Doubling rule

When the differential is decidedly kept equal to or less than the designed-in value, the life of the elko is then determined by the familiar doubling rule—every 10°C fall in core temperature (from its maximum rated) the life doubles. That is how we can finally get the required 44k hours. For example if the core is correctly estimated to be at 65°C, then the calculated life of a 2000 hour capacitor is actually 2000 x2x2x2x2x2 = 64k hours. 

Capacitor manufacturers recommend that in general we don t pass any more current than the maximum rated ripple current. This ripple current is the one specified at the worst case ambient (e.g., 105°C). Even at lower temperatures we should not exceed this current rating. No temperature multipliers should be used. Because only then is the case to core temperature differential within the design specifications of the part. And only then are we allowed to apply the simple 10°C doubling rule for life. 

If the measured ripple current is confirmed to be within the rating, we can then take the case temperature measurement as the basis for applying the normal 10°C doubling rule, even if the heat is coming from adjacent sources. Again, that is only because the case to core temperature differential is actually within the capacitor s design expectations. 

However, in direct customer communications, Chemicon has, at least in the past, allowed a higher ripple current than the rating. But the life calculation method given is then slightly different. This amounts to a special doubling rule every 5°C, which we will describe below using a practical example. 

The Access term in the previous equation should be omitted if the ripple current is equal to or less than its rated value. In other words, Access is not allowed to be negative. In that case we revert to the usual 10°C doubling rule (i.e., just omit the 2 /5 term in the equation above). 

The succinct, empirical rule lifetime is halved for each 10 °C increase in temperature is widely used in the electrical industry and elsewhere. A detailed comparison with Arrhenius formula shows that for the range of activation energies (50 to 150 kj/mol) and temperatures (20 to 200 °C) commonly encountered in electrical engineering the doubling rule is usefully correct [8]. 

Nontronite is the exception to this double rule. It can, however, be accepted for all smectites having a predominance of A1 or of Mg in the octahedral layer. 

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