Radio today (Sept 1935-Dec 1936)

REPLACING ELECTROLYTIC CONDENSERS Surge, peak, and working voltage ratings explained • APPLICATION of the voltage characteristics of electrolytic condensers to replacement work* is a subject understood only by a relatively few radio servicemen. One of the features at the recent New York I.E.S.M. Show was an explanation of electrolytic condensers by G. V. Peck of the P. E. Mallory Co., Inc. Mr. Peck's talk, which follows, describes the various voltage ratings and interprets their use. "There seems to be a rather general misunderstanding of the various terms used to describe the voltage ratings of dry electrolytic condensers. A complete understanding of these terms and their proper application will unquestionably allow the condensers to be used more intelligently, and will result in their having a longer life with marked freedom from field trouble. "The object of this talk is to give a clear picture of the voltage ratings of dry electrolytic condensers and point out how each affects the operation of a condenser in the usual radio set. "Dry electrolytic condensers have •When available exact duplicate replacement units should be used, for they are designed especially to replace the defective unit. — Service Editor. three different voltage ratings as follows : "Working voltage is the maximum DC voltage that can be applied to a condenser continuously without overheating (see Fig. 3 on drawing). This is the voltage read by the usual high resistance DC voltmeter. The working voltage rating is determined by the voltage used to form and age the anode foil during manufacture. This, however, has nothing to do with the point at which the condenser starts to spark internally on voltage overloads. The actual working voltage applied to each condenser in a radio circuit should be measured at the highest input voltage which might ever be encountered on the radio transformer primary. The working voltage rating of the condenser should always equal or exceed the measured value. Peak voltage rating "Peak voltage represents the total DC working voltage plus the maximum allowable peak AC ripple voltage superimposed thereon. In most circuits the peak AC ripple voltage amounts to 10 or 20 volts (see Fig. 4 on the drawing). The maximum al SURGE VOLTAGE WITHOUT CAPACITOR -SPARKING POINT OF AVERAGE CAPACITOR RATED 450 VOLTS OR ABOVE ' 375 385 1 20 30 40 50 60 TIME IN SECONDS Figs. 1 and 2 show how leakage of condenser regulates the surge voltage — note that surge with 450-volt condenser is greatly reduced. Excessively high working voltage ratings do not necessarily mean added safety. lowable peak AC ripple voltage is dependent on the capacity of the unit and, therefore, must be rated differently for each value of capacity. In general, it may be said that the peak voltage rating of a condenser is relatively unimportant and should not be incorrectly emphasized as a DC working voltage rating. However, if the peak AC ripple voltage encountered in a radio set exceeds 25 volts, it should be taken into consideration. Surge voltage defined "Maximum surge voltage is the maximum voltage a condenser will stand for 5 minutes without breakdown, when the voltage is applied through a series resistance equal to 20,000 ohms divided by the capacity of the condenser in microfarads. It is an arbitrary formula designed to obtain a standard of comparison. However, the resistance value approximates that encountered in radio set filter circuits. The maximum surge voltage rating is not connected directly with the working voltage rating of a unit. H the applied voltage exceeds the maximum surge voltage rating of a condenser, sparking will result and destruction of the unit may follow. Surge voltage measurements may be made with a high resistance DC voltmeter and readings should be taken at the highest input voltage which might ever be encountered on the radio transformer primary. Remove all the tubes from the radio set except the rectifier and disconnect all electrolytic condensers when making the surge voltage measurement. A paper dielectric condenser of one mid. or more must be connected across the rectifier output terminals or the meter will not read accurately. The maximum surge voltage rating of the condenser should exceed the measured surge voltage reading. "The surge voltage rating of a condenser, as is now obvious from the above explanation, remains relatively fixed and does not move up in value in step with increased working voltage rating. This is particularly true of condensers rated 350 volts DC and over. "If the measured surge voltage (as described above) does not exceed the actual surge voltage rating of the eon 38 Radio Today