Electronic – LC circuit + diode problem ! (without resistors)

dcdiodesltspicespicesteady state


What will be steady state capacitor voltage?

My hit at the solution:

The static resistance of the diodes come into action causing a large current to flow across the diodes ( assuming diode isnt damaged) the voltage initailly at the input of LC circuit is 6v which appears across the inductor causing the current to increase ,eventually charging the capacitor to 6v. Also,when the current in LC goes negative,the Diode D2 does provide a path which would mean current in LC ciruit should be sinusoidal!

My doubts:

1) What happens after the cap reaches 6v ,the inductor current is still non-zero right?
which would mean the cap should continue charging?

2) How is steady state achieved in the circuit?

3)Why does cap stop charging at 6 volts?

4)Will the current oscillate in the LC part?

Best Answer

Since this question essentially became about a [LT]spice simulation, here's the answer to that "mystery".

The diode model in SPICE does have an ohmic resistance parameter (RS in the figure below) to simulate bonds/wires/contacts. At DC it looks like:

enter image description here

(Source: http://www3.imperial.ac.uk/pls/portallive/docs/1/7292572.PDF)

Since this circuit used an 1N4148 model (not the ideal diode model), the ohmic resistance (Rs) was set to a non-zero value; actually to 0.568ohms.

enter image description here

If you put a single 1N4148 (forward biased) in series with a 12V DC source, you'll get about 19.3A through it in LTspice (so with two you'll get about half that). A trivial calculation/approximation with Ohm's law (ignoring the voltage drop over the current source || GMIN) shows that 12V through 0.568ohm will give a 21.12A current limit.

Of course, as correctly emphasized by several others on this page, in real life, the 1N4148 diode catches fire (bonds vaporize and what not) long before reaching those levels of current.

Also, it seems there's a large variation between SPICE-based simulators' databases on diode RS values. For example TINA-TI has only 2miliohms for 1N4148, so in that sim you get 5.3kA through it.

Of course it would be nice if SPICE-based simulators implemented power dissipation limits for component packages and flashed a newbie-friendly warning when it's exceeded... but alas they typically don't have that feature because it's not standard in the academic & free SPICE codebase they're based on. (If someone knows of a counterexample, please leave a comment.) So in SPICE-based simulators it's normally left to the user to figure out that the current or power dissipation limit is exceeded for a component and that it catches fire in real life.