10,000 Users (in 10 Weeks)

May 10 2012, 9:15 PM PDT · 0 comments »

In just under ten weeks after our public launch, we're happy to announce that last week we crossed the 10,000 registered users milestone! We decided to reach out to a few of our newest users and see who they were. Here's what a few of them told us about themselves:

Jason Goodman, from Wheaton College in Norton, MA is a physics professor drawing simple circuits for exams and problem sets.

Alexander S. is an engineer born and living in Munich. Alexander says: "I am proud to be one of the users 10000+ who recently signed up to this site. I have worked in Russia for quite some time, now I work as an test field engeneer. I still have my own company and I am developing small circuits and interfaces for various applications like telephone, signalisation and survillance. A friend told me about this site and I am impressed about the possibilities without needing to install something. I had some experiences with spice before but I think this site is good for beginners and students to get in touch with electronics. When I was young I started to get in touch with electronics by soldering parts together. With CircuitLab I want to do some things with my son on this site for him to get into touch with electronics."

Sergio Andrade is from Monterrey, NL, Mexico. Sergio says: "I'm an Electronic Eng interested in Robotics, Sensors and RF mainly."

Grey K. is an electrical engineering student in New York interested in power, RF and robotics. Grey says: "I'm two weeks short of finishing my junior year studying electrical engineering at NYIT. I have these lab classes wherein I need to simulate some simple analog electronics and then compare those simulation results with experimental measurements. [...] What I love about [CircuitLab] is that it's easy to use, especially as compared to PSpice when all I basically need is a frequency response graph. At the same time though, it isn't lacking in features; Last week I found out you can actually change all the specific attributes of a BJT."

We're excited to be attracting a diverse worldwide userbase across the entire online electronics community of students, educators, hobbyists, and professionals. Over the coming months, help us unite those users into a community -- one with literally tens of thousands of years of combined electronics experience.


Piecewise Linear (and Piecewise Step) Sources

Apr 13 2012, 6:30 PM PDT · 0 comments »

As another facet of our Arbitrary Behavioral Sources release, we've now added support for voltage and current sources to generate piecewise linear or piecewise step signals inside the simulator. This allows you to inject the specific signals you need to test your circuit.

The PWL, PWS, PWLREPEAT, and PWSREPEAT expressions (described in the documentation) take lists of time/value pairs, so you can describe any function you want, and let the simulator interpolate between control points if desired.

Take a look at this Capacitively Loaded Emitter Follower demo which uses a PWL voltage source:

The PWLREPEAT function used to define V2 sets up a pulse that has both postive-going and negative-going parts, with defined edge slopes between the two and the off-state. Open the circuit and run the time-domain simulation to see the piecewise-linear source in action, summed with the +2.5V static bias to produce the voltage waveform V(in). Then, take a look at the V(out) trace, and you'll see that an emitter follower isn't always great at following signals with large, quick changes!

Piecewise sources, plus our new behavioral sources, expose the power of the CircuitLab simulation engine and make it easier to apply CircuitLab to your specific engineering design problems.


Arbitrary Behavioral Sources (Experimental)

Apr 13 2012, 1:45 PM PDT · 1 comment »

We're happy to announce a major upgrade to our simulation backend: voltage and current sources can now become arbitrary behavioral sources. For those not familiar with the tech lingo, it means that voltage and current sources are now "programmable". This feature is currently experimental within CircuitLab, but we wanted to provide you with the power you need to go beyond our included device models.

Let's take a look at a simple modeling example: a TL431 programmable shunt voltage regulator. It's sort of like a Zener diode in that it's a shunt-type device where the incremental resistance becomes tiny past the "on" voltage. However, the TL431 has a third terminal which allows it to be adjustable, and it basically draws as much current from the "cathode" terminal as needed to keep the "reference" terminal at 2.5 volts above the "anode" terminal. The user can then build a simple feedback network, like a resistor divider, to have a programmable shunt voltage reference. Beyond the programmability, the TL431 has another advantage over Zener diodes, which is that its incremental "on" resistance is quite low, so its voltage stays quite steady over varying load currents.

Here's a very simple model built in CircuitLab (from a bigger test circuit we'll link below):

You can see that I2 is a normal current source, drawing 2uA into the reference terminal. But I1 is a current source that's defined by an expression, "MAX(0, (V(REF)-V(ANODE)-2.5)/1m)". That expression means that the current flowing through I2 is the maximum of 0 and the quantity on the right side -- meaning that if the right side is less than 0, the current is just zero, so the current is never negative. The other piece of the expression is "(V(REF)-V(ANODE)-2.5)/1m", which means to take the voltage difference between REF and ANODE nodes, subtract 2.5 volts (which is the built-in reference voltage of the TL431), and then divide by 0.001. That 0.001 is almost like a resistance, where the current varies by 1000 amps for every volt of difference. But here, it's really a transconductance, because the terminals that the current flow through aren't the same terminals whose voltages are being measured.

This is part of a test circuit:

We can run a DC sweep, and demo the parameter sweep options as well to adjust the resistors connected in the external circuit:

Here, we can see that the output voltage is adjustable by the feedback resistors R2 and R3.

Behavioral sources are experimental, but feel free to go ahead and give them a try! Read more about allowed expressions and behavioral sources. Beware that behavioral sources make it very easy to cause convergence problems. We work hard to make sure that CircuitLab-defined built-in models converge in most cases, but arbitrary behavioral sources means that you're on your own. If you're having trouble, feel free to ask for help on the forums and see if another user can help re-structure your equations to achieve the desired effect.