| Cold cathode type-only used for decorative lighting | ||
| Hot cathode | ||
| a, High pressure fluorescent | ||
| (i)Mercury vapour types )Used mainly for street lighting | ||
| (ii) Sodium type , )but sometimes used for deck lighting | ||
| b, Low pressure Hot Cathode Fluorescent type discharge Lamp | ||
Principle- A length of glass tubing contains a small amount of mercury vapour and argon gas, at a very low pressure ( 10-6 atmospheres ). A heater element forming an electrode is situated at each end of the tube. These electrodes may be coated with an oxide to improve thermionic emission. The interior of the tube is coated with fluorescent powder ( a Phosphor coating)
If a suitable voltage is applied between the two electrodes a DISCHARGE strikes between them and the mains voltage is then sufficient to maintain the discharge. This occurs in low pressure so that the lamp will run at a comparatively low temperature and so will not effect the fluorescent coating. The electrons from the electrode collide with the mercury atoms. This dislodges an electron from the atom making the mercury atom a positively charged ION. As the dislodged electron returns to the influence of the ION ( i.e. the electron changes from one energy level to another) a certain amount of electro-magnetic radiation (i.e. a photon) is given off in the form of Ultra-violet light. These rays activate the fluorescent coating and the luminous surface provides a glare free efficient light.
Operation -With switch start circuits to start a discharge across the tube a large Voltage Impulse is required.
There are also methods using starterless circuits, referred to as rapid start or instant start,. where a drop in potential between the electrode and an earth strip is sufficient to ionise the gas adjacent to the electrode and this ionisation then spreads across the whole tube.
C1- Radio suppresser
Shortly the bimetal strips cool sufficiently to break contact, This sudden reduction in current flow causes a large e.m.f to be generated in the choke ( typically four to five times mains)
The voltage surge across the tube is sufficient to ionise the gas, reducing the resistance to electron flow and allowing the discharge to occur and be sustained by the mains voltage.
Operation may still occur if the mains voltage is reduced, however the tube is unlikely to start hence this type of light is not used for emergency lighting
The choke has a second purpose other than providing the start voltage. It maintains a constant correct potential difference across the tube when the mains is an alternating current.. If a d.c is used then a ballast resistor ( which may be an incandescent light) must be used
This may be obtained by the following methods;
C2- Power factor correction
Glow type switch starter circuit
When the control switch is closed the contacts on the bimetal strips which are open form the electrodes of a small discharge lamp. The mains voltage is sufficient to cause a glow discharge in the starter which warms the bimetal strips. The strips bend until contacting and a large current flows through the electrodes of the main tube, forming an electrode cloud around their cathodes (thermionic emission)
Advantages-
Disadvantages-