current operating vs voltage operating electronic devices

[Mr RB]

I pretty much agree, but I can think of at least one example. By reversing your inductor example, consider an uncharged capacitor. We can (and should) charge the capacitor with a current source, and in that first instant, there is no voltage. The field (and voltage) is a result of the charges getting on the plates. Of course, in that instant when even those first electrons are moved, an electric field is present, but it is the charge movement that generates a net electric field which manifests the potential voltage. ...
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Thanks for replying. I expected a capacitor argument in rebuttal and understand your point about the "charge movement" but I don't think it is a perfect inversion of the voltage-first argument. With the inductor we can start from zero and apply an instantaneous voltage, which will eventually cause a current. With a capacitor how do you apply an "instantaneous current" to its two pins? No current can flow between two pins UNLESS there is a voltage differential.

Or for a clearer example that the inductor what about a spark gap or ionised gas device like a neon bulb? Enough voltage must be applied before the device is eventually "activated". I would classify that as a voltage activated device, and it is roughly comparable to a LED or PN junction that requires enough voltage to activate it to cause it to function, and once activated the amount of current then determines the effect. Likewise the bandgap point raised by Mr_Al above.

So in a face off of current vs voltage activated/operated/controlled I think the vast majority of electronic devices are really voltage controlled, especially if the context is general electronics where we are more concerned with HOW we control a component, rather than what might be happening inside a component from a pure physics viewpoint.

But I would still describe something like a LED as "current controlled" to a beginner as the important thing for them to know when controlling a LED is to make sure they get the current right!

 

[steveB]

I don't think it is a perfect inversion of the voltage-first argument. ... With a capacitor how do you apply an "instantaneous current" to its two pins? No current can flow between two pins UNLESS there is a voltage differential.

I don't think it's a perfect example either, which is one of many reasons why "I pretty much agreed with you". I also agree with MrAl that energy is an important factor and both current and voltage are working together. There are ways to move charges without a voltage differential, by mechanical means, but that's not typically how we use the capacitor in practice. However, a key point is that it is not enough to have a potential difference. One also needs to establish a conduction path. So yes we establish a small voltage initially because trying to separate the charges on the plates generates a reverse electric field that would cause the charges to go back to their equilibrium positions. However, the charge flow is the impetus that must be provided as the input to generate the output electric field between the plates. The electric field between the plates and the charge separation are essentially the same thing and represent the stored energy. That energy came from both the voltage and current.

For the inductor, yes again the voltage would seem to be the drive input, with current as the response. However, it is the flow of current that generates the magnetic field which is storing the energy. It's just too hard to separate voltage and current and call one the absolute control. But, I do feel the voltage control viewpoint is typically much more useful. Voltage, by definition, is energy per unit charge, and with energy being such a key concept, it's hard to argue against that viewpoint.

But I would still describe something like a LED as "current controlled" to a beginner as the important thing for them to know when controlling a LED is to make sure they get the current right!

I would also take that approach. Further, I would describe the capacitor as current controlled to a beginner because the capacitor law i=C dv/dt does not like to have instantaneous changes in voltage. So current should be stressed and the voltage dilemma can be resolved with the idea that we drive a capacitor with a voltage ramp (dv/dt) and not a voltage step. If we do apply a voltage step, then it is typically with an in line resistance to allow the capacitor voltage to ramp up. A voltage source with an inline resistance essentially is a varying current source in this mode of operation.

 

[MrAl]

Hello again Roman and Steve,


Good example bringing up the neon bulb i think Roman. The band gap in insulators is much higher than in the semiconductors so it takes more voltage, but the voltage cant get there without some current flowing. But yes this is an example where you would want to be careful not to emphasize the current too much.

The LED is on the other side of the coin, there the current make the most difference and is the most controllable variable, so that too makes sense to explain mostly on the basis of a controlled current.

What i was trying to do is help to reduce the mistakes made by mostly newcomers when they do things like try to drive MOSFETs. As i said before i think maybe in this thread, when they look up MOSFETs on the internet they see, "The MOSFET (or just FET) is a voltage controlled device", and that turns off their thoughts about what the current is, and that ends up with a high speed circuit with a 100k resistor driving the base
I've seen this so many times now that the only explanation is that most of the web sites specify it as voltage controlled and so the reader looses site of the current requirement. And i see others still making this mistake by quoting junction theory with limited viewpoint.

So if any of you see another "MOSFET high speed switching circuit" with a 100k resistor driving the base, you'll know why it got there

 

[Mr RB]

Thanks SteveB for explaining that, of course you are right the perfect theoretical cap is a pretty good inversion of the perfect inductor, and that voltge and current really do work as a pair (once they are working).

And again more good points form MrAl.

I think our industry has tied itself in a bit of a knot, in that a lot of the ways we explain things and even calculate things is based on old assumptions that are completely ingrained in the industry. (Let's not even start on conventional current!). And where we have to do some things in a particular way because our components and instruments are not "perfect".

Instead of concrete statements "this component IS current controlled" maybe we should say to beginners "we should think of this component as current controlled, to be best inline with how we would use it". Maybe acknowledging a separation between what we really need to focus on to model/use a component and what is really happening inside the component. For example the "current through a capacitor" arguments.

 

[MrAl]

Hi again,


Yes, that's all i have been saying here also.

I think the most concise statement that can be said about current vs voltage for operating a device is that for any device the current and voltage follow a TRAJECTORY in the voltage/current plane. That says it all in a nutshell.

 

[MikeMl]

...
I think the most concise statement that can be said about current vs voltage for operating a device is that for any device the current and voltage follow a TRAJECTORY in the voltage/current plane. That says it all in a nutshell.

Oh, you mean what I said in post three of this thread?

 

[MrAl]

Oh, you mean what I said in post three of this thread?

Hello Mike,

Oh another wise guy in the bunch huh? [just kidding here ]

Really though what i did was plot the V I for just about any device and showed that the trajectory was the most concise view from the standpoint of external control. What you did was mention that the slope was important. The slope can be derived from the trajectory, but i think you were right about mentioning the slope too.