Silly question about voltage and current

[dieter]

I have a question: Do the voltage and the current for a cetrain load have to come from the same source?


Or could I, example given, use two independent sources like photopholtaic and wind or whatever, then go out of phase with each one by -90 resp. +90 degrees with a cap and a coil, then ac- sync this so in none of them is actally both (v+a) present, then simply but these together , maybe trough some diodes, to get a complete, working source of V+A ?

[TinselKoala]

I have a question: Do the voltage and the current for a cetrain load have to come from the same source?


Or could I, example given, use two independent sources like photopholtaic and wind or whatever, then go out of phase with each one by -90 resp. +90 degrees with a cap and a coil, then ac- sync this so in none of them is actally both (v+a) present, then simply but these together , maybe trough some diodes, to get a complete, working source of V+A ?

Huh? Voltage is the pressure that pushes current through a load. It is impossible to separate voltage and current. Even the highest impedance voltmeter there is (electrostatic voltmeter) needs some tiny current put into it by the voltage it is measuring. And even the slightest possible current needs some tiny voltage difference between the source and sink in order to flow.

Now.... you can certainly do weird things like putting sources in series. For example, my old RM503 oscilloscope has a CRT with a 12 volt filament, that draws a few hundred mA when operating. BUT.... this filament is raised to 3000 volts over the system's ground level, because it plays a role in the emission of the electron beam in the CRT. This is done by feeding the filament with a winding from the main power transformer, biasing it from the scope's HV section and limiting the voltage _difference_ across the filament so that only the small current flows, not the current that would happen (very briefly!) if the filament was hooked directly across the 3000 volt source. The scope didn't work when I first got it, it kept blowing fuses and I finally figured out it was because of insulation breakdown within the main (expensive and complicated) power transformer: the 3000 V biased winding was shorting to another winding within the transformer somehow. How to fix this without replacing the transformer (impossible to do anyway). So I got a good quality 12 volt transformer from Radio Shack, powered its 120 v primary from the mains, and let the secondary be elevated to the 3000 volt level by the scope's voltage multiplier, and I drove the filament from that. It works 100 percent fine now. So... in this sense yes, the 3000 volt elevation is provided from the HV section of the scope, and the actual current through the filament is provided by the 12 volt winding on the RadioShack transformer. Since the RS transformer's secondary is not inside the main transformer the original short is eliminated, and the insulation in the new transformer is good enough to keep the 3000 volt bias elevation of the secondary from leaking through to the primary and causing the same problem as before. The filament current is running on the voltage difference between 3012 volts and 3000 volts and this current comes from a different source than the 3000 volt elevation.

There--- are you completely confused even more now? Don't feel too badly, it took me quite some time to track down this problem and understand it to the point where I could repair the scope. It's a classic antique though and has some unique features, so I really was motivated to fix it, and I learned a lot while doing it.

[dieter]

So. that's a yes?
actually, isn't voltage and current sperated when they are out of phase? As in a coil the voltage is established earlier than the current and in a cap later, there is a moment in the ac cycle where both, the coil and the cap contain only v or a (?)


What if we tap this very moment only, by switching? Both is there, v + a, although maybe not as a perfect sinus wave.

[TinselKoala]

So. that's a yes?
actually, isn't voltage and current sperated when they are out of phase? As in a coil the voltage is established earlier than the current and in a cap later, there is a moment in the ac cycle where both, the coil and the cap contain only v or a (?)


What if we tap this very moment only, by switching? Both is there, v + a, although maybe not as a perfect sinus wave.

Well, not exactly. Yes, you can certainly "have" a voltage without current, as in a disconnected battery.... but any _measurement_ you make of the voltage will require a tiny current at least to flow through or into your instrument. This also means that any _use_ of the power requires current to flow. The phase shift in AC power feeding a reactive load means that the overall power is split into Real or True power (that powers or is dissipated in a load) and Reactive Power which isn't dissipated in the load,  and Apparent Power which is the total power. The distribution of the input into these parts is dependent on the phase angle.
http://www.allaboutcircuits.com/vol_2/chpt_11/2.html

[Turbo]

That is of cource the pre historic view on electricity and most of the old lad's think of it that way.
But in the real world, there is no voltage nor is there current flow and these elements only show up when the time component is taken into the equation.
What it means is that if you were to freeze time, all voltage would dissapear, and all current would stop flowing.
If you look at for example the pre historic equation, Power(in Watts) = Voltage(in Volts) x Current (in Amps) you can already see that there is no mention of the time component, and that therefore this equation is pre historic and incomplete.
Because from this equation, you can not tell if the total sum is per second, minute,hour,day or whatever and if you need to build a perfect model in a perfect world where time does not exist, you will run into problems with this.
In the classical sense it would be Joule per second, but when you are unable to measure the time component, you will run into problems with this.
I think it is interesting to see that the only place where this is really important, is in the area most people on this board are intrested in, and for the rest of the world this would not make any difference as long as the light comes on when they flip the switch...
But only so few understand this and only so few know what it is really about it all depends on your point of view so, from which point you are looking and measuring, to understand this you need to develop a complete shift in perspective because the 'normal' view is too limited to be able to explain and understand it.

It is impossible to separate voltage and current.

Only if you think it is.