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I recently started using circuit simulation software that is warning me that I have a "capacitor loop with no resistance" and then refuses to run any simulations until I correct the problem. I can trigger this warning simply by creating a circuit with a battery and a capacitor tied together. I don't understand the prohibition of connecting a circuit this way as it does produce measurable effects in the capacitor.

Can anyone help me to understand what this warning may mean and why I should avoid circuits that are connected this way?

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    \$\begingroup\$ Wouldn't it be better to just say it's Falstad's CirSim (www.falstad.com/circuit/‎) ? It's just a simple Java sim meant for educational purposes, it's not a real sim per se. Go for e.g. one of SPICE variants if you want to do a real sim... And NO, if your circuit doesn't work without explicit ESR, it probably won't work anyway. \$\endgroup\$
    – user20088
    Aug 12, 2013 at 20:34
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    \$\begingroup\$ I suppose it would be better if it were true: the software is called iCircuit (iOS and Mac). I didn't really want to distract with the name of the software since the error sounded like some fundamental electrical principle that I was trying to understand. From what I can tell based on the responses so far, the problem appears to be overly-simplified resistance assumptions in the simulation. \$\endgroup\$
    – Mikey
    Aug 24, 2013 at 19:02
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    \$\begingroup\$ iCircuit is based on Falstad's CirSim; quote from icircuitapp.com: "iCircuit wouldn't be possible if it weren't for the pioneering work and generosity of Mr. Paul Falstad. Check out his Circuit applet and his other wonderful ideas."; iCircuit has long pages of code copied and ported to mobile devices. \$\endgroup\$
    – user20088
    Aug 25, 2013 at 18:37
  • \$\begingroup\$ I stand corrected. \$\endgroup\$
    – Mikey
    Aug 26, 2013 at 19:41
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    \$\begingroup\$ No harm done, mate; it's just that to get a precise answer, it's best to ask a precise question... \$\endgroup\$
    – user20088
    Aug 27, 2013 at 10:46

3 Answers 3

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Guesstimate the resistances you would have in the real circuit, and insert resistors in the sim circuit to represent them.

A little more explanation of why the simulator reports an error without them may be useful : the instantaneous current when the battery and capacitor are connected is independent of the capacitance; it is purely the voltage divided by the resistances in series. Which, with ideal components, is the well known "divide by zero " error.

The capacitance will have some value of "ESR" = equivalent series resistance, which you may find from datasheets. (Some datasheets model ESR as loss, or "tan delta" instead) This page allows you to select capacitors by ESR; values from 0.003 ohms to 1.6 kilohm with 0.015 to a few ohms being common.

The battery will also have an ESR which will increase as the battery drains; again, see the datasheet. A fully charged NiCd or NiMh may have 0.1 ohm or more ESR, up to tens of ohms for a small button cell.

For the wiring or PCB traces, you can find the resistance in ohms per meter for approximately the right wire, (e.g. 0.1 ohm/m for 26SWG) and estimate the length of wire required (say 10cm, or 0.01 ohm).

Any one of these will break the loop and produce a circuit that can be simulated; modelling all of them will produce a more accurate simulation (may not change the results very much)

There may also be better simulation models for components such as capacitors, that allow you to set the ESR (and other parameters) and other features of real components.

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There are two 'ideal' circuits simulators cannot simulate: shorted capacitors and open ended inductors.

Say, have a capacitor charged to 1V. Then it short-circuited, it could discharge with infinite current. Even if computer can operate with infinities, this could produce totally impractical result. So simulators simulators refuse to work with such circuits.

To avoid warning do not short-circuit capacitors, do not connect them directly to power source, do not connect them in parallel (replace them with one of summary capacitance). In case to simulate real circuit having ones, add resistances in series, as others answered.

Open ended inductor is similar. It could produce infinite voltage on its ends to discharge, which is also totally impractical. Avoid this also.

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Learning to respect that all parts have ESR is the 1st to understanding how real parts work. The TL78L05 needs a cap with very low ESR to be stable but not zero. Then you get low ripple and near zero idle current in an LDO. The key is understanding the cap is a critical part of the design. Zero ESR in a cap means infinite current to a Voltage source (hence error) and zero ripple feedback to an LDO (unstable).

If you want to analog electroncs, get familar with the ESR in all common parts and understand why it is the most critical quality factor for most power designs.

Consider the effects and cost of 5,50,500, or 5000 milliOhm in simulation of transient loads.

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