Well, because the C is directly across the voltage source, the instantaneous current is theoretically infinite!

One would be a pretty poor theorist to ignore an entire fundamental property of the universe -- magnetism!

In fact, source impedance, wire impedance, and so on, give the limit, but it is going to be pretty high. And of course, the finer the time resolution of the measurement, the higher the current you are likely to observe.

The current peak is given, to second order accuracy (hard to beat that!

), by adding the source resistance, and wiring inductance.

In practice, the source won't be an ideal voltage source, but the lowest impedance it has will be, guess what -- another capacitor! So to be accurate, one must have the C and ESR of that, and connect it in series with the load side ESR and C. (If Cload >> Csource, then it could take so long to charge that the transformer, rectifier, and maybe AC mains source, have to be taken into account. But the current will be much lower.)

ESL is the tricky one, though. You can guesstimate stray inductance based on wiring length: the inductance of free space (that is, the inductance due to a current simply flowing through a distance) is 1.257uH/m (more correctly, mu_0 == 4*pi x 10^-7 H/m). The inductance of most cables is around 1/2 to 1/4 of this (because the magnetic field inside a cable isn't in free space, but it's trapped between conductors that oppose!), so you can guesstimate 0.3 to 0.6 uH/m.

So, a typical 2m cable is on the order of 1uH, and paired with 47uF, the sqrt(1uH/47uF) ~= 0.14 ohms. (So, ESR > 0.14 ohms gives good damping, and the peak current from 24V will be under 160A.)

But this has been a really instructive exercise, as it led me to the (now obvious) idea of putting an RC snubber right on the input, to limit voltage (damping oscillations) without too much of a current pulse. And the DSO is invaluable as it allows me actually find good values of R and C by observation and measurement, rather than guesstimating, which is where I'd be at without it. Great tool!

Yup. And, you also know the value of RC required: the C needs to be more than double the non-resistor C value, and R needs to be equal to sqrt(L/C).

sqrt(L/C) shows up so often, it has a name: characteristic impedance, Zo. Note that it does indeed have units of ohms (Google Calculator will show this works). Which should tell you something interesting will happen when you add a resistor of similar value to the circuit!

In practice, electrolytic capacitors have enough ESR to avoid ringing except for very long cables (but then, the cable resistance itself may be significant, too). The inrush current can be quite large, so it needs to be directed away from sensitive circuits. Tantalum capacitors have a range of ESR available (so can be used for damping, or can be prone to more ringing), but more importantly, shouldn't be exposed to surge currents and voltage spikes, which can

*ignite* them! Polymer caps usually have very low ESR (like a tiny little 25V 47uF aluminum polymer having an ESR of 20mohms!), so they can cause big problems with ringing in power supplies and have to be used appropriately.

Nothing at all more complicated than algebra-level arithmetic!

Tim