Thread: just simple common sense

well ..with all the future scientists surfing this site...i would just ask a few basic common sense questions
1)why is it that a bulb glows at the instant of switching on the switch while a tubelight 1st flickers and then glows..(in india)?
2)what does a basic camera flash gun circuit have the components in it(just give the description)??
3)ideally how(small or large) should the input and output impedances of a transistor be ??....(no kudos for guessing)

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Let me just deal with your second question, that of the electronic flash.

Electric energy from a battery or accumulator (or other sources, such as household current) is converted to high voltage (300 volts or more) and is used to charge a capacitor. The converter often makes a high-pitch sound which you can hear when the unit is charging. The capacitor is permanently connected to two electrodes in a glass tube ("bulb") filled with xenon gas. At this stage, the gas does not conduct electricity and emits no light.

Another, small, capacitor is charged at the same time as the big one. When the flash unit needs to fire, this small capacitor is discharged through a transformer, which generates a pulse of very high voltage (several thousands of volts). This voltage is applied to a third electrode in the xenon tube. The high-voltage pulse causes the gas to ionize. Ionization makes the gas conductive, and the big capacitor starts to discharge through the xenon gas.

Bright light is emitted by the xenon gas during this process. Since the resistance of the gas is very low at this stage, the discharge is rapid, with the current following an exponential curve.

About 1/1000 - 1/200 seconds later the capacitor is essentially empty, and the voltage has dropped so low that the xenon stops to conduct electricity, and the light pulse dies off. At this point, the process can be started from the beginning.

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Here is the answer to part 1.

A fluorescent lamp works by a flow of electrons through the tube which ionizes mercury vapor causing it to emit ultraviolet radiation which is converted to visible light by the coating on the inside of the tube. The tube conducts electricity quite readily once the current begins to flow, but in a conventional lamp, the line voltage is not high enough and it needs to be started in stages.

First the cathodes at each end of the tube are heated so that they emit electrons which ionize nearby gases. Then a surge of high voltage is applied. This is conventionally done by the starter contacts opening, which breaks the circuit to the anode heater and interrupts the circuit to an inductor in the ballast causing an inductive kick. This provides enough voltage to get the current flowing, and the line voltage is then sufficient to keep the current flowing. The ballast then limits the flow of current.

There are instant-on fluorescent lamps, which apply a high enough voltage to start the current flow without a need for the starting sequence described above.

well what mr stevef has written is for a very high voltage ...ihad asked for the one used in cameras i.e.two cells each of 1.5v =3v.how does this work??

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Yes, I was indeed referring to flashes operating on only one or two cells of 1.5v each. The electronics in the unit is quite capable of converting this small voltage to much higher voltages.

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Here is a good explanation of the flash circuit.

http://electronics.howstuffworks.com/camera-flash3.htm

Originally Posted by SteveF

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Let me just deal with your second question, that of the electronic flash.

Electric energy from a battery or accumulator (or other sources, such as household current) is converted to high voltage (300 volts or more) and is used to charge a capacitor. The converter often makes a high-pitch sound which you can hear when the unit is charging. The capacitor is permanently connected to two electrodes in a glass tube ("bulb") filled with xenon gas. At this stage, the gas does not conduct electricity and emits no light.

Another, small, capacitor is charged at the same time as the big one. When the flash unit needs to fire, this small capacitor is discharged through a transformer, which generates a pulse of very high voltage (several thousands of volts). This voltage is applied to a third electrode in the xenon tube. The high-voltage pulse causes the gas to ionize. Ionization makes the gas conductive, and the big capacitor starts to discharge through the xenon gas.

Bright light is emitted by the xenon gas during this process. Since the resistance of the gas is very low at this stage, the discharge is rapid, with the current following an exponential curve.

About 1/1000 - 1/200 seconds later the capacitor is essentially empty, and the voltage has dropped so low that the xenon stops to conduct electricity, and the light pulse dies off. At this point, the process can be started from the beginning.

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Some neon sign transformers output about 7,500 volts.

Sincerely,


William McCormick