|
This circuit demonstrates the principle of a (old) starter of a fluorescent lamp like in the following diagram:
https://en.m.wikipedia.org/wiki/Fluorescent_lamp#/media/File%3AFluorescent_Light.svg
The following parts will match with each other, first the part from the picture, then the part in the circuit:
A: the fluorescent tube. In the circuit seen as the combination of the horizontal lamp and the relay in series with the resistance;
B: the AC supply. In the circuit seen as the AC supply in the top left corner;
C: the starter. In the circuit seen as the combination of the capacitor, relay in series with the resistance and the DC circuit with potentiometer. The N.C. contact of this relay in series with the resistance represents the gas inside the starter;
D: bi-metallic switch inside the starter. In the circuit seen as the N.O. side of the relay mentioned in subject C;
E: capacitor parallel over the starter. In the circuit seen as the capacitor mentioned in subject C;
F: the filaments to heat up and keep lit the fluorescent tube. In the circuit seen as the 2 combinations of transformers and lamps;
G: the ballast, or coil. In the circuit seen as the coil in the top right corner.
When power is first applied to the circuit, there will be a glow discharge across the electrodes in the starter. This heats the gas in the starter and causes one of the bi-metallic contacts to bend towards the other. When the contacts touch, the two filaments of the fluorescent lamp and the ballast will effectively be switched in series to the supply voltage. The current through the filaments causes them to heat up and emit electrons into the tube gas by thermionic emission. In the starter, the touching contacts short out the voltage sustaining the glow discharge, extinguishing it so the gas cools down and no longer heats the bi-metallic switch, which opens within a second or two. The current through the filaments and the inductive ballast is abruptly interrupted, leaving the full line voltage applied between the filaments at the ends of the tube and generating an inductive kick which provides the high voltage needed to start the lamp. The lamp will fail to strike if the filaments are not hot enough, in which case the cycle repeats; several cycles are usually needed, which causes flickering and clicking during starting (older thermal starters behaved better in this respect). A Power Factor Correction (PFC) capacitor draws leading current from the mains to compensate for the lagging current drawn by the lamp circuit.
Once the tube strikes, the impinging main discharge keeps the cathodes hot, permitting continued electron emission without the need for the filaments to continue to be heated. The starter switch does not close again because the voltage across the lit tube is insufficient to start a glow discharge in the starter.
How to use:
1 Close the switch top left. A small current runs through the circuit, no lamps will light up yet;
2 The N.C. contact of the bottom relay in series with the resistance represents the current flowing through the gas. The potentiometer circuit represents the temperature of the gas. Move the wiper from 1% to 100%. The relay will change over to the N.O. contact and represents the bi-metallic switch closing. Now the bi-metallic switch is closed a bigger current will flow through the circuit, lighting up the 2 lamps representing the filaments to heat up the fluorescent tube;
3 The gas will now cool down. To simulate this, move the wiper from 100% to 1% again. At a certain time, the bottom relay changes over from the N.O. contact to the N.C. contact, representing the bi-metallic switch opening again. This will open the circuit forcing the coil top right to induce a peak voltage of around 1 kV.
4 The fluorescent tube will strike. This is simulated by the top relay in series with the resistance and horizontal lamp. Once struck, the only way to switch it off is to turn off the switch in the top left corner.
5 If the fluorescent tube is not struck, repeat above mentioned steps from point 2.
|
