Dear All,
With all the available literature on Film and Aluminum electrolytic capacitor comparison, it is clear that direct replacement (one to one) is not possible. We know that the capacitance requirement for Film type will be much less as that of Aluminum Electrolytic one. Is there any thumb-rule or standard procedure for replacement from Film to Al Electrolytic or vice-versa?
Your views will be much helpful.
PS: I have referred the attached IEEE paper (Available as open source) by Prof. Dr. G. Narayanan from Indian Institute of Sciences, Bangalore to find the capacitance value for my application. This will be a Film capacitance value.
Regards,
Aalok
Probably the thumb rule is that here is no thumb rule; only hints. Film capacitors can easily handle 1A/µF, inelectrolytic it is rather 0.01A/µF. Film capacitors are much cheaper and smaller if the design relates to current. But In energy storage electrolytic capacitors are much better than film. In practice, the film capacitors are designed with a high ripple voltage at 100Hz. So this requires a different type of converter, it is even worth to change even the topology.
Well said, Alex: There is no thumb rule, only hints !!!
Aalok,
I will just add a bit to Alex's response - again there are no guidelines for this.
Since the Al-electrolytic capacitors are polarised elements, in many DC applications you will find the voltage across them to be marginally higher than what one expects it to be going by theory of a particular circuit configuration !
Just by how much does the voltage get boosted by electrolytic effect ? Again this depends on the specific technology, and there are no standard rules !!
-Sanjay
Additional consideration is related to the long term reliability - aluminum electrolytic capacitors have expected life time at elevated temperatures significantly less than film capacitors and due to the higher losses they dissipate more power. A set of film capacitors equivalent to one electrolytic (based on the same ripple current) have higher surface to volume ratio and less internal temperature rise critical to the converter lifetime.
Alexey,
OK, but you can also put a number of electrolytic capacitors in // to have a larger surface, it is done.
The electrolytic capacitors corrode internally, and have a 'bath tub' failure probability, at the end of life the internal resistance increases, finally getting into a thermal runaway and more HF ripple (feedback systems get unstable by a higher gain). LC filters with film capacitors are much less damped. This means that a PID controller might be needed where a PI controller could be enough with electrolytic capacitors.
So even if you replace an electrolytic with the same value in capacitance, it will not be the same, unless one adds series resistance to the film capacitor, which is normally "not done" for a design engineer.
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