[bobs bots]

============== KL7212 Power supply lifetime calculation =========
OK for the curious , I will do a worked example of a lifetime calculation.
(a) assume capacitor is the life limiting component, (2000hrs at 85C at 6A each with 30milliohms)
(b) transformer should be OK as they will usually run at 125C for decades
(c) assume the current drawn is full rated for 1minute and 25% of rated for 9 minutes in every 10minute period. (Typical for CNC applications)
(d) assume self heating in the capacitor due to rated current and ESR produces a 40C increase in still air.
(c) assume a 20C ambient, as the power supply is not inside a larger sealed enclosure, and assume at sea level.
(d) assume you mount the supply vertical, with modest natural ventilation with 12 x 20mm holes in the bottom and top

===Step one , work out cap temp rise with our duty cycle====
The self heating effect is due to ohmic heating due to the ESR (Equivalent Series Resistance) times the current squared.
For continous current of 6Amps per capacitor, (which have a 30millohm ESR) the power is 6^2 * 0.03 = 1.08W (which we assume produces an internal temperature rise of 40C in the cap).
At continuous 25% rated current (1.5 A per cap) the power is 1.5^2 * 0.03 = .26W , which will raise cap temperature by only 10C.
For our duty cycle we need to add the 10% of heating at full current to 90% of heating at 1/4 current. or 0.1* 1.08W + 0.9*0.26W = 0.35W , raising cap temperature by 13C.

=== Step 2 work out cap temp ====
We need to work out the ambient temperature in the vicinity of the cap, which will always be higher than that in the room. I'm just going to pluck a number out of the air and say the bridge rectifier heats up the local air by 10C, so local ambient is 20C + 10C =30C, now add the cap's temp rise of 13C to this and get 13C + 30C = 43C

==== step 3 work out the capacitors life ===
This goes according to the Arrhenius equation where
life is proportional to exponential of temperature ratio, too complicated for this forum, so just use the doubling for every 10C approximation.
As the capacitor has a 2000hr lifetime at 85C , and it is now 42 degrees lower (this is 4 sets of 10C plus a bit) , it will last for 2 x2 x2 x2 longer (4 sets of doubling) or 16 x 2000hrs or 32000hrs
=== step 4 work out how many years it will last ======
OK , it's only used 20Hrs/week so 32000hrs/20hrs/wk = 1600wks=31years
This is pretty good, and adding a fan will probably double this again.

On the other hand, consider the exact same power supply, delivering 100% rated current, 24/7, mounted horizontally, inside a sealed cabinet, at a steel foundry at 15000' elevation, at 30C air temperature. Then the deltaT of 40C gets doubled by the density effect of high altitude to 80C, which is added to ambient of 30C, which is added to the temp rise inside the sealed cabinet of maybe 30C, for a capacitor temperature of 140C, this is 55C hotter than the capacitor rating (i.e.5.5 sets of 10C) so the life is reduced by a factor of 45, from 2000hrs to 44hrs, i.e. less than 2 days.

Pretty amazing difference, Huh?

This is why you get so many different opinions on the usefulness of cooling, in fact adding a fan (sucking in outside air) to the foundry example will increase the operating life to about the rated life of 2000hrs.

Cheers, Bob T