A split out question from my other project. Trying to pick out some of the basics I might be missing.
If I put a cap directly across a DC barrel jack input, then when I connect the power to it, I will get a spark and the supply will basically see a 0 Ohm short for a fraction of a second and a load which seems to want to pull an ideally infinite amount of current.
So, it seems to make sense to put at least a "token" value resistor in series before it. That or one of them magical to me, inductors.
As it is a bulk input cap, it has to be able to pass at least the average current of the load.
True an ideal capacitor will "Draw" infinite current provided their is an infinite supply. But if restricted to 5A then the maximum is 5A. Now even 5A seems to be a lot, so Lets look at the mechanical structure of the circuits leads and internal plates of the capacitor and compare that to the fusing current. Typically you would see that 20AWG (capacitor lead gauge for a 100uF capacitor) is capable of 5A. But the fusing value of 20AWG is 158 A for 1second or 882 A for 32 mili seconds.
https://en.wikipedia.org/wiki/American_wire_gaugeAs you can see the amount of current that a simple piece of 20AWG can handle is incredibly high for short periods of time.
The Time Constant RC is variable as this is dependent on the series resistance and capacitance of the circuit.
https://en.wikipedia.org/wiki/RC_time_constantBut typically the charge time of 1RC (63.2% total charge or peak supplied voltage) is only a few micro seconds. By that time the amount of current across the capacitor has also drop near 63.2%.
Of course you can lengthen the time constant by adding a resistor in series with the capacitor there by increasing the ESR, but keep in mind that that resistor will also slow the discharge of the capacitor. So in a PSU filter this becomes undesirable because slowing the capacitors charge and discharge will increases output ripple voltage. Also depending on the value of the resistor and the desirable RC time constant that resistor begins to heat up over repetitive charges and discharges of the capacitor. Typically you don't want the capacitor to discharge lower than 63.2% per cycle. You can work out the dissipation factor of the capacitor if you like.
https://en.wikipedia.org/wiki/Dissipation_factorSo for filter capacitors, in your PSU, you want low ESR with a low dissipation factor. A capacitors operating properties are listed in the data sheet and you would look at that to see if the capacitor your using is adequate for the application.
Home
Help
Search
Login
Register
Recent Posts