Charging capacitor with constant current, troubles simulating and implementing

Thread Starter

Lincoln Sprague

Joined Feb 4, 2016
I'm trying to build a crude scale-model of an electric vehicle charging load by using a constant current source to charge up a capacitor. I would like the time constant to be large, so that an observer can sit and watch it charge for 20 seconds or so. Here's what I've done:

1) I built the JFET current supply provided in a tutorial on this website. It works great. I used a J211 and and it supplies ~22mA as per the data sheet. Only problem is my biggest capacitors on hand are 1000uF, and I'm working at 12V, so by I=C(dV/dt), they are charging in half a second.

2) I figured my options are a smaller current or bigger caps. I decided to explore smaller current. I've been trying to simulate possible circuits in LTspice and that's where the trouble begins. The simulations work with resistive loads (I see constant current as load varies), but when I swap in a cap or just put one in series, I do not get the linear ramp in voltage across the cap (zero to Vsupply) that I would expect. In fact, the voltage tends to stay fixed and tiny femto currents flow. Numbers that make me certain I'm messing something up. I don't have enough experience to know what's going on.

I've attached the circuits I've been simulating. I'd appreciate insights into what I'm doing wrong as well as advice on completely different ways to go about this if I'm on the wrong path. Thank you in advance.



Joined Mar 14, 2008
When you do the transient response, select the option "Start external DC supply voltage at 0V".
Otherwise it does a DC operating point before it starts the transient simulation, which charges the cap to its final voltage.