Some noob questions

[rstofer]

Dave has a playlist of Fundamentals videos.  w2aew has some great videos on circuits.  Both of these sources are somewhat beyond Ohms Law but they are both excellent resources.

Digilent has an entire curriculum:
https://learn.digilentinc.com/classroom/realanalog/

MIT has OpenCourseware
https://www.google.com/search?q=mit+opencourseware+electronics+video+lectures

Khan Academy has an Electrical Engineering track but they are best known for their math lectures:
https://www.khanacademy.org/welcome?learn=1

I can't begin to list all the excellent resources, there are tutorials all over the Internet.

[Mr D]

In Dutch to make it more clear for the TS: "Iets belast een voeding meer, de belasting is hoger omdat de weerstand lager is. Er kan dus meer stroom lopen, dat maakt de boel warmer en vormt zo een zwaardere beslasting.

Haha, thanks, i can read it but i'm actually English!

Thanks all, now to bed. Sorry for coming across as a dumb-ass, i have zero background in this stuff.

I can't promise not to resurrect this one tomorrow!

[rstofer]

My brain hurts.

Just one more before i go to bed:

I don't get it. You're talking about a load drawing the current. The greater the load, the more current.

But at the same time the circuit has resistance.

So it's pulling and pushing back at the same time??

Load isn't a fundamental unit of electricity.  Volts, amps and resistance are fundamental units.

Intuitively, we think of a heavy load as drawing more current than a light load and for equal voltages, we get to the idea that a heavy load draws more current and therefore must have less resistance because, for a fixed voltage, current and resistance have an inverse relationship.  E = I / R   Hold E constant and to increase current, you need to decrease resistance.  Increase resistance and current must decrease.

We could also consider load in terms of power and this would allow for different voltages as well as different resistances and therefore different current.  We don't need to go there.

Ohm's Law does it all!
https://www.electronics-tutorials.ws/dccircuits/dcp_2.html

[rstofer]

Haha, thanks, i can read it but i'm actually English!

Thanks all, now to bed. Sorry for coming across as a dumb-ass, i have zero background in this stuff.

I can't promise not to resurrect this one tomorrow!

Going to bed at midnight?  That's prime time for doing homework as I recall!
Don't worry about resurrecting the topic.  It is the most fundamental discussion you will ever have on the topic and it needs to be nailed down or you simply can't move forward.

[Jwillis]

The water analogy is correct if explained  correctly .
Think of a tank of water with a pipe attached to the bottom. The amount of water in the tank is the "charge". The pressure of the water is called "volts " .The more pressure the higher the voltage. The flow or rate that the water moves  is called "Amps" or Current. The size of the pipe is called resistance. The bigger the pipe the less resistance. Now voltage and current are dependent on each other. No pressure on the pipe no flow or current. Put a plug in the pipe Even with increased pressure there is no flow.

Now lets consider your circuit is a paddle wheel at the output of the pipe.The paddle wheel can only handle so much flow before the water pours over the sides. This is the flow rating or current rating .no mater how much water is dumped into the paddle wheel it will only except so much.Lets say 1 amp . So your paddle wheel requires a certain amount of pressure and flow for it to move but also has a maximum pressure and flow before it breaks.
You set your pressure an flow at a required amount to make the paddle wheel move. Just like 3 volts and  1 amp.Every thing works fine.
Now lets say the flow remains the same but you increase the pressure .12 volts at 1 amp and like dirt in front of a pressure washer your paddle wheel  promptly explodes and flies apart .

Just like a circuit only excepts a certain amount of flow or amps .If you increase the pressure or voltage beyond its rated requirements it will burn out .A circuit .when working correctly ,will only take so many amps no matter what the voltage is. This is also why voltage is the dangerous part of electricity. Very high voltage with very small amps will kill where as very high current and very small volts will do little.

Now lets consider the size of pipe .Just like wire ,the bigger the pipe or wire the less resistance there is.

Hope this helps

[Mr D]

Thanks, i appreciate you taking the time, it helps.

One thing i don't understand though:

Imagine you were designing a complicated plumbing system in a building, moving water around the building through various pipes, valves and tanks at various pressures.

Say an alien landed and asked you to explain the physics of the system to him.

You wouldn't start to talk about analogies with electricity, volts, current etc, right?

So, when talking about electricity, why isn't it possible to explain it in terms of what it actually physically is?

Why is it always necessary to explain via analogy?

This is not meant as a criticism of your attempt to explain it. I just don't understand why it's never explained without resorting to analogy!

   

[agehall]

Why is it always necessary to explain via analogy?

It is not.

HOWEVER, going down the physics route to explain quickly becomes very complicated and if you understand that, you probably would not be asking the question in the first place. So the easiest way to get a good understanding of what is happening, it is usually easier to use an analogy to something most people already know.

[Mr D]

The flow or rate that the water moves  is called "Amps" or Current.

Ok, but what is being measured here? The physical speed that the current flows? Because water can move at different speeds. Can charge also move a different speeds?

Or is it the amount of electric charge per second that passes a certain point?

Because in the water analogy, you could have the same volume of water moving past a fixed point, whether it's a wide, deep, slow moving stream, or a 1 meter diameter pipeline at extremely high pressure.

[agehall]

Now you are getting into things way beyond basics.

Current is charge moving over time. Wikipedia does a good job of describing what it is. https://en.wikipedia.org/wiki/Electric_current

[Mr D]

Going down the physics route to explain quickly becomes very complicated and if you understand that, you probably would not be asking the question in the first place. So the easiest way to get a good understanding of what is happening, it is usually easier to use an analogy to something most people already know.

OK, but when talking about flow of water, you're not really explaining it at a fundamental level, in terms of molecules and atoms, you're also generalizing.

So what is it about electricity that is so fundamentally less intuitive, that makes it so hard to understand, one needs to resort to an analogy of water?

[Ice-Tea]

So what is it about electricity that is so fundamentally less intuitive, that makes it so hard to understand, one needs to resort to an analogy of water?

Have you seen water? I would assume so. Have you actually seen electricity? I would think not.

So, there

[PA4TIM]

It is possible to explain it on a fundamental level but that makes it very complex unless you are an ace in Physics.
Now we only talk about DC voltage and current.
The water model represents the functional model. Water as current is more or less usable to explain the effect.

In real life electrons are travelling from minus to plus, besides that they travel very slow while the energy is moving very fast. Then there are electric and magnetic fields, static or dynamic. Water won't induce a water flow in a pipe next to it. A current flow can do that. Water will flow in the pipe, DC current does, AC, too but AC can also move on the surface of the wire and to make it even more complex, move outside the wire and that brings us to transmission lines and things like wave behavior that leads us to quantum mechanics. There is no way to understand that straight away and the amount of things you need to know is huge. There are only a hand full of people on this forum who really know the deepest fundamentals.

I had the same problem when I started. I started with radio and every time I got my head around something "that opened a door" it turned out there were even more closed doors after that. And then is the problem, what path to follow. There is not one path, there are many running parallel and they are forking out constantly too. You need a very high drive to follow a lot of paths parallel

Now you talk about a simple restive load and that is already hard, but in real there is not just resistance, there is "complex resistance" called impedance and split out in reactance and resistance. You need to know this. But to know this you need to know a lot about inductance and capacitance. A real resistor also has inductance (and capacitance to other parts of the circuit) A trace on a pcb is a resistance for DC but has transmission line behavior for AC. Without that knowledge you can understand a lot of things practically, the rest is called voodoo by many and they can do a lot without knowing that part. Knowing that makes life more easy.
And that also brings in a lot of math

You are now struggling to learn maybe 0,001% of the fundamentals. By far the easy part and a lot of people do not understand more then this and still can do a lot.   

[hamster_nz]

Maybe a car analogy would help (not very likely, but I'll try  ).

Think of a car as being an electron (all identical cars, because all elections are exactly identical), and motorways are wires. You pick a reference point across the motorway and count each car as it passes, and how fast it is traveling.

In this analogy, current is the count of cars crossing that line in a standard unit of time. (this is a good analogy, as one amp is 6.242×10^18 electrons passing in one second).

Voltage could be thought of as the average speed of cars (this part isn't a very good analogy, as voltage is more a 'difference in pressure' between two points).

But what is clear in this analogy is that the two things (count of cars & average speed of cars) can be independent of each other - you can have a few cars moving slowly (low voltage and low current), or a few cars moving very fast (high voltage and high current), or you could lots of cars moving slowly (high current, low voltage), or lots of cars moving fast (high current high voltage). It all depends on the local conditions at the time - there might be heavy fog with snow and ice, or it might be a clear day.

So when you measure 2A on your plugpack, that is measuring 12,484,000,000,000,000,000 electrons entering the meter every second (and the same number exiting too).

The confusing bit is the voltage, and it is not just because of my broken analogy. The plugpack is engineered such that it will at most only let a given number of electrons flow per second, regardless of the voltage setting. So should the plugpack is set to 12V and more than 2A wants to flow, then the electronics in the plugpack reduces the output voltage (the energy per electron) until only 2A will flow.

[PA4TIM]

http://wiki.c2.com/?SpeedOfElectrons
Electrons moving through a cable is a big simplification on its own.  There is a drift speed and a thermal velocity. The electrons move slow but the charge moves fast.
From the link:

For example, for a copper wire of radius 1 mm carrying a steady current of 10 Amps, the drift velocity is only about 0.024 cm/sec!

[alanb]

Mr D. It sounds from your original question that you are connecting your multimeter directly to the power supply whilst in the current mode. This is something that you should not do. In current mode you should connect the meter in series between the power supply and the load. With the meter connected directly to the power source in current mode the current is only limited by the maximum current capability of the supply and this can blow the fuse in the meter or worse. In your case the maximum current from the supply appears to be 2 amps thats why you get the same result on both voltage ranges.

[rstofer]

All of these complications with the analogy is why it is used for the first half hour of the first class and then put away, never to be discussed again.

All that is hoped for is an understanding that volts pushes current through resistance.  The analogy should stop there because all the other electrical effects can't be described with the analogy.  I had forgotten about mutual inductance, mentioned above.  That clearly doesn't happen with water in a hose.

It is sufficient to understand that higher pressure in a hose will result in more water flow.  Add to that the fact that more water will flow through a big pipe than a small pipe for the same pressure and you have just run out of the analogy.  We're done, it's time to move to Ohm's Law and start playing with DC circuits.

OTOH, it isn't necessary to understand the physics behind electron flow.  At least not until AC circuits and probably not until RF or semiconductor physics.  Ohm's Law and Kirchhoff's Laws are sufficient for most anything a hobbyist is likely to run across unless they are into amateur radio.

The analogy is useful in that most people have played with a garden hose but beyond the trivial pressure, flow rate, pipe resistance, the analogy isn't helpful.  Even flow resistance isn't a linear kind of thing.  There are boundary conditions, laminar and turbulent flow and other issues that are not appropriate for consideration in the analogy.  That's why there is the entire study of fluid dynamics.  I wonder if they use the electrical analogy for the first five minutes of the discussion.

[asmfan]

It doesn't matter much because Ohm's Law relates all 3 variables and we can swap them back and forth at will. 

For all practical purposes this statement is a useful simplification, and holds true when working with ohmic materials.
This simplification does not work when working with non-ohmic materials such as when experimenting with static electricity.

Usually you see Ohm's law written in the following form:

V = I x R

However, in this form it is tempting to define voltage in terms of resistance and current. It is important to realize that R is the resistance of an ohmic material and is independent of V in Ohm's law. In fact, Ohm's law does not say anything about voltage; rather, it defines resistance in terms of it and cannot be applied to other areas of physics such as static electricity, because there is no current flow. In other words, you don't define voltage in terms of current and resistance; you define resistance in terms of voltage and current. That's not to say that you can't apply Ohm's law to predict what voltage must exist across the a known resistance, given a measured current. In fact, this is done all the time in circuit analysis.

- Paul Scherz and Simon Monk. "Practical Electronics for Inventors" Fourth Edition pg 24

[rstofer]

It doesn't matter much because Ohm's Law relates all 3 variables and we can swap them back and forth at will. 

For all practical purposes this statement is a useful simplification, and holds true when working with ohmic materials.
This simplification does not work when working with non-ohmic materials such as when experimenting with static electricity.

And perfectly adequate for the purposes of a newbie thread.  There is no point in expanding the discussion at this point in the learning process.  Close enough...

[Mr D]

Many thanks to all for being exceedingly patient with me, i'm gonna take my time and read through this thread a few times.

I want to ask one more favour. Could someone tell me the values of a few resistors i should buy that i can use to connect to my Fluke and power source so i can experiment with calculating ohms law? I guess the resistance should be in a range that it'll be clear on my multimeter, but not so low that i risk damaging my power supply or Fluke.

And does anyone have some links to a good lab power supply that will be useful for testing Ohm's law? Doesn't have to be very cheap, but i also don't want to spend more than necessary. There's so many available, i have no idea what to get.

[Rick Law]

The flow or rate that the water moves  is called "Amps" or Current.

Ok, but what is being measured here? The physical speed that the current flows? Because water can move at different speeds. Can charge also move a different speeds?

Or is it the amount of electric charge per second that passes a certain point?

Because in the water analogy, you could have the same volume of water moving past a fixed point, whether it's a wide, deep, slow moving stream, or a 1 meter diameter pipeline at extremely high pressure.

Mr. D,

We need to back up a bit here.  Your wall-wart has the potential to supply energy to whatever you plug the power output into.

That potential to provide energy is measured in Voltage and Current it can supply to the load. Voltage is measured in V (volts), and Current is measured in A (Amps or Amperage).  Current is generally denoted as I.  When your output is merely your DMM, the DMM itself is a load.  This is actually unsafe, but lets ignore that safety issue for the time being and just understand what went on.

* When the wall-wart is set to 3 Volt.  The 2A reading means the DMM is taking up 2Amps when the DMM itself is the load. 2A*3V=6Watt, it is using up 6 watts of power.  The DMM is a heater putting out 6 Watts of power somewhere inside the DMM.
* When the wall-wart is set to 12 Volt.  The 2A reading means the DMM is again taking up 2Amps when the DMM itself is the load. 2A*12V=24Watt, it is using up 24 watts of power.  This is a lot of power, when applied for enough time, it may melt the whole darn thing.

Even at 6 Watt, it is a lot of heat to dissipate.  Your DMM can cook itself.  Since your DMM so far survived, lets continue to see what is going on.

Case 1:
(Your DMM is set to measure current)

Imagine you have a FAN connected to that output with the DMM in-line (in series) like this:
  power+  to  DMM (red)
  DMM (black)  to  FAN+
  FAN-  to  power-

Now your DMM is measuring the current flowing through the DMM itself.  Since the DMM is in series with the FAN, the current is the same as the current flowing through the FAN.  (Just as a garden hose, the amount of water going through the hose is the same as the amount of water going through the nozzle.)

Case 2:

(First make sure your DMM is set to measure voltage)
On the other hand, if you have the FAN and the DMM connected in parallel and then to power like this:
 power+  to  DMM(red) and DMM(red) is also connected to FAN+
 power-  to  DMM(black) and DMM(black) is also connected to FAN-

Now, your DMM is measuring the voltage being supplied by the power.  Consider voltage (aka potential) is the pressure (how hard it is pushing).  The higher the voltage (the harder it pushes), the more current would flow assuming the darn thing didn't burst.  The amount of energy being delivered would be V*I (ie:  Wattage = voltage times current, recall, current is denoted as I).  With your DMM, it seems pushing at 3V or 12V results in the same current (2Amps), so it was likely designed limit - just as the diameter of a garden hose limits the amount of water that can flow.

Why your initial test was unsafe:
If you understand what is described thus far, I can explain why your initial connection (with DMM measuring current directly connected to the output) is unsafe.  Without the FAN in between, nothing is there to use the power.  So, the DMM is taking the full load.  Good that your DMM limits that to just 2A - it is still not good for long, but it survives for the short duration of heating.

Case 1 and 2 wrap up:
Now if you want to try that yourself, find a FAN that operates in 12V and 3V - or use an incandescence light bulb such as a bulb for your car.  Make sure it is incandescence bulb.  (LED bulbs designed to function at specific voltage.  It would not do well in our voltage-changing experiment.)   When you connect the bulb at 12V, you should see it lights up - you can measure the current (when in series as depicted in case1 above), or you can measure the voltage (when in parallel as depicted in case2 above), now you can evaluate the Wattage the bulb is putting out.

Incandescence auto bulbs (as resisters) is a good way for your experimentation until you can buy some real resisters.  Remember, resisters (bulb) will heat up.  A 24watt 12volt bulb will dissipate the heat properly as long as you don't over-volt the darn thing by applying more volt than it can take.

Depending on where you are, mail order may be the best way to get some real resisters.  Tayda is good place to get electronic parts.  Very few real resisters would take 24watts (as your initial connection).
https://www.taydaelectronics.com/

If you understand the post thus far, you can dig into Ohms law where you can begin to figure our how V=I*R works.


Case 3:

If you got this far, an interesting thing to see would be to remove the light bulb with your DMM (set to voltage) connected to the power alone.  This is the NO-LOAD voltage put out by your wall-wart.  Depending on design, this should be higher than when the FAN was also connected.

Footnote:  when in series, the DMM actually caused a very small voltage drop, but that is too deep a lesson for now.

[rstofer]

Many thanks to all for being exceedingly patient with me, i'm gonna take my time and read through this thread a few times.

I want to ask one more favour. Could someone tell me the values of a few resistors i should buy that i can use to connect to my Fluke and power source so i can experiment with calculating ohms law? I guess the resistance should be in a range that it'll be clear on my multimeter, but not so low that i risk damaging my power supply or Fluke.

I bought an assortment kit from Jameco many years ago.  Maybe something similar is available to you?
https://www.jameco.com/z/00081832-540-Piece-1-4-Watt-5-Carbon-Film-Resistor-Component-Kit_81832.html

100, 220, 330, 470, 1k, 2.2k, 4.7k, 10k, 22k, 47k, 100k all 5% ought to cover it.  Notice how the values are just 10x multiples of the original sequence?  That's a property of common resistors

https://ecee.colorado.edu/~mcclurel/resistorsandcaps.pdf

And does anyone have some links to a good lab power supply that will be useful for testing Ohm's law? Doesn't have to be very cheap, but i also don't want to spend more than necessary. There's so many available, i have no idea what to get.

I didn't have a lab type supply for decades.  I used batteries or wall warts and occasionally a dedicated open frame power supply.  I finally broke down and bought the Rigol DP832.  It is not inexpensive and there are many cheaper alternatives but it works really well and I'm glad I bought it.

A 3 output supply is handy when working with dual rail op amps where you need +15V, -15V and maybe 5V for some kind of supporting logic.

Having a supply with adjustable current limiting saves on parts.  It does no good to have an analog knob with no display of the limit value, you can have it set too high and not even know it.  A true lab supply has an indication of the current limit value.  Very important feature.

You can search the forum for 'power supply' and find MANY threads.  A lot of people want to build their own and there are a number of users recommending it.  Maybe so...  I buy power supplies, the only one I built was an old Heathkit back around '70.  There are some power supplies on eBay if that's a help.  Some are older HP supplies and these are probably quite good.  There are also some Chinese versions of dubious quality.  So, prices are from cheap to expensive with features to match.

Of course, I have built the project level supplies with LM7805s and things like that.  They were specific to the project, not general purpose.

[rstofer]

Why your initial test was unsafe:
If you understand what is described thus far, I can explain why your initial connection (with DMM measuring current directly connected to the output) is unsafe.  Without the FAN in between, nothing is there to use the power.  So, the DMM is taking the full load.  Good that your DMM limits that to just 2A - it is still not good for long, but it survives for the short duration of heating.

Well, we know the 2A number but we really don't know the voltage at the DMM.  The power supply has some internal resistance (known later in your studies as the Thevenin Equivalent Resistance) so some of the voltage was dropped internally and some of it was dropped in the meter.  We don't know the resistance of the current measuring circuit in the DMM but it's going to be LOW.  Say it's 0.1 Ohm...  If so, the meter was only dissipating 0.4W (I squared times R).  The rest of the voltage was dropped internally within the power supply and there are a number of ways that can occur and not all of them result in excess heating.

Why do we care about this?  Well, Thevenin is going to play an important part in circuit analysis but more important, it's going to lead us to the fact that we need 2 DMMs (minimum).  It does no good to measure the current through a device unless we can simultaneously measure the voltage across it.  The power supply may be the limiting factor, not the device.

This is all a lot of fun.  Ohm's Law, Kirchhoff's Laws, Thevenin's Theorem - these are the tools of circuit analysis.  You go nowhere without understanding them.

[Rick Law]

Why your initial test was unsafe:
If you understand what is described thus far, I can explain why your initial connection (with DMM measuring current directly connected to the output) is unsafe.  Without the FAN in between, nothing is there to use the power.  So, the DMM is taking the full load.  Good that your DMM limits that to just 2A - it is still not good for long, but it survives for the short duration of heating.

Well, we know the 2A number but we really don't know the voltage at the DMM.  The power supply has some internal resistance...
...
...
This is all a lot of fun.  Ohm's Law, Kirchhoff's Laws, Thevenin's Theorem - these are the tools of circuit analysis.  You go nowhere without understanding them.

Don't scare the guy...  Let him in gently.  I think he has to figure out a lot more before he gets there.

I merely want to let him know he can't stick the probes into just anywhere when it is set to current...  Like sticking it into the AC outlet.

Now we can relabel the DMM settings as Crispy and Extra Crispy.

[rstofer]

Don't scare the guy...  Let him in gently.  I think he has to figure out a lot more before he gets there.

I merely want to let him know he can't stick the probes into just anywhere when it is set to current...  Like sticking it into the AC outlet.

Now we can relabel the DMM settings as Crispy and Extra Crispy.

One thing we all know: If you try to measure volts with the probes in the current position, the fuses may blow.  Or the meter may just do an imitation of a hand grenade.

The user just has to pay careful attention to where they put probes and what they expect to have happen.  LOOK at the meter and verify the test lead connections BEFORE sticking a probe somewhere.  Some meters (like my Fluke 189) will flash a warning if the probe position isn't correct for the range selected.  That's a nice feature

There was a time before meters had fuses and CAT ratings are kind of a new thing as well.  In spite of that, many of us are still here.

NOTE:  The EEVblog Brymen BM235 warns when the test leads are in the wrong position.  So does the new EEVblog 121GW.  These are professional meters.

The Aneng 8008 does NOT warn of incorrect test lead connections.  Apparently, it is expendable.  As is the user...  I like the meter and use if in preference to my other meters but I do need to be aware of the limitations.  Since I work on low energy projects, it's not much of a risk.

[Brumby]

Firstly, let me address the water analogy.

It is a very good analogy when taken as a guide to help your thinking, but - like all analogies - it will break down if you try and take it too far.  This is what you are doing when you start talking about the speed of the water flow.  You will get caught up with details that simply are not part of the analogy and you will get totally confused.

It would be better (not perfect) to think of the quantity of water flowing - not the speed.


Secondly, you absolutely have the right idea of wanting to understand Ohm's Law!!  Once you do, the water analogy will make a lot more sense and you will probably see where a few of its limitations arise - but you will also be less likely to need it.


Third - but perhaps the most important at this juncture - DO NOT put your meter into current mode and connect it directly across a power source unless you really have to and already know what to expect.  It can be very dangerous and it is not a habit that you want to develop.  To understand why, you really need to know Ohm's Law and apply it in the real world.