What is the proper way to determine the resistance value of the capacitors when plugging in the data?
There's a default value of 2 ohms on C1, C2, and C3. I do I know what vlue to enter there?
There's a default value of 2 ohms on C1, C2, and C3. I do I know what vlue to enter there?
An externally hosted image should be here but it was not working when we last tested it.
Get the ESR off the caps' spec sheet, then add the predicted trace or wire resistance for your layout (remember both traces, to and from, will be in series!). 2 ohms is the default, but it's much higher than what you'll typically use.
The manufacturing tolerance of the VT rectifiers and/or the series resistance of the choke will swamp out the effect the ESR of the filter caps, they're the least of your worries... Remember that you are not building for NASA, just a geetar amp ;-)
That resistance value has bearing on how the circuit operates, surely. 2 ohms would basically be like putting voltage straight to ground, no?
Furthermore, when I read a 60uF cap that I have on hand with an ohmmeter I get 20m or so. How could I possibly ignore the difference between 2 ohms and 20 megs?
Furthermore, when I read a 60uF cap that I have on hand with an ohmmeter I get 20m or so. How could I possibly ignore the difference between 2 ohms and 20 megs?
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No it's not a DC resistance, it's an AC impedance - you're really MASSIVELY over thinking this - just look what other amps use, and copy those (there's no calculations involved, and no need for any).
Basically capacitors in the PSU's were decided on by what was available, their physical size, and their cost - no calculations at all.
I just thought that since the program allows the editing of that parameter, it must have some meaning to the circuit. Just trying to fill in the blanks of my understanding here...
from memory simulations I have done in spice the esr of the caps will make a *small* difference to the total voltage you get out of the supply. lower ESR means higher voltage. Typical electrolytics you will be using in a power supply will have ESR much lower than 2 ohms.
I just substituted 2 ohms for ESR in my spice simulation using 10,000 uF caps with esr of 0.05 ohms and ripple voltage went from 0.7V to 10V.... RMS voltage dropped from 57V to 52V.
Whether PSUD uses this figure the same way or not I don't know but 2 ohms esr for a cap seems way off, I'm surprised that is the default.
Tony.
I just substituted 2 ohms for ESR in my spice simulation using 10,000 uF caps with esr of 0.05 ohms and ripple voltage went from 0.7V to 10V.... RMS voltage dropped from 57V to 52V.
Whether PSUD uses this figure the same way or not I don't know but 2 ohms esr for a cap seems way off, I'm surprised that is the default.
Tony.
You need to learn basic electronics and how to use the test equipment, as both statements above show that you are not yet ready to fiddle with the default values of the program. Small steps...
The two things to bone up on- equivalent circuits and esr in the cap model plus the basic formula for capacitive reactance.
No wait, I messed up that calculation. Didn't convert to farads and forgot to divide 1 by the sum.
Should be somewhere around 132 ohms...
Should be somewhere around 132 ohms...
0 frequency. Direct current doesn't have frequency because it's a straight line. Right? So the capacitor has infinite resistance for DC, but ~132 ohms for AC?
OK, you're halfway there! 
Yes, infinite resistance for DC. The resistance (actually, it's reactance or impedance) for AC depends on frequency (Xc = 1/(2piFC)). This is in series with the Equivalent Series Resistance (ESR). I'll let you compute the values for 60Hz, 120Hz, 240Hz, and 960Hz.
Now, to complicate things slightly, the total impedance of the cap is NOT the sum of the AC reactance and the ESR because they're 90 degrees out of phase. It's actually sqrt (Xc2 + ESR2). What that means is that the ESR only comes into play at high frequencies.
Yes, infinite resistance for DC. The resistance (actually, it's reactance or impedance) for AC depends on frequency (Xc = 1/(2piFC)). This is in series with the Equivalent Series Resistance (ESR). I'll let you compute the values for 60Hz, 120Hz, 240Hz, and 960Hz.Now, to complicate things slightly, the total impedance of the cap is NOT the sum of the AC reactance and the ESR because they're 90 degrees out of phase. It's actually sqrt (Xc2 + ESR2). What that means is that the ESR only comes into play at high frequencies.
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