Applications of inductor


Camera Flash Lamp

The inductor (or coil as some prefer to call it) played important roles in the camera flash lamp circuitry. For the camera it was the important component that resulted in the high spike voltage across the trigger coil which was then magnified by the autotransformer action of the secondary to generate the 4000 V necessary to ignite the flash lamp. Recall that the capacitor in parallel with the trigger coil charged up to 300 V using the low-resistance path provided by the SCR. However, once the capacitor was fully charged, the short-circuit path to ground provided by the SCR was removed, and the capacitor immediately started to discharge through the trigger coil.
An SCR, or Silicon Controlled Rectifier, is a semiconductor, or integrated circuit (IC), that allows the control of current using a small current. Basically, it is a simple direct current (DC) light switch.

Flash camera: basic circuitry
Fig. 1: Flash camera: basic circuitry
Since the only resistance in the time constant for the inductive network is the relatively low resistance of the coil itself, the current through the coil grew at a very rapid rate. A significant voltage was then developed across the coil as defined by Eq.
$$ v_L = L({di \over dt})$$
When you first use the camera, you are directed to press the flash button on the face of the camera and wait for the flash-ready light to come on. As soon as the flash button is depressed, the full 1.5 V of the dc battery are applied to an electronic network (a variety of networks can perform the same function) that will generate an oscillating waveform of very high frequency (with a high repetitive rate) as shown in [Fig. 1].
The high-frequency transformer will then significantly increase the magnitude of the generated voltage and will pass it on to a half-wave rectification system (introduced in earlier chapters), resulting in a dc voltage of about 300 V across the $160\mu F$ capacitor to charge the capacitor (as determined by $Q = CV$). Once the 300V level is reached, the lead marked in [Fig. 1] will feed the information back to the oscillator and will turn it off until the output dc voltage drops to a low threshold level.
When the capacitor is fully charged, the neon light in parallel with the capacitor will turn on (labeled flash-ready lamp on the camera) to let you know that the camera is ready to use. The entire network from the 1.5V dc level to the final 300V level is called a dc-dc converter. The terminology chopper network comes from the fact that the applied dc voltage of 1.5 V was chopped up into one that changes level at a very high frequency so that the transformer can perform its function.
Even though the camera may use a 60-V neon light, the neon light and series resistor Rn must have a full 300 V across the branch before the neon light will turn on. Neon lights are simply bulbs with a neon gas that will support conduction when the voltage across the terminals reaches a sufficiently high level. There is no filament, or hot wire as in a light bulb, but simply conduction through the gaseous medium. For new cameras the first charging sequence may take 12 s to 15 s.
Succeeding charging cycles may only take some 7 s or 8 s because the capacitor will still have some residual charge on its plates. If the flash unit is not used, the neon light will begin to drain the 300-V dc supply with a drain current in microamperes. As the terminal voltage drops, there will come a point where the neon light will turn off. For the unit, it takes about 15 min before the light turns off. Once off, the neon light will no longer drain the capacitor, and the terminal voltage of the capacitor will remain fairly constant.
Eventually, however, the capacitor will discharge due to its own leakage current, and the terminal voltage will drop to very low levels. The discharge process is very rapid when the flash unit is used, causing the terminal voltage to drop very quickly ($V = Q/C$) and, through the feedback-sense connection signal, causing the oscillator to start up again and recharge the capacitor. You may have noticed when using a camera of this type that once the camera has its initial charge, there is no need to press the charge button between pictures-it is done automatically. However, if the camera sits for a long period of time, the charge button will have to be depressed again; but you will find that the charge time is only 3 s or 4 s due to the residual charge on the plates of the capacitor.
The 300 V across the capacitor are insufficient to fire the flash lamp. Additional circuitry, called the trigger network, must be incorporated to generate the few thousand volts necessary to fire the flash lamp. The resulting high voltage is one reason that there is a CAUTION note on each camera regarding the high internal voltages generated and the possibility of electrical shock if the camera is opened.

Line Conditioner

In recent years we have all become familiar with the line conditioner as a safety measure for our computers, TVs, CD players, and other sensitive instrumentation. In addition to protecting equipment from unexpected surges in voltage and current, most quality units will also filter out (remove) electromagnetic interference (EMI) and radio-frequency interference (RFI). EMI encompasses any unwanted disturbances down the power line established by any combination of electromagnetic effects such as those generated by motors on the line, power equipment in the area emitting signals picked up by the power line acting as an antenna, and so on. RFI includes all signals in the air in the audio range and beyond which may also be picked up by power lines inside or outside the house.
Fig. 2: Line conditioner: general appearance
The unit of [Fig. 2] has all the design features expected in a good line conditioner. [Fig. 2] reveals that it can handle the power drawn by 8 outlets and that it is set up for FAX/MODEM protection. Also note that it has both LED (light-emitting diode) displays which reveal whether there is fault on the line or whether the line is OK and an external circuit breaker to reset the system. In addition, when the surge protector is on, a red light will be visible at the power switch.
In the line conditioner, the primary purpose of the inductors is to "choke" out spikes of current that may come down the line using the effect described under the discussion of Lenz's law in this chapter. Inductors are such that a rapidly changing current through a coil will result in the development of a current in the coil that will oppose the current that established the induced effect in the first place. This effect is so strong that it can squelch current spikes of a significant number of amperes in the line. An undesirable result in line conditioners, however, is the voltage across the coil that will develop when it this rapidly changing current through the coil.
However, as mentioned in the previous chapter, there are two coils in the system that will generate opposing emf's so that the net voltage to ground is zero. This is fairly clear when you carefully examine the two coils on the ferromagnetic core and note that they are wound in a way to develop opposing fields. The reaction of the coils in the line conditioner to different frequencies and their ability to help out with the blocking of EMI and RFI disturbances will have to wait until we discuss the effect of frequency on the reaction of an inductor in a later chapter.

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