe, I was thinking about the body's resistance a couple of days ago, so I took my Fluke meter to check my resistance and got nothing. Then I wet my fingers and grabbed the leads, and got nothing. Am I non-conductive? I was hoping to at least be a decent semiconductor.

Can you expand on the concept that all the world's a voltage divider? Can you expand on how the human body would be a voltage divider?

Larry, thanks for your comments about having to over-drive a speaker with voltage, that helps.

Trumpy, thanks for your understanding.

SMF

Just a guess.

Meter not on auto range or wrong range selected?

Meter set to continuity not ohms?




If yes then you are truly a freak of nature.

I doubt you are non-conductive.

Spark,
I've got some bad news for you buddy. You're DEAD! Not only that, you've been dead for quite some time if the Fluke can't see some R across you. But before we start calling your next of kin, is there any chance that you have your meter on "diode check", instead of "ohms" or inadvertantly gotten into a manual, low resistance range? For now, let's stipulate that you are among the living. I haven't checked out Winnie's link yet but let's consider why it doesn't seem too credible. A modern DMM will usually have an imput impedance of about 10 million ohms. You would probably be hard pressed to find one with less than a million ohms. This is a good thing because we wish to measure a circuit without influencing it. To consider the possibility of a DMM being a shock hazzard, we have to think of it as a 9 volt power supply with a source impedance of 10 million ohms. Our unknown voltage is Vdead (Vd). Our poor victim's resistance is Rdead( Rd),(maybe we should use URdead). Total resistance (Rt)= Rsource (Rs) + Rdead (Rd). We can use voltage divider equations to note that Vt/Vd = Rt/Rd. If you happened to look like 100,000 ohms of resistance, you would plug & chug & come up with 9/Vd = 10,100,000/100,000. Vd = 0.089 volts in this case. Your current is 8.9^-7 A. So my gut feeling is that the only way that a DMM set to R is going to kill someone, is if Jet Li is using it.

Why don't you try taking a 9 volt battery and putting yourself in series with your Fluke on DC uA across it. Then if you can get a hold of a Simpson 260 or other analog meter that has its input R expressed in thousands of ohms per volt(R depends on voltage scale that you use), try this. Measure your battery voltage with the analog volt meter and your DMM. Now put a 1M resistor in series with the battery and measure between the other terminal and the free end of the resistor. Quite a difference between the 2 readings now, right? That's because, "All the world's a voltage divider."
Joe


[This message has been edited by JoeTestingEngr (edited 12-24-2005).]

Sorry folks, got suckered by the second page.
Joe
By the way, I-wire posted before I did and I agree with him completely.

While it is non-conclusive, the body's structure is non-conducive to being non-conductive. Since further speculation is non-constructive, I'll just say MERRY CHRISTMAS!!!

[This message has been edited by JoeTestingEngr (edited 12-24-2005).]

Winnie posted:
Joe, there is a story (urban legend? I have no proof, here is a Darwin Awards reference http://www.darwinawards.com/darwin/darwin1999-50.html ) of someone electrocuting themselves with a 9V source using a multimeter to measure body resistance.

Yikes! I just went to the link and see how it might be possible. He used a VOM and flunked his bloodborne pathogen practical test. I just asked my fellow Testing Engineers to test themselves with a hi-z DMM. I didn't ask them to become my blood brothers and sisters.
Joe

I'm using a Fluke 336.

http://us.fluke.com/usen/products/specifications.htm?cs_id=30405(FlukeProducts)&category=ELW(FlukeProducts)

Hey Spark,
Everything makes sense now and you don't have to worry about hanging out with my friend, "Dave the reptile" That is sort of a special purpose, stuck in the middle meter, that you have there. It will probably serve you very well as an electrician. If you want to do the kind of mad scientist, disturbed mind, experiments, that I tend to come up with, the 70 & 80 series of their meters work better. Yours won't indicate resistances higher than 6000 ohms and low currents. You'll probably want to pick up another meter, down the road, to delve into experiments such as these.
Joe

I don't how true that story is......

I was in the AF in '84, and heard the same story, but it was a trainee in the AF.


Dnk....

[This message has been edited by Dnkldorf (edited 12-24-2005).]

Has there ever been a study done on humans being shocked? Certainly somebody must have done that kind of thing long ago. I've seen those machines with two brass handles, you're supposed to hold the handles and the machine shocks you with more and more intensity until you either let go or somehow "win" - I guess it maxes out at some predetermined point. Sounds dangerous to me, since from one hand to the other, it goes through your heart. I wonder if there was ever an experiment to see how much voltage it takes to "shock" somebody, if the voltage was fed through a variac. Where does a person start feeling a tingle? Where does a person start feeling shocked? Where does a person get jolted and burned? Killed? Was there ever a study done where people might know something about this?

Iwire, to answer your question, my Fluke 336 meter gives you continuity and ohms on one setting. It also does AC volts, DC volts, AC amps and DC amps with an amp clamp. 600 volts, 600 amps.

I thought I had a decent meter. It's better than the ones I've seen on the job. Joe was kind enough to let me know that it's a middle-of-the-roader on the grand scheme of things. Basically I'm trying to take a de-tuned four stroke enduro out to Carlsbad.

Joe, thanks for your help. What meters would you recommend for someone with a sick mind?

I see, 70 & 80 series...I'll look for that.

Thanks!

Quote:
" I wonder if there was ever an experiment to see how much voltage it takes to "shock" somebody"

If you ever want to really be sickened by human greed, go study the competition between Edison and Westinghouse in the beginning days of electricity. Edison, promoting DC, went to extraordinary means to prove how deadly Westinghouse's AC was. This involved "scientific" studies in the torture of dogs and anything else they could electrocute to prove how deadly AC was. I believe there was even a film of them electrocuting an elephant.
I wouldn't swear to it, but I believe the first criminal execution involving the electric chair also stemmed from this feud.

If you can imagine a "scientist" shocking a dog repeatedly while gradually increasing the voltage until the animal finally died, then you have an idea of how sick some individuals are.



[This message has been edited by WFO (edited 12-24-2005).]

I'm sure somebody learned all about the limits of electro-animal and electro-humanoid interaction, maybe I can find something on that so I know. There oughta be a chart at least.

If anybody knows where to find information on this, please let me know!

Well Spark, I think that meter of yours is a good choice for your field work. You might consider checking out a local hamfest for a used bench meter. It's amazing the things that can be found there. I used to go to the Dayton Hamfest. It was like Oshkosh for airplanes or Daytona for NASCAR.
Joe