Vehicular Power Bus – Hell on Earth?

For a good feel on how sad things can be on a perfectly normal automotive electrical system, we should all familiarize ourselves with this military standard…

Yes, it is for 28 V vehicles, but the ideas scale well to our 14 V systems by dividing most of the voltages by 2. There is an SAE standard for 14 volt systems…

…, but I haven’t had the need to pay the $68 for it. Some of the SAE highlights are discussed in the “Background of the Present Invention” section of this particular patent concerning a DC Power Conditioning product.

Skipping over the part about Electromagnetic compatibility, the rest of 1275 reads like a Steven King short-story for anything that connects to the vehicular power bus. The following details are found by dividing the voltages by 2.

Vehicle power bus steady state values

This one we all know…

• Steady State Voltage Limit – 12.5 to 15 Volts (Note SAE J1211 says 16V max)

Vehicle power bus surge/dip values

Did you know the following voltages are perfectly legitimate power bus specifications as well?

• Surge of 20 V for 50 ms
• Surge of 16-20 volts for 1/2 a second
• Peak steady state DC voltage can regularly reach 16 V when you include allowable ripple (up to 1 V peak)
• Dip down to 3 V during the first second of cranking
• Dip down to 8 V during the cranking attempt – up to 30 seconds in this spec

The above surges are not low energy noise spikes, but hard driven by low impedance sources.

Vehicle power bus spike values

Reading into the next chapter of the horror story, there are spikes caused by switching of reactive loads: motors, solenoids and, yes, resistive loads at the end of long wires (wires have enough inductance to cause measurable spikes). I don’t know if we can divide by two here, but even if we do, the story is chilling…

• +/- 125 Volts for 70 us…
• …sloping to +/- 20V in 1 ms

Yes you read correctly… plus AND minus.

Ackpf!

The good news is these spikes contain relatively low energy.

…but but the battery is the world’s best capacitor… right?

Many folks (including me many years ago sadly) assume the vehicular power bus is a peaceful quiet place just like our good ole trusty regulated PS on our bench. They assume this simply because an enormous lead-acid battery tames, somewhat, the voltage monsters lurking about. The battery certainly helps (compare the above specifications with the generator-only mode in 1275), but is too big and powerful to quickly react to spikes and is only marginally good at taming surges quickly. The next figure highlights the “capacitance” of a battery and how it diminishes very quickly with rising bandwidth…

This is a bit like expecting your trusty mule, that can powerfully plow like no other, to dance like Fred Astaire.

We need to remember a starter battery is a big chemical monster that takes a finite amount of time to change its reaction rates to changing demands. It is certainly not a capacitor with zero series resistance and zero series inductance as some seem to think. Indeed it is unfair to expect the size and might of automotive lead-acid cranking batteries to nail our bus voltage to 13.8.

MILSPEC not so MIL after all

While designing power systems for UAVs, I was given stern looks while suggesting we follow 1275 and a close cousin standard MIL-STD-704. The response would go something like… “John, this particular UAV doesn’t need to be built to MIL-SPEC.”

Whereupon I responded “You misunderstand… 1275 isn’t pushing us to over-design loads to some hardcore military limits, it’s describing how loose the vehicle power system can be (and is) and what our loads should expect to tolerate in the real world.”

Another UAV story involved a computer vendor who completely misunderstood what we meant by 28 V aircraft power system. We received a prototype with a max voltage rating of 28.5 volts! Reject.

Hurl energy towards the load

The ultimate purpose of a power bus is to deliver energy from source to load. The unwise think it does so with ballet like footsteps. The reality is something more like a General George Patton style mass march.

Commercial power supply manufacturers understand

Professional vehicle power supplies within high-end equipment are always rated to work between 9-18 Vdc (or 18-36 Vdc for 28 Volt systems) and tolerate the spikes and surges. MIL-STD-1275 suggests why.

Equipment doesn’t necessarily have to function as the voltage dips, but certainly should not sustain damage or blow fuses during these moments. We should expect our equipment to function perfectly during the spikes and surges one can expect (as 1275 suggests) on any generator-battery DC power system.

Certainly car manufacturers deal with this topic to protect their engine control computers and other critical electrical loads.

Are you sure the manufacturer of your radio understands the above realities? Most have been at this for quite a while and our 2m/440 mobile rigs seem to survive just fine with one exception being the power cycle during engine cranking. A study of the schematics of recent offerings, however, makes me reluctant to just plug-and-play some of the newer gear into my car.

Caveat Emptor.

Conclusion

Specifications like MIL-STD-1275 consolidate decades of observations of vehicular bus behavior into a quick read. It provides power system designers/users with a practical set of achievable voltage specifications and reminds the load designers of these limits.

We cannot expect a car’s powerful alternator, enormous lead-acid cranking battery and tens, if not hundreds, of feet of wire (distributed series inductance) to act identically to a desktop regulated power supply with a two foot power cord.

Connecting our radios and accessories to our vehicles is a terrible terrible thing to do… but connect we must. It is up to us to verify our gear can take the punishment. You cannot assume clean power in a vehicle no matter where you tap it… so don’t.