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In Reply to: Where goes time factor of "joules" in a PSU? posted by cheap-Jack on April 16, 2007 at 12:14:24:
But maybe you want to talk coulombs which = farads(voltage). One amp is one coulomb per second. Average power in a cap (in watts) is [farads(volts^2)]/2(seconds).Does this answer your question?
Russ
Follow Ups:
Hi.This is what the poster asked. I asked for a calculation example for its joule value of a given PSU.
c-J
Actually the original post asked for the formula for energy storage in an inductor. That question has been answered, incorrectly by me and then correctly by several other people. What is your problem?
So I wouldn't say you were "incorrect". Truth be told, it was the first time I ever saw you make a "slip". I just felt duty bound to make the small correction.As to what CJ wants....Jeez I gave him any number of ways to relate joules, watts, and time. Now just how joules of storage one needs is a matter of opinion (to some degree).
Russ
Energy in a CapacitorWe know that:
1. Energy = charge × voltage
2. Q = CV.
This second relationship tells us that the charge – voltage graph is a straight line.
The capacitor is charged with charge Q to a voltage V. Suppose we discharged the capacitor by a tiny amount of charge, dQ. The resulting tiny energy loss (dW) can be worked out from the first equation:
dW = V × dQ
If we discharge the capacitor completely, we can see that:
Energy loss = ½ QV
By substitution of Q = CV, we can go on to write:
E = ½ CV2
I'll leave you to do the units ........
Hi.Assuming your dW=V.dQ is correct (relevant to the electrical defination of a joule), how many joules are you talking about a given PSU say 350V with its final cap being 8uF ?
There are tons of similar formula around, my question is how to apply them in the joule calculation to assess a PSU.
c-J
E joules = 1/2 C V 2350 volts, 8 μF
8 μF = 8 * 10 -6 F
E = 1/2 * 8 * 10 -6 * 350 * 350E = (1/2 * 8 * 350 * 350) * 10 -6
E = 490,000 * 10 -6
E = 4.9 * 10 +5 * 10 -6
E = 4.9 * 10 -1
E = .49 joules
Cheers, John
Hi.This makes the didfference btween actually knowing it or just hearsay about it.
c-J
a clue:1 joule = 1 watt.sec
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