Theory
Voltamper characteristic
The Voltamper characteristic is one of the basic characteristics of the electrical and electronic elements. The volt-ampere characteristic is a graph of the current flowing through the appliance to the electrical voltage connected to the appliance. It is just the basic shape of the characteristic and the general course of the current and voltage dependence that many tell us about the electrical element. The shape of a characteristic can be influenced by many parameters. Thanks to this, the "voltamper characteristic" can also be the whole system of graphs of dependence of the current on the voltage whose shape is influenced by some other variable as a parameter - eg voltamper characteristics at different temperature, pressure, magnetic induction etc.
Most components or appliances have a linear voltamper characteristic. In some cases, the voltamper characteristic is not completely linear, but for simplicity, small nonlinearities are neglected and the components are considered linear. For example consider a resistor. Conversely, non-linear components have a voltameric characteristic in the shape of a non-linear curve, such as exponential but also other curves (diode, thermistor, stabilizers, rectifiers…).
Electrical static and dynamic resistance
Static resistance:
For conventional appliances, such as resistors, the relationship between electrical voltage and current is referred to as the electrical (static) resistance of the appliance. If the resistance of the appliance is constant, the voltage-to-current dependency of the Ohm law and the current-to-voltage ratio are straight. The electrical resistance is given by:
where U is the voltage on the appliance and I is the relevant current flowing through the voltage at the given voltage.
Since the graph is a straight line, the resistance is still the same, it can be computed by the relation (1) from any pair of corresponding values. The resistance value R also corresponds to the linear voltamper characteristic.
Differential (dynamic) resistance:
Fig. 1 – Example of a non-linear V-A characteristic
Its significance can be shown as follows: If there is little change in the voltage ΔU around the value of U around the given point P, the resulting current will change by changing ΔI as if moving along a line fixed at point P (see Figure 1) . The value of the directive in this line determines the magnitude of the differential dynamic resistance. Its value is again dependent on the position of the point P (compare the slope inclined at points P1 and P2 in Fig. 1) and is determined by the non-linear dependence of the element by measuring the differences ΔU and ΔI or theoretically by calculating the derivative:
The differential resistance curve tells us the tendency for the element to increase or decrease. Specifically, for the bulb, the electrical resistance (static and dynamic) of the bulb filaments depends on the temperature ("ignition"). The bulb has a metallic filament which heats up strongly with the passage of the current and its resistance is greater than cold during operation - see Temperature dependence of the metal resistance.
Bulb
Temperature light source
The largest and oldest group of artificial sources of light are thermal sources, so called incandescent. Inkandescence is the phenomenon of light emitted by heat excitation. In these sources, light is produced as one of the components of electromagnetic radiation induced by the high surface temperature of a body (burning paraffin candles, passing the electric current through the fiber… etc).
Conventional incandescent bulbs work on the principle of so-called electroincandescent – inducing high temperature through the passage of electric current through a high-melting, conductive substance such as carbon or tungsten. The solid is heated to the desired temperature, where visible light is emitted. Tungsten (in the case of the first carbon light bulbs) fires in the light bulbs.
The common properties of temperature sources are:
Very low power efficiency of light energy conversion (efficiency increases at power consumption),
large proportion of the energy emitted in the form of heat (the bulk) compared to visible light,
continuous distribution of light in the spectrum,
subjectively pleasant perception of light by the human eye.
Specific power of the source
The specific power of the light source indicates the efficiency of the transformation of the electrical energy into the light. It is equal to the ratio of the emitted light flux (lm) of the light source and its electrical input (W). Specific power is used to compare the efficiency of light sources. It is denoted η [eta] and is given in lumens per watt (lm/W). If it was possible to prevent the source from emitting at other wavelengths than visible, we would have a luminous efficiency of 251 lm/W. This value represents the theoretical maximum to which artificial white light sources may approach. Only 37 % of the maximum efficiency of the monochromatic source is 683 lm/W.
Table no. 1: Approximate values of some conventional light sources.
| source |
type |
Specific output power
(lm/W) |
Wattage
(W) |
| bulb |
classic |
6 – 16 |
15 – 200 |
halogen
(mains voltage) |
24 – 30 |
60 – 2000 |
halogen
(12 V voltage) |
11 – 19 |
5 – 75 |
| compact fluorescent lamp |
"Energy saving bulb" |
50 – 87 |
5 – 55 |
| LED white |
|
20 – 150 |
0.04 – 180 |
High pressure
sodium lamp |
conventional
street lights |
70 – 130 |
50 – 250 |
source: Měrný výkon světelných zdrojů aneb účinnost podruhé. ARIGA S.R.O. FotonMag.cz:
aneb nejenom LED, svítilny, baterky a baterie v jiném světle [online]. 28. 12. 2007 [cit. 2015-08-14].
Dostupné z: http://www.fotonmag.cz/osvetleni/merny-vykon-svetelnych-zdroju-aneb-ucinnost-podruhe/
Bulb as light source
Light bulbs are the most common representatives of temperature sources of light. Due to its wide range, small installation and maintenance requirements are nowadays widespread sources of light. Classical light bulbs have a specific output of only 6 – 16 lm/W. For halogen bulbs, the efficiency of the halogen regeneration cycle is up to 30 lm/W. A major drawback of the bulbs is the switching current, which is up to ten times the nominal current (see below) due to the low resistance of the cold fiber.
The temperature of ordinary light bulbs in the range of40 – 200 W ranges from 2000 °C do 2640 °C. For lower power bulbs, the temperature is lower. Threading the fiber at such high temperatures causes a gradual erosion of the fiber when the evaporated tungsten is deposited on the inner wall of the flask. This reduces the cross-section of the fiber and thus changes the parameters of the entire bulb. In order for the fiber to not burst immediately, it is placed in a glass of ordinary glass from which air is exhausted. For standard 15 W lamps, the bulb is usually vacuum-vacuumed (vacuum-filled), with stronger bulbs filled with nitrogen and argon, but also (less often) krypton or even xenon. These fillings allow for higher operating temperatures of the fiber and reduce aging due to sputtering or evaporation. For standard and large bulbs, the charge is chosen so that the pressure in the bulb is approximately equal to the atmospheric pressure. The statistical lifetime of conventional bulbs is about 1000 hours of lighting.
- glass fask
- tungsten fiber
- carrier hooks
- inlets
- pumping (evacuation) tube
- socket
- central contact
Fig. 2 – Main parts of the bulb
Fig. 3 – Effect of voltage on basic bulb parameters.
source: HABEL, J. a kol.: Světelná technika a osvětlování.
FCC Public, Praha, 1995, ISBN 80-901-9850-3
The luminaire's luminosity is dependent on the current that heats the fiber. Each heat source should be operated at rated parameters (most commonly the rated voltage). When reducing the supply voltage by 5 %, the bulb's light output decreases by about 18%. When increasing the rated voltage by 5 %, the luminous flux increases by about 24%, but the lamp life decreases by 50 %! (See Figure 3) Overloading of the bulbs thus results in a significant reduction in service life.
The luminosity of the bulb also significantly affects its frequent switching on. For the lamp, the worst current peak is when it is switched on when the fiber is cold and the shock is very strong (current up to ten times the nominal). Halogen bulbs are even more sensitive to this start. The current-on-off impact can be limited, for example, by using a so-called soft-start circuit which slowly (1 – 3 s) flashes the bulb and thus increases its lifetime (up to 5×). Or, for example, in directional car lights, the flicker frequency is set to increase the life of the bulbs so that it does not completely cool down due to the thermal inertia of the filament - note that the bulb does not completely dimming when the directional lamp breaker is working properly.
