Voltage, Current, Resistance: Simple Illustration & Ohm's Law

[cabraham]

An example of a current source is an energized inductor. If the terminating impedance suddenly changes, the current will be maintained but the voltage will abruptly change. Of course the current is sustained for only a finite time, as the inductor gives up energy. In switching power converters, LED drivers, & motor drivers, this principle is made use of. I will elaborate if need be. The capacitor is an example of a device that emulates a voltage source. If the energized capacitor discharges into a resistance, then that resistance changes, the voltage will be sustained while the current abruptly changes to a new value. Of course this is for a finite time as well.

A photodiode works very well as a constant current source. Light incident upon the device produces current. Usually, a PD is placed across the inputs of an op amp, with a resistor as a feedback element. Light on the PD generates current, which must pass through the feedback resistor. The non-inverting op amp input is usually at ground, so that the output is simply I*Rfdbk.

Another example is a generator, such as the alternator found in a car. Since most batteries operate much better in constant voltage mode, the regulator adjusts the alternator field current to maintain constant terminal voltage. But said alternator could just as well have its current regulated to a constant value. As loading changes, field current could be adjusted for steady current, & the voltage varies with load resistance.

Constant voltage vs. constant current regulation are options we have when building power sources like batteries or generators. Batteries seldom are built for current source operation, losses are too great. But nuclear battery cells, not commercially available yet (maybe never), are known to work best as current sources. Generators are easily regulated for current or voltage.

I can elaborate if needed. BR.

Claude :-)

 

[LvW]

A photodiode works very well as a constant current source. Light incident upon the device produces current.

Hi Claude - is there really no E-field involved? Which force then allows the movement of charges which were set free by photons?

 

[cabraham]

LvW - Yes there is no E field involved. I am at work. When I get home tonight I will scan my text (Sze - semiconductor physics). A charge can move under influence of an E or B field, as well as photonic interaction. E/M waves through space or through solid material such as conduction is both wave-like & particle like, neither model is complete. When photons strike a conductor, electrons are elevated from valence to conduction band, & currnet is generated. This is the photoelectric effect for which Einstein received a Nobel prize. Photons have Energy per Planck's Law E = hf. To raise an electron from valence to conduction requires an increase in energy which the photon provides.

An LED works in the opposite manner. Electrons in conduction drop to valence due to recombination. Valence being a lower energy state (more negative) than conduction means that the difference in energy must be accounted for. In this case photons are emitted, E = hf. All the energy is accounted for.

I hope this helps, I will elaborate if needed. Best regards.

Claude

 

[cabraham]

Reference: Sze & Ng, "Physics Of Semiconductor Devices", Wiley Interscience, 2007, 3rd Edition

Claude