In Part 1, Part 2, and Part 3 of this post series I used an incandescent lamp example to introduce a few of the nonelectrical 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:
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).
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, realworld 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.
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 VHDLAMS.
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Post a CommentJohn Fitch
7:03 PM Dec 14, 2010
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