Transforming Spice

March 2, 2016 by Al Williams 13 Comments

Spice is a circuit simulator that you should have in your toolbox. While a simulator can’t tell you everything, it will often give you valuable insight into the way your circuit behaves, before you’ve even built it. In the first installment of this three-part series, I looked at LTSpice and did a quick video walkthrough of a DC circuit. In the second, I examined two other parts of Spice: parameter sweeps and AC circuits. In this final installment, I want to talk a bit more about real-world component performance and also look at modeling transformers.

Recap

Last time we looked at a low pass filter, but it wasn’t practical because the components were too perfect. Only in simulation do voltage sources and wires have zero resistance. There was no load resistance either, which is unlikely. Even an oscilloscope probe will load the circuit a little.

The resulting AC analysis showed a nice filter response that was flat to about 1 kHz and then started roll off as the frequency increased. Suppose the source had an 8 ohm series resistor. How does that change the circuit response?

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Spice Power

February 29, 2016 by Al Williams 8 Comments

Spice is a circuit simulator that you should have in your toolbox. While a simulator can’t tell you everything, it will often give you valuable insight into the way your circuit behaves, before you’ve even built it. In the first installment of this three-part series, I looked at LTSpice and did a quick video walkthrough of a DC circuit. This time, I want to examine two other parts of Spice: parameter sweeps and AC circuits. So let’s get to it.

In the first installment, I left you with a cliffhanger. Namely the question of maximum power transfer using this simple circuit. If you run the .op simulation you’ll get this result:

--- Operating Point ---
V(n001): 5 voltage
I(R1): 0.1 device_current
I(V1): -0.1 device_current

The power in R1 (voltage times current) is .5 W or 500 mW if you prefer. You probably know that the maximum power in a load occurs when the load resistor is the same as the source resistance. The Rser parameter sets the voltage source’s internal resistance. You could also have created a new resistor in series with V1 and set it explicitly.

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Adding Spice To Your Workbench

February 26, 2016 by Al Williams 43 Comments

Most of us didn’t fight in World War II, drive a race car, or fly the Space Shuttle. But with simulation, you can experience at least some of what it would be like to do those things. Granted, playing Call of Duty isn’t really the same as going to war. No matter what you are simulating, it only goes so far. However, you can get a lot of value from a simulation. I’d bet the average kid who has played Call of Duty knows more about WWII locales and weapons than my high school history teacher.

When it comes to electronics, simulation is an excellent way to get insight into a circuit’s operation. After all, most circuits operate in the abstract–you can’t look at an audio amplifier and see how it works without a tool like a scope. So simulation, when done well, can be very satisfying. You just have to be careful to remember that it isn’t always as good as the real thing.

That’s Spicy

One of the best-known electronics simulators is Spice, which Berkeley created in 1973. In its original form, you had to punch cards that described your circuit and the analysis you wanted to perform. Modern PC versions sometimes replace the deck of cards with a text file. The best modern versions, though, give you a GUI that allows you to draw a schematic and then probe it to see the results.

There are several paid and free versions of Spice (and other simulators) that include a GUI. One of the best for a casual user is the free offering from Linear Technology called LTSpice.

Linear makes LTSpice available and populates it with models for their devices in the hopes you’ll buy components from them. However, the software is entirely usable for anything, and it has a powerful set of features. Linear produces the software for Windows, but I can attest that it runs just fine under Wine on Linux. The Web site will invite you to register, but you don’t have to if you don’t want to.

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