The difference between same spec capacitors

Thread Starter

Mark Flint

Joined Jun 11, 2017
129
When I look for an aluminium electrolytic 4000uF 50v capacitor I can find them ranging in price from £2 to upwards of £50.

So they have the same voltage and capacity rating but they are physically much different in size.
How are they different - is it just their life expectancy?

Thanks.
 

ajune

Joined Oct 24, 2017
10
Hi,
Made from what and where ,country contribute a price difference. So you only concern to the price...?, not the functions.
What you worry on that.
 

WBahn

Joined Mar 31, 2012
26,398
When I look for an aluminium electrolytic 4000uF 50v capacitor I can find them ranging in price from £2 to upwards of £50.

So they have the same voltage and capacity rating but they are physically much different in size.
How are they different - is it just their life expectancy?

Thanks.
If they are different in size, then they are NOT the same spec capacitors since dimensions are part of the spec. But even if we ignore physical specifications and only consider the electrical specs, there is much more to it than just capacitance and voltage rating. What they had to do to either the physical size to achieve some of the electrical specs or what they had to do to the electrical specs to achieve a certain physical size (and the same goes for cost), is going to make them quite different. That's not to say that the difference matters in any particular application, but you can pretty well bet that it matters in at least some applications, otherwise the two different caps probably wouldn't exist.
 

Thread Starter

Mark Flint

Joined Jun 11, 2017
129
Let's say the application is a buffer, or temporary storage, and I'm trying to figure out how much time a capacitor can power a load, do the capacitance and voltage specification give info I can use to calculate this - or do I have to take some other attribute like physical dimension?

Eg, if I have 2 aluminium electrolytic caps, both rated at 4000uF 12v; one is 3" high and 3" diameter, the other is 0.5" high and 0.5" diameter, which one will power the same DC 12v load the longest?
 

kubeek

Joined Sep 20, 2005
5,772
That depends on the load, but for light loads with the capacitance the same the time would also be the same.
But it would be best if you could show which capacitors are you choosing between. If they are not new stock, it could be also the age difference, where a 30 year old cap will be physically larger than a new one with the same rating.
 

Thread Starter

Mark Flint

Joined Jun 11, 2017
129
Thanks kubeek - I don't have a particular project in mind - just trying to fill some of the many gaps in my understanding :)

It makes sense to me that the frequency which it is charged/discharged would be an important factor and affect the temperature and life expectance perhaps drastically. In my mind I would expect to need a physically larger cap to comfortably handle, say 20Khz vs 50hz.
 

MrChips

Joined Oct 2, 2009
26,097
You need to back up a bit and cover some basic electronics fundamentals.

A 4000μF 12V capacitor will not supply a DC 12V load for very long.
Here are some fundamentals. The basic parameter of a resistance/capacitance circuit is called time constant. You can calculate this by multiplying the resistance times the capacitance.

For example, if the resistance is 1kΩ (this would draw 12mA from a 12VDC source),
time constant = 4000μ x 1k = 4 seconds

So what does this mean?
It means that in 4 seconds the voltage across the 4000μF capacitor charged to 12V will drop to 37%, i.e. to about 4V when there is a 1kΩ load connected across the capacitor.

The time for the capacitor voltage to drop from 12V to 11V is about 0.4 seconds. Not very good, is it?
This is the same regardless of which 4000μF capacitor you choose.
Bear in mind that of you want a 12V supply, your capacitor should be rated much higher that 12V, at least 16V or 25V.

When you introduce the frequency of charge and discharge into the equation, that is a different analysis altogether.
At 50Hz, there is a longer time between charge cycles and hence the voltage is going to drop much lower than at 20kHz.
Hence at 50Hz you will need a much larger value capacitance (actually, 400 times larger).

Furthermore, we have to look into the heating effect of the AC voltage on the capacitor. The deeper discharge scenario will cause the capacitor to generate more heat. This is where the physical size and the ESR of the capacitor play the most importance.
 
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