dimensioning an electrolytic capacitor for an impulse current source

Thread Starter

uhf737

Joined Dec 9, 2017
14
Hi
a) Calculate the capacity of an electrolytic capacitor (Tolerance 20%). V0 of the capacitor is 33V, the voltage after discharging is V1=30V, the supplied current should be 100A for 100us

b) Also calculate the minimal capacity required so that there is enough capacity for discharging until the capacitor reaches the end of its useful life (assume 2000h at 105°C with deltaC/C = +-45%).

a)

First I calculated the capacity required for the discharge of 3V.

Q = 100us*100A = 10mC

C = Q/3V = 3.33mF (which is correct according to the solution)

next is b)

The capacitance changes by +-45% during its useful life at 105°C but why is there a plus and a minus here ?

The solution says 3.33mF = C * 0.8 * 0.55 solved for C it gives 7.58mF so they assumed both tolerance and the capacitance change over time to be lower but why ?
 
Last edited:

WBahn

Joined Mar 31, 2012
29,979
Hi
a) Calculate the capacity of an electrolytic capacitor (Tolerance 20%). V0 of the capacitor is 33V, the voltage after discharging is V1=30V, the supplied current should be 100A for 100us

b) Also calculate the minimal capacity required so that there is enough capacity for discharging until the capacitor reaches the end of its useful life (assume 2000h at 105°C with deltaC/C = +-45%).

a)

First I calculated the capacity required for the discharge of 3V.

Q = 100us*100A = 10mC

C = Q/3V = 3.33mF (which is correct according to the solution)

next is b)

The capacitance changes by +-45% during its useful life at 105°C but why is there a plus and a minus here ?

The solution says 3.33mF = C * 0.8 * 0.55 solved for C it gives 7.58mF so they assumed both tolerance and the capacitance change over time to be lower but why ?
The first answer isn't correct if they are wanting to know what the minimum size of the capacitor that needs to be purchased in order to ensure achieving that performance. Let's say that you found a 20% tolerance capacitor that was marked 3.33 mF. Since it has a 20% tolerance it's actual value could be as low as 80% of this, or 2.66 mF. But that would result in the the 10 mC draw dropping the voltage by 3.76 V, which violates the requirement.

Hopefully that will give you enough to understand their answer to the second question.

In addition (and this is apparently not part of the expectation), you can't find a 3.33 mF capacitor (standard value). The nearest standard value is 3.3 mF, which would be too small even if it were exactly the stated value. The next standard value up is 4.7 uF.
 
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