In Part 1 and Part 2 of this post series I described some of the non-electrical behaviors of an incandescent lamp and fuse, and then explained how these behaviors might be accounted for in a simulation model. Now it’s time to combine these two models, with a handful of others, to analyze the thermal behavior of a basic automotive emergency flasher system. Here is my circuit’s schematic:
Though a simplified version of an actual flasher system, this circuit contains the main system elements: four incandescent lamps representing one at each corner of the vehicle; a length of wire connecting each lamp to the fuse box; the flasher system fuse; a length of wire connecting the fuse box to the car’s battery; a switch representing the flasher on/off switch inside the car; and the battery. Note that the switch model allows me to control on/off switching using a simple digital clock. There are certainly other ways to model a flasher system, but this circuit is adequate for our discussion.
To add a bit of realism to the component parameters in my system, I dusted off the maintenance repair manual for a Jeep Cherokee I used to own and browsed the wiring diagram for the emergency flasher circuit. Here is what I found:
Light bulbs: Type 2057 with an “on” (i.e. hot) filament resistance of 6.1Ω. Rules of thumb estimate a lamp’s on resistance to be on the order of 10x greater than the cold filament resistance — a parameter required by the lamp model. Using this rule, I set each lamp’s cold resistance to 600mΩ.
Wire gauges and lengths: For wire size I used AWG12 between the battery and fuse box, AWG18 between the fuse box and the front flashers, and AWG20 between the fuse box and rear flashers. For wire length, two meters of wire between the battery and fuse box, and three meters between the fuse box and each of the lamps (yes…all wire lengths are simply estimates based on my knowledge of the Jeep’s size – the wiring diagrams do not give wire lengths).
Fuse: According to the wiring diagram, my fuse must have a 15 amp minimum blow current. I left the element melting point at the model’s default: 400 °C.
Battery: I left the battery model’s parameters at their default values: 12.6 VDC open circuit voltage; 15mΩ effective internal impedance.
After assigning these design parameters, I used SystemVision to analyze my flasher circuit’s performance against the following specification ranges:
Flashes per minute (FPM): 70 – 100
Duty cycle (DC): 35% - 55%
Along with taking a general look at my circuit’s thermal profile, I wanted to see if I really need a 15 amp fuse for circuit protection. I will show you the results in my next blog post.