DOSSpice.jpgIn college I found Spice very frustrating.  I was still learning the basics of circuits.  Spice won’t tell you that a typical SMT discrete can’t dissipate several watts.  Some of the analyses won’t even tell if you the circuit isn’t biased properly; they provide output as if it were biased right.  So I came away with the idea that Spice is more trouble than it was worth.  I thought if the circuit was so close to the edge of meeting spec that you need Spice, you needed to re-design it. 


When I revisited Spice five years after college, I found it could solve circuits that were tedious to analyze by hand.  It could actually calculate things like the drop across a diode that I would others estimate based on roughly how much current I guessed would flow through it in a particular circuit.  Armed with a knowledge of how circuits behave, Spice was more useful.


There is a new web-based online circuit simulator call PartSim.  It’s amazingly powerful for something that runs in a web browser.  You can create schematics by dragging parts onto the schematic area.  You can save your schematics on the site, but there is no way to download them to your computer. 

I ran it on a slowish machine with an internet connection data rate of 5Mbps downstream / 0.5Mbps upstream.  I could not detect any lagging or delays. 


If you’re new to Spice, PartSim is actually easier and more intuitive than LTSpice.  LTSpice is the free circuit simulator from Linear Technology that most engineers have tried.  Despite its being freeware, LTSpice is a powerful program.  Linear Technology engineers use LTSpice for simulation in the development of their ICs. 


PartSim is certainly not a replacement for LTSpice.  There are many nice things you can do in LTSpice but not in PartSim:

  • You can do math on graphs of transients to view things like voltage squared.  You can calculate averages, such as average power dissipated during a transient event.
  • It’s a simple matter to map a schematic symbol to a Spice model privded by a vendor.
  • You can measure the transfer function of a circuit.  You can do a DC sweep, i.e. see how the bias changes as DC supply values change. 
  • If you want to measure the transient current in any wire, you just hold down alt and the icon changes to ammeter that can be applied to any branch.
  • You can easily add non-idealities like capacitor ESR or inductor resistance to discretes.
  • You can export a CSV file from an oscilloscope and import it into LTSpice as a piecewise linear voltage source.
  • If you want to share with someone a circuit you simulated along specific plots you generated, you can save the plots as .plt files.  If you open the circuit file in the same directory as the .plt files, it will automatically open the plots.



PartSim has different strengths:

  • You can be up and running in seconds.  You don’t even need to install an applet or plugin.  Just go to the webpage.
  • The interface is very intuitive.  Just drag parts from the list on the left to the schematic on the right.  Many OrCAD hot keys have the same function in PartSim.
  • If you save your simulations in PartSim you can access them on any computer connected to the internet.


The opportunity cost of trying PartSim is so low that I suspect even Bob Pease, whose book contains a photo of him tossing a computer from a parking ramp in frustration with Spice, would at least give it a try if he were still with us.  If you try PartSim and like it, download LTSpice to get even more functionality.