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| | | | | | | | | | | | | | | | | | | | Jntroduction | | | | | | | | | | 1 Introduction | | | | | | | | | | 1.1 Principle of operation In order to obtain a brief intense flash, a very high transient current is passed into a xenon bulb designed for this purpose. The current is generated by discharging a bank of manufacturers of capacitors, and may reach hundreds of amps but will flow for times on the order of a millisecond or less. The capacitance value can be as high as 4,000 microfarads (uF), charged to voltages as high as 400 volts. Between flashes, the capacitance is recharged. The charging circuit passes a smaller current over a correspondingly longer period. A separate igniter circuit is needed to trigger the flash. A xenon bulb normally has a very high resistance, and will not allow any current to flow even when it is directly connected across the fully charged storage capacitors. However, if a much higher trigger voltage [on the order of tens of thousands of volts] can be induced across the bulb, some of the gas in the bulb will ionise and a spark will jump between its two electrodes. This spark provides a conduction pathway through which the capacitance can discharge, and the energy released by this current flow both maintains and propagates the ionisation of the gas in the tube. The current thus rises very rapidly and the bulb effectively presents a short circuit across the capacitance. The capacitance thus discharges extremely rapidly. As the capacitance discharges, the current falls, until it is no longer sufficient to maintain the gas in its ionised state. The bulb then reverts to a high resistance, allowing the capacitance to be recharged and the process to be repeated. Although the principle is simple, the problem is that the igniter circuit must appear in series between the capacitors and the bulb. Not only must it generate a very high voltage (which appears across the bulb because that is the high-resistance part of the circuit), but it must also present a very low resistance (or more strictly, impedance) to the flash current. Some other designs have tried to avoid this problem by isolating the trigger circuit from the discharge pathway, but that just transfers the demands to the isolation circuit. We solved the problem by devising an igniter circuit of sufficiently low impedance for it to be placed directly in the main current discharge pathway. As well as being a more elegant solution, our design is much more compact. and it is small enough to be incorporated within the lamphouse itself. The charger circuit and storage capacitor bank are also very compact. This has been made possible by using switchmode power dosing technology for the charger circuit, and modern capacitor types that are relatively small in size, without any detriment to performance - in fact they both probably give a performance improvement over other designs. | | | | | | | | | | 1.2 Minimising electrical interference A particular problem with flash photolysis is the generation of electrical interference. This can be especially troublesome when sensitive electrical measurements are being made at the same time, as is often the case in physiological applications. We have done our best in designing this unit to keep the generation of such interference to an absolute minimum. The steps that we have taken are: 1. Careful design of the igniter circuit. The igniter necessarily generates several tens of thousands of volts, and signals of this type seem to be particularly effective at crashing professional computers. We have used a "balanced" igniter design, in which the high voltage is produced symmetrically with respect to earth, in order to minimise its pickup by other equipment. Furthermore, the igniter voltage is produced within the lamphouse, so the lead lengths are short and the lamphouse provides a degree of electrical screening. 2. The main discharge pathway carries very high currents as noted above. The current-carrying leads in the interconnecting cable are screened to minimise the radiated interference from this | | | | | | | | | | ___Page 1 | | | | | | | | | | | | | | | | | | | |
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