What are the Adavantages of High Amperage?

Drakkith

If you look at Ohm's law, you will see that the current is determined by the voltage and resistance. When the resistance is very very low you can get a high current from even a low voltage source. However many things, such as the insulation around wires and human skin don't have low resistance. When you crank the voltage waaaay up it stars to have the ability to discharge through even high resistance objects. Once this happens you get very large currents through things that shouldn't have them. As Phinds said, a 10 volt source will not push a large current through your hands if you touch the leads, while a 10,000 volt source will.

cabraham

Only if the power source is constant voltage. With a constant current source, voltage is determined by current & resistance, but CCS is not how batteries & generators are designed. Insulators are very good, while conductors are not, so it is better to source constant voltage, i.e. the voltage is always full value, and the current varies from 0 to 100%, depending on loading.

When current is determined by voltage & resistance, that is a man made condition. It's not Mother Nature. We could design the power grid so that I is constant, V varies with R. We don't for a good reason, but we could. Voltage does not "push" the current. I don't wish to be pedantic, but I feel this needs to be mentioned.

Claude

mfb

Well, the constant current source, attached to a human body (with high resistance), would have to deliver a high voltage. And, as stated before, this is bad for humans.

Drakkith

I'm not sure what you are getting at. A circuit with a constant current must have V vary with R, as Ohm's law states. Just as a constant voltage must have I vary with R. As to voltage "pushing" current, I don't know enough about electronic circuits to be certain, but I thought that was exactly what happened.

phinds

That seems to be saying that Ohm's Law is just some made up words that have no bearing on reality. Surely that's not what you mean?

russ_watters

No, cabraham is saying that we make voltage the independent variable and amperage the dependent -- that that is an arbitrary choice and it could just as easily be the other way around.

I beg to differ: I don't think a passive constant current source is possible. We didn't choose constant voltage supplies arbitrarily, we chose them because they are what happens naturally.

Pharrahnox

I have not read into Ohm's law much, so I never thought of current as a byproduct of volts and resistance. Does this mean that a 100 volt power source could vary its current simply by adding a thick wire to get less resistance?

Studiot

Yes this is true.

You need to understand what is meant by a voltage source and a current source. Both of these have precise meanings so if you do not understand these meanings you can easily draw false conclusions.

A source of electrical energy is either a current source or a voltage source.
It cannot be both and in general you cannot have both in the same circuit.
Both are actually idealisations never achieved by real world components - although some are better than others.

A voltage source maintains the voltage it presents between its terminals, at a predetermined level, regardless of the current drawn from it by the circuit.

A current source maintains the current through itself regardless of the voltage the circuit imposes on its terminals.

cabraham

Not possible? Are you serious? Take an ac generator used by the utility company. It is spun at constant speed so as to produce constant voltage. If it were spun at constant torque, the result would be a constant current source. The power company could just as well deliver to us a power source with constant current, & voltage that varies with load.

It isn't done because conductors have higher losses than insulators. A CCS generates, & transmits at full current all the time. A CVS produces ful voltage all the time, which is less lossy.

Which one is independent vs. dependent is indeed arbitrary. It could be either way. Batteries can be produced as CCS, but the performance is inferior. Nuclear batteries OTOH, are more suitable as a CCS. Search using key words "nucell", "nuclear battery".

Because CVS works better, generators & batteries are optimized for CVS operation, but this is man made. A capacitor, however, tends to maintain CVS operation. This is Mother Nature, not arbitrary. Likewise an inductor has a natural CCS modus operandi, not arbitrary.

Claude

Integral

Sound like it is time for you to go read up on Ohms Law. Without knowledge of that most of what has been said in this tread is meaningless.

Andrew Mason

The only way this could occur is if voltage and power output decreased as load increased. That would not be very practical for a system that is supposed to provide more power if load increases.

For example, suppose you have a generator producing 100 watts of power at 100 volts and 1 amps current supplying a 100 w. lightbulb. If you added another 100 watt lightbulb in parallel, thereby doubling the load, the voltage would have to drop to 50 volts in order to keep the current a constant 1 amp (1/2 amp for each light bulb). This means the power supplied to each lightbulb would decrease to 1/4 of its previous value.

What does this statement mean?

I have to agree with Integral that you need to learn about Ohm's law before we can have a meaningful discussion here.

AM

willem2

The only way it could work, is if all your appliances and outlets were in series. To switch something off, you'd have to short-circuit it. A 100 W lightbulb would have twice the resistance of a 50 W lightbulb. An outlet would always have both sides connected, only to be broken, if you inserted a plug.
Instead of a fuse that procected against too large currents, you would have to have a fuse that protected against too large voltages and short-circuited if the voltage became too large.

cabraham

Dude, I hate to tell you, but you are the one in need of studying. I am in the final stage of the Ph.D. in EE, & I am in my 34th year as a pro EE. You're lecturing me on what's what, so how much education do you have. BSEE, MSEE, Ph.D.?

With a CCS operating mode, the lamps are wired in series. The switch which turns the lamp on & off is placed across the lamp (in parallel). With both switches shorted, both lamps are off, since the constant current goes through the 2 switches. Opening switch A results in constant current through lamp A. Lamp B remains off since the constant current is diverted through switch B. If the lamp is a 100 ohm value, and the CCS is 1.0 amp, we get 1.0 amp always, & the voltage is 100 volts. Total power is 100 watts.

If we wish to turn on lamp B, we open switch B. Now there are 2 lamps in series, with a 1.0 amp constant current. The voltage is now 200 volts, & the power is now 200 watts. Constant current can be made to work very well, but there is a problem with it.

In the miles of transmission lines from the power plant to the home, there are insulator losses, & conductor losses. The insulator loss is V²G, & the conductor loss is I²R. It so happens that insulator loss is way smaller than that for the conductor. If we generated at transmitted at full current all the time, & variable voltage depending on load, loss is higher than the CVS method.

It's less lossy to generate & transmit at full voltage all the time with current depending on load. This is why we do so. In addition we get another benefit with CVS operation. Constant voltage is a result of constant turbine speed. A side benefit is constant frequency. Since the CVS bus is also CF (constant frequency), synchronous motors can be used for applications where fixed speed is needed. Clocks are a great example.

In addition, it pays to use transformers to raise V & lower I reducing I²R loss further. In doing so we increase V²G loss, but that loss is so tiny in comparison, it's worth it. All of these issues have been thoroughly examined for over a century. Many people do not have a need to know this, but if you have never dealt with power, sources, energy conversion, etc., how can you tell an old pro that they got it wrong?

As far as my needing to learn Ohm's law goes, all Ohm states is that the relation between V & I is R. The V/I ratio equals R. That doesn't explain the pros & cons of CVS vs. CCS operation. What I explained above requires more than Ohm's law gives us. I don't mind people asking for clarification, but I advise all reading these posts to be careful before telling someone they are wrong.

A person w/ limited technical education should think twice before rebuking someone. Learning this stuff takes many years of very intense study. Very few know this topic as well as they would like to believe. Nothing personal.

Claude

truesearch

cabraham: 'current is determined by voltage and resistance, that is a man made condition. It is not mother nature'..... what do you mean by that, what physics principles are you using?
How does 'mother (father?)' nature sort this out, are we close to understanding?

cabraham

What I said was that our power sources, batteries, ac wall outlets, bench top lab supplies, car alternators, etc., are intentionally designed, built, & optimized for CVS operation. The terminal voltage is monitored & regulated to a constant value. A load placed across this CVS terminal pair has the fixed voltage value impressed upon it. The current is then computed per Ohm's Law, I = V/R.

If V is fixed, & R is fixed, then we easily can compute I per Ohm. It's pretty simple. But we need to remember that V being independent & I being dependent is so because the utility company intentionally holds the line voltage as the fixed value. It could work the other way. The power plant could configure their turbines so as to output a fixed value of current, i.e. a CCS. If our wall outlets were CCS instead of CVS, then a load plugged into an outlet would have a fixed current impressed upon it, and with a resistance value of R, the voltage is given by Ohm as V = I*R.

In the CCS case, I is independent, & V is dependent. It's a man made condition. The CVS mode is preferred because conduction loss is way lower than insulation loss. Not only is CVS used, but the value of said fixed voltage is stepped way up with transformer to great values in order to decrease the values of current. This greatly reduces conduction loss even further.

Anyway, that is my point. The notion that "V is fixed, I varies inversely with R", is a man made condition, not Mother Nature. As I said earlier, however, inductors behave like a CCS, whereas capacitors behave like a CVS. That is not man made, but that is Mother Nature. Did this help? I will elaborate if desired.

Claude

truesearch

As I understand it you cannot have a current without an emf but you can have an emf without a current. I think that power supplies are 'intentionally designed' to generate an emf rather than a current, a current needs a complete circuit.... what if no one connects!!...It then seems natural to design electrical appliances to operate on a constant voltage rather than a constant current.
Could a power company generate a 'constant current', waiting for consumers to connect? Are their any examples of such a thing?
I think I am also correct in saying that a constant current source has a very high (infinite) effective output resistance. A constant voltage source has a very low (zero) effective output resistance.
Look forward to enlightenment

mfb

Superconductors can do that.

This is a good approximation.


I think so, too.