afesheir

AllVol

Re: current source

See wikipedia for a good article.

__________________
All those who believe in psycho kinesis, raise my hand.

stevez

Regarding the dielectric I'd have to say that it all depends on the dielectric. Many plastics can serve as dielectrics - and the properties of many plastics are profoundly affected by things like temperature, moisture and stress. Glass makes a good dielectric and moisture probably doesn't affect the dielectric properties. It's a pretty broad question - is it homework or for something else?

__________________
stevez

ljcox

Transistors are current sources.

There was a mechanical device called (I think from memory) a Van de Graff generator. It has a moving belt that carries charge to a receiver and it is a current source since the charge carried on the belt is constant per unit of time.

__________________
Len

Russlk

There is a current source diode, I think it is a field effect device.

__________________
see my website: www.geocities.com/russlk

Sceadwian

Currents are created by a voltages so even a 'current source' is still really just a variable voltage source that self regulates to provide a steady current by varrying the voltage. In the case of the transistor the varying voltage effect only exists on a molecular level. That's one of the reasons transistors are such magical devices, they can control current by the selective application of voltage, the current that flows through the base junction of a transistor is actually just a byproduct of the voltages and the non-issolated nature of a bipolar transistor.

__________________

ljcox

Agreed, a transistor in the active region is a voltage controlled current source. The collector current is an expotential function of the base - emitter voltage. FETs (both JFETs and MOSFETs) are also current sources in the active region. The drain current is controlled by the gate - source voltage.

__________________
Len

Sceadwian

A tunnel diode also functions in a similar manner. After a certain point increasing voltage results in decreasing current.

__________________

ljcox

Yes. I've never studied the tunnel diode in detail, but I know they have a negative resistance region.

I once saw a circuit that used a tunnel diode as a threshold detector.

__________________
Len

Othello

Until the charge accumulation on the upper terminal repels the charges on the belt with such force, that the belt slows down.

Uwe

afesheir

thanks lot for your answers ...
but I want further information than steves said about my first question (about the factors affecting the dielectric constant)

stevez

I'll just add a tiny bit more about plastics - the chemical purity/composition and molecular weight of plastics is quite variable even within the same batch out of a reactor. I think it is likely that plastics or more specifically, the materials used for dielectrics are made with tighter controls on the things that most siginficantly affect dielectric properties. What those things are would likely be well known by capacitor manufacturers.

Again, it's a pretty broad request.

__________________
stevez

Roff

I think the current source question was about 'native' power sources that have a high output resistance, as opposed to voltage sources, which have a low output resistance. Solar cells (and photodiodes) are the only thing that comes to mind.

Attached Images

solar cell i-v curve.gif (8.4 KB, 5 views)

__________________
Ron

RadioRon

Ok, here's some quotations from "The New College Physics" Albert Baez, Freeman and Co, 1967:

"when a substance is placed into an electric field, E, every single electric charge, q, within it experiences the force qE; positive charges are pulled in the direction of E, and negative charges are pulled in the opposite direction. Whether the charges move depends upon the substance, and the motion itself depends on the strength of the field and the time during which it acts. In metals there are many so-called free electrons that can travel over large distances within the sample. The atoms of dielectrics, compared with those of metals, have so few free electrons that they are virtually non-conductors, or insulators. The molecules of a dielectric bear positive and negative charges, of course, and each of these experiences a force when the dielectric is placed in a field, but only a slight separation of the charges within each molecule takes place. The effect is called POLARIZATION. Mobility is limited to molecular dimensions."...

they then present an example...
" When an uncharged slab of glass, G, is placed between the charged plates, A and B, of a capacitor, there is no migration of electrons, but the effect of the charge separation induced by the field is a net accumulation of polarization charges on the upper and lower faces of G."
now put a picture into your mind of a simple circuit containing a battery, a switch and our capacitor with glass between the plates.
"when the slab of glass being absent, the switch is closed, charges flow to the plates of the capacitor until the potential difference across the plates is the same as that of the battery." No current flows when this is completed." When the glass, G, is brought between the plates, polarization charges appear on the upper and lower surface of the glass."
Negative polarization charges appear right next to the positive plate of the capacitor, and positive polarization charges appear next to the negative plate of the capacitor.
"The appearance of positive polarization charges on the lower (negative) plate weakens the resultant repulsive electrostatic force, and more electrons can therefore come from the battery to reside on the plate. The capacitance of the combination has thereby been increased."

there in a nutshell is the reason why a dielectric material can increase capacitance. The relative dielectric constant is the number that states how much more the capacitance has increased compared to the case where there is only a vacuum between the plates.

__________________
RadioRon