Voltage Regulating Devices

Radio Record, Volume V, Issue 15, 23 October 1931, Page 16

Voltage Regulating Devices

Methods of Preventing Fluctuation

By

Cathode

| HERE are two main types of \ voltage regulating devices. One is concerned with the ‘ac. input to a receiver and operates to maintain the voltage of this substantially constant notwithstanding line yoltage variations. The other is applied across the output or part of the output of a B supply de vice, and is intended to: maintain. the B voltage approximately constant i: spite ‘of variations in the current dra: from the unit. The two types of device may als«. be distinguished by the receivers in which they are incorporated, Th first type is usually found in a low. priced receiver in which. the safety margin of transformer and condenser: has been made so small as to render protection against line fluctuations essential. The second is seldom found in any commercial receivers, but is the more expensive and is intended to improve the results of the receiver rather than to protect any of its components against breakdown. Line Voltage Regulator. Wwe shall deal with the first type. tl: line voltage regulator first. In essentials all these devices are th game. Whatever their outward appearance, they contain a resistanc element, usually a wire winding, havying an extremely high temperature co efficient. Their operation is simple. Assume that we have such a device connected in ‘series with the primary of a power transformer feeding a set. Under normal operating conditions, say, th« set draws half an amp at 100 volts. Under these circumstances the voltage regulator has a working resistance of 20 ohms, thus reducing the line voltage of 110 volts to the vequired 100 volts and running just = slightly

warm. Now, owing to a temporary reduction in the load on the -supply line, the voltage rises to, say, 120 volts. The first and obvious result is that the set will draw more current through the regulating resistance. The second and less patent effect is that th« inereased current will heat the voltag: regulator, the resistance of which will by reason of its high temperature coefficient, increase to, say, 80 ohms. The drop in voltage across a 80-ohm re sistance will be 15 as compared with 10 volts across a 20-ohm resistance Thus the voltage applied to the power ‘ransformers will rise from 100 to 105 ius compared with the rise to 110 whic! might have been anticipated. Fig. 1 shows a typical set of curve: for a device of this kind. Fig. 2 shows the simple manner } which it is connected. a

It is unfortunate that the available types’ of regulators are designed for insertion in a 110-volt line and not for the 230-volt supply common in New. Zealand. One particular regulator (i.e., Amperite 545) is suitable

lor sets dcdrawilbg Dail all AIMpere, Uy eyer, so that the larger receivers tak: ing 100 watts or more from the supply jine may .have this unit incorpor..cd in them. © Where a receiver is wired for 110 volts and this is de rived -from a step-down transformer the appropriate unit may be wired

in series with the transformer in the receiver proper. The step-down transformer. will not. usually require any protection.

Perhaps the writer may be permitted to express his own private opinion that power apparatus should be designed on sufficiently generous lines to render line voltage regulators unnecessary. Internal Voltage Regulator. OMING now to the other type of voltage regulator, this is represented by the UX874 voltage regulator tube, which maintains a constant potential drop of 90 volts over a wide range of current values. It is normally connected ‘in’ parallel with the lower portion of a potential divider across the output of a B supply unit as in fig. 3. .The tap to which éne terminal of the 874 is connected maintains the required voltage of 90 almost irrespective of the drain from this point. It will also be clear that other tappings on the potential divider will maintain a much more constant potential than would otherwise be the case, A’ voltage of 180 can be maintained absolutely constant if desired by connecting two of these tubes in series. If the UX874 is examined it will be found to be simply a neon glow tube of unusually accurate characteristics. To start the characteristic blue glow a potential of 125 volts is necessary, but as soon as the tube flashes the increased current through the potential divider drops the voltage across the tube to 90, where it stays until the current through the tube falls to less than 5 milliamps, a condition not likely to arise if’ the tube is connected across suitable points. The maximum current which the 874 will pass is 50 milliamperes, Apart from its value as a voltage regulator, the UXS874 is extremely helpful in reducing back coupling, partieularly in an audio amplifier. Considered from this viewpoint, it has been found equivalent to a condenser

of 26 mfd. connected in the same position. The UX874 is particularly commendedgto the notice of the amateur transm@ter as a means of maintaining the oscillator voltage constant in a lowpowered M.O.P.A. transmitter. It is entirely possible, with its help, to use the same plate supply for both oscillator and amplifier, the oscillator voltage remaining perfectly steady when the amplifier is keyed. .