Protecting Components

Radio Record, Volume III, Issue 25, 3 January 1930, Page 31

Protecting Components

Avoiding T. rouble with Breakdowns

{VERY radio component has a definite voltage or current limitation. If we impress too much voltage across a fixed condenser we destroy its internal insulation. Likewise, excessive current in the filament of a valve can burn it out. AS a general rule, components ‘are worked well within their safety lines.: In one or two instances, however, care must be taken that the right standard or type of component is used for a definite job. A typical example of this is the fixed condenser which figures in resistancecapacity audio stages. This condenser is joined between the plate of one valve and the grid of another. The grid of this second valve will also ‘be joined through a grid leak via the grid bias, and to A-, earth, B-, and so on. Thus practically the full voltage of the "B" supply will be impressed upon that coupling condenser. Obviously then, this particular component must be of such construction that it can withstand that. pressure. For this reason, one generally sees that for such work a mica condenser is advised in set specifications. Mica condensers are quoted for the simple reason that mica dielectric condensers of any origin are invariably capable of standing moderate. voltages, whereas there are a good many paver dielectric condensers which are not quité as satisfactory as they should be in this respect. . This is unfortunate for the one or two very satisfactory paper condensers that are available. In power packs one: has to be very careful in regard to fixed condensers, Here you are liable to get the full voltage of the A.C. mains impressed across the terminals of such a component. A Very Important Point. Wt A.C. mains one comes against * greater voltages than nominal ratings. The specified voltage of A.C. mains will be calculated on a basis of what is known as a root mean square. The alternating current is rising and falling from zero tO a maximum all the time, and 250-volt mains will have a voltage rising to well over 300. One does not have to take this into account in any ordinary current resistance calculations, but it is of importance in regard to fixed con-: densers, and that is why condensers that are tested at voltages round about double that at which the niains are rated should be used. Additionally, of course, in power packs one comes up against voltage surges which have to be taken into account. A properly constructed air condenser is practically indestructible, but, as the dielectric constancy of air is unity, this scheme can only be employed at

the sacrifice of compactness. A 4-mfd. air condenser suitable for a power pack would assume sometling of the dimensions of a bookcase, Loud Speakers. 'TURNING to the current handling capacities of other components, do not operate on the assumption that, provided the gauge of wire used seems to be such that a certain amount of current can be passed through it, one is safe if he works within those margins. An obvious example is the loudspeaker. , By passing a current of * D.J. character through the wrong way,.: the permanent magnets which figure in many kinds of speakers can be demagnitised. The result will be insensitivity, In the case of an audio choke, too much D.C. may temporarily reduce the inductance to " value worthless for the purpose. Injury can be done to some types of. audio transformers of small sizes. By short-circuiting the primary winding across the "B" supply the core charac-

"St teristic can be completely upset. The smallness of these audio transformers is generally achieved by using a nickel and-iron alloy for the core stampings, instead of Stalloy, which i: a silicon steel, | \ , It should be noted that electrolytic condensers of the type generally met with in these circumstances require careful handling. A common rating is 1500 or 2000 mfds, at 12 volts or so. This 12 volts represents a definite limitation. -If much more than this is impressed across its two terminals the component may be completely ruined. Also, electrolytic condensers, being of a polarised nature, must be connected up in one definite way. And, in. conclusion, a word or two about fuses. An H.T. fuse should figure in every valve set. ‘The purpose of such may be twofold. Valve filaments can be protected against burnouts by shorted H.T., and protect the H.T. battery itself from the injurious results of short-circuit: In the case of a mains unit a fuse should’ figure in each of the input leads, and these fuses should be such that they break down at currents not too much in excess of those dealt with by the apparatus.