CHAPTER ONE
1.0 INTRODUCTION
A Low Power Indicator can be defined as an indicator that is having a very low electrical supply or current situated in an electrical instrument which indicates the amount of power present in the instrument by their colour indication be it red, yellow or green.
1.1 TYPES OF A LOW POWER INDICATOR
A. LIGHT – EMITTED DIODE (LED): - This is a junction diode made from the semiconductor gallium arsenide phosphide which has large band gap and hence a larger forward drop than silicon. Typical flooded panel – type LED indicators look good at 10MA forward current on a board inside an instrument one can usually get away with 2.5MA, particularly if one use a narrow-angle LED. When forward biased it emits red, yellow or green light depending on its exact composition. No conductor or light emission occurs for reverse bias which if it exceeds 5V, may damage the LED.
B. LIQUID CRYSTAL DISPLAY (LCO): - This is a display that is usually is a driven by an a.c wave form otherwise their liquid guts (contents) are ruined. However, it driver chips have some way to generate a square – wave segment drive, synchronized to the LED backplane wave form. An example is the HC4543, the LED cousin of the HC4511 LED latch/decoded driver. LCD come in rather large panels that can display a line or two or text i.e they have ability to store several message and communicate via serial ports.
C. LOW – BATTERY INDICATOR USES FLEA POWER: - This is a low indicator that consumes as little power as possible for a 9V, 450-MAhr alkaline battery, a 50 – PA low battery indication can by itself run the battery down in a little more than a year battery – powered devices that need to run continuously for a long time require battery indicators that consume minimal power.
1.2 PRACTICAL EXAMPLES OF LOW POWER INDICATOR
An example of low power indicators are:
PEAK LEVEL INDICATOR
A simple regulator circuit for use with the peak level indicator is shown in figure below
The sensitivity of the circuit does vary slightly with variations in the supply rail potential, but this is not likely to be a problem. If the unit is powered from a various power supply unit (RSU). Any changes in the supply voltage are then likely to be too small to be significance.
The situation is rather different if the unit is to be battery powered since the voltage of a volt dry battery can vary by more than 20% during its normal working life. It is therefore advisable to use a stabilizer circuit between the battery and the peak level indicator if this form of power source is to be used.
L.E.D ZENER P.S.U
It was mentioned earlier bias a L.E.D has a forward bias characteristic which is rather like that of low voltage Zener diode. L.E.D can serve a dual purpose it can be used as a low voltage (about 1.7v for a Til 209) Zener diode, and will provide excellent results of course, it can also be used as a ordinary On/Off indicator at the same time. The circuit diagram of the L.E.D Zener P.S.U shown in figure 1.1.
The figure 1.1 gives the circuit diagram of a simple low voltage stedilised P.S.U which uses a L.E.D as both a Zener diode and the On/Off indicator. This can provide an output voltage of anything between approximately 2.4v and 12v at a continuous current of up to 100MA.
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