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Operation of Mercury Vapour Rectifiers
On account of the comparatively high voltage drop occurring in most rectifiers of the high-vacuum type, mercury vapour rectifiers are are finding a widening sphere of usefulness for feeding Class B amplifiers, receivers operating at frequencies approaching zero, and for other equipment of fairly high power where the relatively poor regulation and Wattage loss of the ordinary hard valve is a serious drawback.
The rectification of AC for power supply units involves the use of a device which might have the following idealised properties. It must conduct in the required direction with a very low voltage drop and in a manner free from discontinuity which might cause interference. It must be an insulator in the opposite direction, and to be fully effective the transition from these states should occur as the applied alternating voltage passes through zero.
An approach to the ideal rectifier characteristics outlined above is only found in the commutator switch, and here the inclusion of the non-interfering clause renders the device useless for ordinary purposes. To what extent, then, does the mercury vapour rectifier approach the ideal?
It is essentially a high vacuum rectifier into which has been introduced a quantity of mercury giving a 'gas filling' of mercury vapour. On the application of anode voltage, the electrons from the cathode will ionise mercury atoms when a voltage of some thirteen Volts is reached (the ionisation voltage of mercury) and give rise to positive ions as well as negative electrons. The ions, flowing towards the cathode, neutralise the shell of negative space charge surrounding it and remove the barrier to full emission of all available electrons to the anode.
Fig. 1 - Characteristics of vacuum and mercury vapour tubes compared. Note that the latter acts virtually as a short-circuit after the critical voltage is reached.
It is therefore possible to pass anode currents of some Amperes at voltages between 13 and 15 volts, and this represents the only loss of voltage in such a valve - a negligible item in rectifying some hundreds of volts.
Inverse current is zero for voltages in the opposite sense as the anode is a non-emitter and ionisation ceases as the anode voltage is reversed. There is a possibility of radio interference from rectification, and this will be discussed later. It is evident, however, that for rectification of voltages well in excess of 15 Volts the mercury vapour rectifier is ideal, as can be seen in Fig. 1, where anode voltage / anode current curves of a standard type of rectifier and a mercury vapour type are drawn to show the comparative voltage drop at a given anode current. A rough calculation shows that, at 250 mA the mercury vapour discharge represents a loss of some 3.8 Watts, while the hard rectifier wastes nearly 30 Watts, and, moreover, contributes an impedance of some 160 Ω to the rectifying circuit, giving rise to poor voltage regulation. The impedance of the mercury vapour rectifier, under the same conditions, would contribute only 6 Ω and would thus be an inappreciable item in the presence of even a well-designed smoothing choke in its, effect on regulation. Before reviewing a few of the practical aspects of these rectifiers it will be necessary to discuss the precautions necessary for correct operation and freedom from failure.
In the conductive direction the discharge is a close approach to a short-circuit between anode and cathode, and direct application of HT will, unless external circuit resistance exists, result in sufficient current passing to damage the cathode. This will also happen if the filament is operated at too low a temperature, for then a normal value of anode current may represent the total possible emission from the cathode. In these circumstances the positive ions bombard the cathode with considerable energy, there is an accompanying rise in anode-cathode PD, and the emissive coating of the cathode will be destroyed. Actually, visible shattering of the coating can be seen at times under these conditions and it is usually accompanied, when operating with an alternating anode voltage, by breakdown in the reverse direction.
Delayed Anode Voltage
Fig. 2 - A practical HT unit with two gas discharge tubes giving full-wave rectification.
It is therefore necessary when using any power supply unit with such rectifiers to delay the application of anode voltage, preferably by means of a thermal delay switch, until the filaments attain normal temperature before any current is passed. Such a thermal delay switch is now made by most manufacturers of the rectifiers and is operated from a separate LT winding on the transformer, or, if of suitable rating, from the rectifier LT winding. It consists of a bi-metallic strip contact suitably mounted in a high vacuum or inert gas, actuated by a heater connected as indicated. Suitable delayed action mercury switches are also on the market and are suitable for switching a separate primary circuit energising the HT winding.The thermal delay switch, operating in the HT circuit, is shown in Fig. 2, which shows a bi-phase circuit using two rectifiers. The switches are rated for various delay periods at certain heating voltages. For the usual type of rectifier this should be at least thirty seconds. On some large industrial equipments, where large heat-shielded cathodes are found in the rectifiers, the initial heating requires some thirty minutes!
The circuit of Fig. 2 shows a small resistance in the anode lead of each valve. This is an important component in some installations, as, with the application of HT a surge will flow through the smoothing. capacitors during the period of initial charging and may represent an instantaneous short circuit unless some resistance is present. A similar action might be experienced at each half cycle if the rectifier is working under a fairly heavy load, so that the rectifier's output is more of the nature of a series of sharp pulses and imitates the effect obtained when an HT battery is suddenly connected to an uncharged capacitor.
Suppressor Resistances
When operating a radio receiver such a power unit may cause severe interference unless the anode resistances are used in the rectifiers. Quite low values of resistance will be found to produce quiet operation, and 25 Ω will usually be ample, and still sufficiently low to cause no serious regulation troubles in the output voltage.
Fig. 3 - A voltage doubling circuit giving full-wave rectification.
A very useful and interesting rectification circuit, yet one which has not found great favour on-account of its poor regulation characteristics, is the voltage doubling arrangement using two separate valves. In this arrangement, outlined in Fig. 3, two capacitors are connected in series and alternately charged on each half cycle, the DC load being connected to their outer terminals.
The effectiveness of this arrangement is controlled by the ability of the rectifier to fully charge each capacitor under various loads, and the low impedance of the mercury vapour tube makes this possible.
A circuit such as this may be found advantageous in place of the usual bi-phase system, apart from any considerations of voltage doubling. It will have an improved ripple content in the output, for reasons that are easily appreciated. The DC output has a voltage at any instant given by the sum of the voltages to which the capacitors are charged. Suppose the load to be adjusted so that at the instant C2 receives it full charge C1: is drained to half charge; it is evident that the next half cycle will not find C1 drained completely, as this is when its charge cycle begins and C2 begins to drain. In this condition the ripple will be that due to variations between full voltage and three-fourths full voltage, whereas a bi-phase circuit would have fluctuations between full voltage and half full voltage.
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