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Creating prototype circuits from home (Part 2)

14 Sep 2015  | Scott Deuty

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As you progress into creating a circuit from home, schematic capture is an important part of the process. However, it requires a little forethought especially if you are planning to transfer the schematic to a board layout software.

Simulation software has gone beyond the traditional SPICE analysis. The expansion of libraries offers additional information to accompany components. This includes packaging dimensions and other useful information. For example you can choose a particular resistor size such as a 1206.

Figure 1: Simulation Software Libraries Now Contain Packaging and Other Useful Information for Easy Transitioning to Board Layout as Shown by the Chip-R-1206 Footprint Dialogue Box.

In the same way libraries assist you, they can also cause you issues by offering limited parts. Often times these parts are in different packages. In addition, the models cause additional computing that sometimes keeps the simulator from converging. This is especially true in power supplies that have abrupt switching waveforms.

The solution that I use is to create two different schematics. I will start with my basic schematic using parts from the library. As things fail to converge, I will replace them. For example, I will used a pulsed voltage source when my 555 timer runs into convergence problems.

Figure 2: SPICE Has Always Been Known to Have Convergence Issues. Fortunately, I got a Few Valuable Pulses Out of this Simulation Before It Produced the Error.

Convergence problems have always been an issued with SPICE [5]. The difference is you get scolded immediately rather than wait for hours while a cursor blinks as it was in the early days of PCs. In fact, reference [5] "Step-by-step procedures help you solve Spice convergence problems," was written almost 20 years ago in 1996. Not much has changes since then. Models only go so far to simulate the real world. There are however ways to progress without a lot of additional effort.

So at this point, you've got to realise that rather than solve these problems, it's often best to replace the troublesome components with generic models. Some examples are:

 • PWMs and timers with voltage sources or a combination of gates, op amps, and comparators
 • Transistors with voltage controlled switches
 • Transformers with dependent voltage and current sources plus a magnetizing inductance
Of course all of this does not do the best in recreating the parasitics that occur. Those have to be factored in as well. However if you want to get up and running this method of using generic blocks is best for quick results. You can get your parts ordered and sort it out on the bread board which is always advised anyway.

Figure 3: This Representation was for the Board Layout Transition from Multisim to Ultiboard. Notice the Pulsed Voltage Source Has Been Replaced with a 555 Timer. Also, Diodes D1, D2, and D3 are co-packaged in the Q1 Transistor that I used. Both Devices Use a TO220 so the part shown represents the final layout for either component. These Diodes Can be Laid Out While Populating them on the Board is Optional Based on the Choice of Transistor.

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