I've been looking for a capacitor to install when the rest of my system goes in, and I've come across caps rated at 16v w/ 20v surge and 20v ones w/ a 24v surge. Both have identical ESRs, so what will the difference in voltage provide? Quicker recharge/discharge?

Given the choice of a 20v half farad cap and a 16v one farad cap, which one would be better for an amp not likely to draw more then 300w (4x75) RMS?

TIA,

Mike

 

Mike,

In a system that only charges at 14.4V, the voltage difference won't make much of a difference during normal operation. As a rule of thumb, caps are usually spec'd into a design with a 50% higher voltage rating than what they see in normal operation to be prepared for voltage spikes, etc. Unless lightning hits your car, or your alternator (and associated components) gives up the ghost, the voltage should remain below that level. Higher voltages across the cap than they are rated for starts to break down the elctrolytic inside, and significantly higher voltages will spark across the plates and eat them up (thereby derating the cap even further).

So, to answer your question, the 24V surge cap won't really provide you with any benefit, only lighten your wallet. Your system will work just fine with 0.5F/20V, but if the 1F/16V is cheaper, you're better off with the extra leeway for system improvement given by the 1F cap.

 

I'm not the EE here, but I thought I've read that in general, electrolytic caps exhibit varying capacitance as the voltlage is varied. That is, they do not always have the same capacitance, it will change depending on the voltage applied to the capacitor. Of course most electronic devices change values somewhat with temperature and other environmental factors.

Anyway, the point is that I seem to remember that electrolytics exhibit higher capacitance as you approach the rated voltage, so you are usually better off (more capacity for your $) to use caps as close to your maximum circuit voltage as you can get. Probably every electrolytic in your amp and head unit is rated for 16 volts.

All that just to agree with MacGyver.

 

Modifry,

I don't remember ever coming acorss that type of information, so if you happen to come across it again, please send me a copy. I'm trying to reason out why that may or may not happen, but I can't decide one way or the other.

Capacitance is dependent upon numerous factors, but the overriding ones are plate size, plate separation distance, and electrolytic type. The voltage rating is also dependent upon the last two factors for sure...as an example, as the plate separation distance decreases, the chance of arcing across plates increases, therefore the voltage rating must drop.

Using a cap as close to the rated voltage goes against the standard practice of derating cap values by 50%, but I have no information to either support or refute the claim. I know I have detailed articles somewhere in my ton (literally, more than a ton, a friend and I once calculated because he got tired of helping me move them ) of magazines and books about all types of caps, but God knows where they are exactly. Maybe I can do some digging.

These are the types of discussions that really interest, the ones that push my current knowledge to the limit and force me to expand it ever so slightly. Time to do a little reading...