Digital systems
Power sources (current and voltage) for different physical processes

In circuit theory, it is common to work with ideal (i.e. constant) voltage and current sources. It is less common in case of thermal or hydraulic systems, so it is important to understand the real structure while modeling.

The ideal (constant) voltage source gives exactly the same voltage at its terminals, regardless to the load (a current, being drawn) – theoretically, it has a zero internal resistance. Most of common voltage sources in electrical practice approaches the ideal voltage source. Some of them are very close to the ideal (e.g. mains supply, lead accumulators), others are a bit far from the ideal operation (e.g. primary batteries, small transformers).

Fig. 8.9: Hydraulic analogies of voltage source

In hydraulic systems, the counterpart of electric voltage is a pressure, or a pressure difference. The voltage source is then represented by a constant pressure source, which is not apparently influenced by load (consumption). It is often realized as a large source (e.g. a basin, reservoir, lake or the ocean or a water conduit).

The voltage is analogous to a temperature or a temperature gradient, in thermal domain. The ideal voltage source corresponds to a massive heating or cooling source, e.g. a huge source of water of certain temperature, boiler, massive wall, ground, river or a well regulated source with sufficient rating.

The ideal (constant) current source is able to drive a circuit through its terminals by constant current, regardless to the load (resistance, causing a voltage drop). It can be approximated by a source of sufficiently high voltage with a large resistance in series.

Fig. 8.10: Approximation of constant current source

In hydraulic systems, the current source is analogous to a constant flux source, which is not influenced by consumption, e.g. a water conduit or reservoir with sufficient pressure or a filling without interaction (feedback) of filled liquid (eg. if poured from the top).

The analogy of electric current in thermal systems is a heat flux. The current source corresponds to a source of constant heat flux, independent on a temperature of heated object, e.g. a heater without thermal limitation (protection). Such source (e.g. a light bulb, heating, furnace), if not cooled, can reach a very high temperature, often causing a disaster (firemen can share their experience).

Fig. 8.11: Hydraulic one capacitor system with drainage bore

Most of discussed systems (electric and hydraulic), except the astatic one, were supplied from voltage source or its hydraulic counterpart (source of constant pressure). Let us show several modifications of their structure by use of the current source or its hydraulic counterpart (source of constant flux).