I am assuming from over voltage from no load on the power supply. Is there a math model on the amount of ESR you should aim for in a power supply cap? I know with voltage regulators there is feedback and sometimes you need a low ESR cap, but too low and it will also oscillate. I remember a thread that discusses this but I can't for the life of me find the discussion.
Think about what the numbers mean.
Suppose you have a 100uF cap, and it filters 120Hz (Full Wave 60 Hz). Capacitive Reactance, Xc = 2 * pi * f * C Xc = 2 * 3.14 * 120Hz * (100 * 10e-6) = 13.3 Ohms Suppose the ESR is Very large, 1 Ohm. The total is about 14.3 Ohms.
If the load current is 0.05A (50mA), the ripple will be 0.05A * 14.3 Ohms = 0.715 V peak to peak, or 0.506 Vrms.
Now suppose the ESR was 0.0000000000001 Ohms. The total is about 13.3 Ohms. If the load current is the same 0.05A (50mA), the ripple will be 0.05A * 13.3 Ohms = 0.665 V peak, or 0.470 Vrms.
0.506/0.470 = 1.0766, a 7.66% difference in the ripple voltage. Now, use a 110uF cap that has a Very large ESR of 1 Ohm, and you will have better results than the 100uF cap that has 0.0000000001 Ohm ESR.
In either case, you will need more filtering for a practical power supply. C R C, or C choke C, etc. Each filter cap Xc and ESR will have similar effects.
Just one way to look at it.
Suppose you have a 100uF cap, and it filters 120Hz (Full Wave 60 Hz). Capacitive Reactance, Xc = 2 * pi * f * C Xc = 2 * 3.14 * 120Hz * (100 * 10e-6) = 13.3 Ohms Suppose the ESR is Very large, 1 Ohm. The total is about 14.3 Ohms.
If the load current is 0.05A (50mA), the ripple will be 0.05A * 14.3 Ohms = 0.715 V peak to peak, or 0.506 Vrms.
Now suppose the ESR was 0.0000000000001 Ohms. The total is about 13.3 Ohms. If the load current is the same 0.05A (50mA), the ripple will be 0.05A * 13.3 Ohms = 0.665 V peak, or 0.470 Vrms.
0.506/0.470 = 1.0766, a 7.66% difference in the ripple voltage. Now, use a 110uF cap that has a Very large ESR of 1 Ohm, and you will have better results than the 100uF cap that has 0.0000000001 Ohm ESR.
In either case, you will need more filtering for a practical power supply. C R C, or C choke C, etc. Each filter cap Xc and ESR will have similar effects.
Just one way to look at it.
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As you all have figured out, ESR in power supply caps isn’t about ripple voltage. It is about ripple current, as in I^2*R losses. If it’s enough to get the cap hot, then worry about it. If it’s not, don’t.
The real reason to consider a film cap over an electrolytic in a power supply (assuming it’s not overly large or expensive) is so that you don’t have to replace it in 30 years.
The real reason to consider a film cap over an electrolytic in a power supply (assuming it’s not overly large or expensive) is so that you don’t have to replace it in 30 years.
This is a very good reason, since there is no chemical reaction to eventually breakdown or actually not react. But for me i believe that caps like the Panasonic DC LInk are a better choice, because they seem to be more dynamic and realistic than lytics.
OK, Here we go again.
We need to re-calculate the available DC current, versus the transformer's current rating.
This is not a fixed ratio, it depends on the type of rectifier and filter setup.
Here is a good link for those calculations:
Design Guide For Rectifier Use - Hammond Mfg.
And I believe some of those configurations might change the integral of (I)squared * R.
Like the R as in ESR, and the resultant heating of the capacitor.
We need to re-calculate the available DC current, versus the transformer's current rating.
This is not a fixed ratio, it depends on the type of rectifier and filter setup.
Here is a good link for those calculations:
Design Guide For Rectifier Use - Hammond Mfg.
And I believe some of those configurations might change the integral of (I)squared * R.
Like the R as in ESR, and the resultant heating of the capacitor.
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Are you still debating whether to go with film or electrolytic? I have not seen your opinion about your choice. Just wondering....
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