Blocking Condensers

Radio Record, Volume III, Issue 27, 17 January 1930, Page 29

Blocking Condensers

A.C. Resistance N ordinary wire-wound anode re‘sistance, rated at, say, 20,000 ohms, is a hard path for any current to pass through. Even if the wire of which it were wound could carry it, it would take an electrical pressure of 20,000 volts to drive a current of one ampere through, this component. Its resistance would be about the same for alternating as for direct current, A. fixed condenser, however, will not pass direct current, although it will offer a path to alternating currents, and the higher the frequency of the alternating current the lower the re‘sistance a condenser of a given capacity will offer, and the larger the capacity of the condenser the lower its A.C. resistance. ae These are facts the constructor "should bear in mind. It would be no use using a .0002-mfd. fixed condenser ‘in connection with a loudspeaker bypass shunt.. The effective A.C. resistance such a component would offer to a frequency of 1000 is somewhere around eight megohms. For such work a condenser having a capacity of 2 imfds. is generally specified, and, at the same frequency, its effective A.C. resistance would be only 80 ohms (this is small in comparison with the resistance of the average type of loudspeaker), For the .0002-mfd. fixed condenser to assume an effective A.C. resistance cf 80 ohms the current handled would yhave to have the frequency of a million cycles, and this is not a low fre"quency, but a radio frequency such as is encountered when 2L0 is tuned in ‘(at this frequency the 2-mfd. condenser would offer an impedance of a mere 1-12th of an ohm.) (CONDENSERS of the 2-mfd. order _ are frequently used as. H.F. shunts or by-passes. A 2-mfd. condenser is often connected from the "primizig" grid of an 8.G. valve to earth, and, as you can see, it offers a very short-cut’ to H.F. currents. The resistance of the H.T. battery might be 100 or 200 ohms, but not much H.F. would reach this while there is an alternative path available of only 1-12th or so of an ohm in resistance. ae And regarding condensers, comparatively recently the electrolytic variety has become available for general use.

The electrolytic condénser is a remark‘able device in that it achieves an enormous capacity with remarkable compactness. An electrolytic condenser rated at 2000 mfds. will be only a matter of a few cubic inches in size. The Electrolytic Type. N principle it is something like a . battery that has no depolarising agent. When a voltage is applied across its two terminals, thin films of gas are formed across the plates that are immersed in the chemical solution. The plates and the electrolytic solution act as do the plates in an ordinary condenser, while the thin film of gas forms the dielectric. And it is because the. gas film is so thin that the great capacity: is obtained. : You will notice that it is by the application of voltage that the gas film is formed, and that therefore this type of condenser cannot be used purely for A.C. work. A steady potential must be applied for the device to function. It can, of course, deal with alternating currents that are superimposed on D.C., such as are met with across the output terminals of an L.T. unit, but the A:C. must not exceed a certain proportion of the total current,

Hlectrolytic condensers are most effective as smoothers of irregularities in L.T. supplies. Heére one often meets low-pitched hums of the order of 120 or so ‘cycles. At this frequency an electrolytic condenser of 2000 mfds. will have an A.C. resistance of only two-thirds of an ohm, while its résistance to pure D.C. will be at least a thousand or two ohms. You may be surprised that these devices have resistances of such a character and not of almost infinity, as in the case of the ordinary type of condenser, but’ you must remember that it is, as its name implies, of an electrolytic character! At a frequency of 1000 cycles, such as ohne might come across in the commutation ripple of D.C. mains, the 2000-mfd. electrolytic condenser will have an A.C. resistance of about 1/12th of an ohm, and its resistance to frequencies of the order of 8000 cycles will drop to 1/40th of an ohm. These are remarkably low resistances in comparison with the resistance; of a filament circuit, which will at least be of the order of tens of ohms. Thus when the current irregularities divide between the two paths that constituted by the by-pass condenser and. the other by the filaments of the valve, the proportion that flows ‘through the latter is ‘dropped to a negligible degree. Meanwhile the D.C. flows through its usual channel in an uninterrupted manner. It is interesting to note that the resistance of the 2000-mfd. electrolytic condenser to a radio frequency of a million will be about a thousandth part of an ohm, but, of course, there are no duties to be found for a component of that nature on the purely R.F. side of a receiver.