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Simulation Beyond Volts and Amps (Part 4)

In Part 1, Part 2, and Part 3 of this post series I used an incandescent lamp example to introduce a few of the non-electrical points to ponder when designing a system. I introduced a simple automotive emergency flasher system that included simulation models for a lamp, wire, fuse, switch, and battery. To parameterize my system model, I did a little research based on a Jeep Cherokee repair manual from my workshop library, and then looked up additional part specifications. Here is a quick recap of my parameter value selections:

  • Lamp on resistance: 6.1Ω; Off resistance: 600mΩ
  • Fuse minimum blow current: 15 amps; Melting point temperature: 400 °C
  • Flasher period: 700 ms; Duty cycle: 35%
  • Battery open circuit voltage: 12.6 VDC; Effective internal resistance: 15 mΩ

These parameter values define the nominal design and I used them as the basis for my system simulations.

First, let’s take a look at the system’s nominal electrical performance. Since the flasher system is symmetrical, looking at the electrical performance for one of the lamps will tell us the basic performance for the remaining lamps. Here is the voltage across, and current through, the left front lamp:

blog_lamp_electrics 

 Due to voltage drops across other system components, the lamps only see 11.94 volts of the 12.6 VDC battery voltage; note the lamp current of approximately 2.08 amps. Now let’s look at the fuse current and resulting fuse temperature. Remember that the fuse carries current for all four lamps (the current through a single lamp multiplied by 4).

 blog_fuse_electrics

The fuse current at 8.30 amps is, in fact, approximately quadruple the current through a single lamp. The fuse temperature at 58.85 °C is well below the melting point of 400 °C. For the final simulation in my example, let’s focus on how changes in design parameter values affect the fuse temperature.

There are many system parameters that affect the fuse temperature. While the parameters given above are nominal values, real-world values vary within manufacturing tolerances. In this last simulation, let’s analyze the flasher system using the following parameter changes:

  • Clock period: 857 ms (maximum), 545 ms (minimum) (approximately 70 and 110 flashes per minute, respectively)
  • Clock duty cycle: 35%, 45%, 55%
  • Minimum fuse blow current: 5 A, 7.5 A, 10 A, 15 A

Using a parametric sweep analysis, SystemVision combines these parameters in simulations that cover all possible combinations.

blog_fuse_temperature

Since some of the numbers are hard to see, I’ll briefly summarize the results. In Part 3 of this series I mentioned I’m interested in seeing what size of fuse I really need for this system. Recall that the nominal minimum blow current for the fuse is set at 15 amps. But the simulation results show that only when the fuse rating is reduced to 5 amps does the fuse melt. Fuse temperature with all other minimum blow current ratings (7.5, 10, and 15 amps) is well below the 400 °C melting point. So if the flasher circuit is the only system protected by this fuse, I can adjust the rating down from its 15 amp nominal value to 7.5 amps – not something I would have easily discovered without the benefits of a standard modeling language. And even though the fuse is really carrying just over 8 amps, the additional system parameters that affect the fuse temperature (flashing period and duty cycle, for example) allow me to use the 7.5 amp rating. A next step might be to apply deratings to the fuse where I might discover that I need to add a little cushion to the fuse rating – say bump it up to 10 amps – but this is an example for a future post.

This is just a simple example of electrical system analysis beyond the typical, and sometimes information limiting, volts and amps performance metrics. Getting at the additional details without a lot of fuss often requires the flexibility of hardware description languages like VHDL-AMS.

 

IEEE 1076.1

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About Mike Jensen

Mike JensenMost career paths rooted in high technology take many interesting (and often rewarding) twists and turns. Mine has certainly done just that. After graduating in electrical engineering from the University of Utah (go Utes!), I set off to explore the exciting, multi-faceted high tech industry. My career path since has wound its way from aircraft systems engineering for the United States Air Force, to over two decades in applications engineering and technical marketing for leading design automation software companies, working exclusively with mechatronic system modeling and analysis tools. Along the way, I’ve worked with customers in a broad range of industries and technologies including transportation, communications, automotive, aerospace, semiconductor, computers, and consumer electronics; all-in-all a very interesting, rewarding, and challenging ride. In my current gig, I work on technical marketing projects for Mentor Graphics' SystemVision product line. And in my spare time I dream up gadgets and gizmos, some even big enough to qualify as systems, that I hope someday to build -- providing I can find yet a little more of that increasingly elusive spare time. Visit Mike Jensen's Blog

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Comments 1

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Note in the old days flashing was done by an electromechanical device that would wear out and fail to "blink", just stay on. This would blow the 7.5A fuse. You might note US nomenclature is a 1A fuse is guaranteed to carry 1A and not blow; Euro nomenclature is a 1A fuse is guaranteed to blow at 1A. I worked at Polaroid for an ex-RCA military engineer who said in aerospace work they often did four fuses in paralleled series-pairs so if any one failed (by opening or failing to do so)the combination would still provide (albeit relaxed) protection. You can't test a fuse. Nice article.

John Fitch
7:03 PM Dec 14, 2010

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