06-08-2013, 01:14 PM
Yes, while the ESR only indirectly relates to the ripple current rating, it does give an easily measured indication of the quality and condition of the cap.
Many restorers buy their replacement electrolytics online from sources and brands of unknown quality, so the ripple current rating is an unknown. Unless you are buying name brand caps from a reputable distributor, there are usually no published specs for the part. So at least, by measuring ESR, you can get some idea of the suitability of the replacement cap, especially if you have a cap with known specs to compare it to.
My other point is that ESR measurement can give an indication of the condition of the original or replacement cap. Typically electrolytics fail by loss of electrolyte by evaporation, resulting in a gradual increase of ESR. So a quick check of capacitance and ESR can give an indication of imminent failure. I recently checked some NOS twist prong caps, and while most were reading an ESR of less than an ohm, one read 140 ohms even though its capacitance was well within tolerance. Of course, this particular cap was obviously bad and would have failed completely upon use.
Ripple considerations are also very important when replacing small value paper filter caps with electrolytics. I recall a posting recently where someone replaced the 1 and 2 uF filters of a model 20 with tiny electrolytics more intended as bypasses. Needless to say, these replacements will not last long, and mylar film caps would have been a much more suitable replacement.
Many restorers buy their replacement electrolytics online from sources and brands of unknown quality, so the ripple current rating is an unknown. Unless you are buying name brand caps from a reputable distributor, there are usually no published specs for the part. So at least, by measuring ESR, you can get some idea of the suitability of the replacement cap, especially if you have a cap with known specs to compare it to.
My other point is that ESR measurement can give an indication of the condition of the original or replacement cap. Typically electrolytics fail by loss of electrolyte by evaporation, resulting in a gradual increase of ESR. So a quick check of capacitance and ESR can give an indication of imminent failure. I recently checked some NOS twist prong caps, and while most were reading an ESR of less than an ohm, one read 140 ohms even though its capacitance was well within tolerance. Of course, this particular cap was obviously bad and would have failed completely upon use.
Ripple considerations are also very important when replacing small value paper filter caps with electrolytics. I recall a posting recently where someone replaced the 1 and 2 uF filters of a model 20 with tiny electrolytics more intended as bypasses. Needless to say, these replacements will not last long, and mylar film caps would have been a much more suitable replacement.
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