Questions about automotive load dump

I've been designing a circuit which must be capable of running off automotive batteries, possibly while the vehicle is running, so I need load dump protection. I've implemented surge suppressors in the prototype, and the circuit works fine under normal circumstances, but I have no way to test the functionality of the load dump protection. Is there any way to replicate load dump transients similar to those specified in ISO7637, without a specialized piece of equipment? Would it be adequate to just charge a large iron inductor and dump it onto the supply rail?

Also, I may have to work with truck batteries as well, which are nominally 24V. But when the battery is in a vehicle, and is being charged by an alternator, should I expect the battery voltage to rise significantly? What maximum DC voltage would I expect to see on a truck battery? I want to make sure the suppressors have sufficient standoff voltage to not conduct under such circumstances.

Finally, for truck systems, are there different load dump specs than for "normal" automotive applications?

Thank you.

mtwieg said:

............. with truck batteries as well, which are nominally 24V. But when the battery is in a vehicle, and is being charged by an alternator, should I expect the battery voltage to rise significantly? What maximum DC voltage would I expect to see on a truck battery? I want to make sure the suppressors have sufficient standoff voltage to not conduct under such circumstances.

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Hi mtwieg,
The battery voltage can go upto 26.4V when it is fully charged. so the alternator output will be near that voltage.

You can expect 28-29V when batteries are connected in system, without batteries expect over 33V. You should check data for that alternator.

tpetar said:

You can expect 28-29V when batteries are connected in system, without batteries expect over 33V. You should check data for that alternator.

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Hmm, this sort of worries me. I don't think we'll ever intend to operate off an alternator with no battery, but if load dump occurs, then I suppose that the battery will no longer be there afterwards, and we should be prepared for a larger voltage from the alternator? Unfortunately this design is not meant for use with any specific automobile or battery, it's something that should be versatile enough to work on general 12V or 24V systems.

And 33V sounds extreme for a 24V system, sounds like much more than would ever be necessary (or safe) for charging 24V batteries.

Another question I had: why are the properties of load dump transients characterized by certain peak voltages and durations? If the transient is the result of alternator inductance, then shouldn't the transient be modeled as such, i.e. a decaying current source (with a duration depending on the load) rather than a decaying voltage source? Or does some mechanism in the alternator somehow give rise to a more voltage-source-like behavior?

Car/Truck electrical system is designed to work with battery in system. If you disconnect battery, alternator will make lots of high voltage peaks, which are deadly for newer electronics in modern cars. Battery in that system have additional role to make voltage stable.

tpetar said:

Car/Truck electrical system is designed to work with battery in system. If you disconnect battery, alternator will make lots of high voltage peaks, which are deadly for newer electronics in modern cars. Battery in that system have additional role to make voltage stable.

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So if this is the case, then wouldn't that make TVS devices useless for protection against disconnected batteries? Because if the battery is removed, then the suppressors may clamp the initial load dump transient, but if after the removal the alternator continues to produce a high DC voltage there's nothing a suppression device can do about that (at least not without dissipating tremendous DC power).

And shouldn't alternators regulate their output voltage to something reasonable, even without a battery? From what I've read, 13.5-14.5V for 12V systems seems standard, since that's in the range needed to charge 12V batteries.

As I understood it, load dump happens when the battery is removed while being charged, but to be honest I've never heard a great explanation of why that would ever happen in the first place. Seems strange that people would pop off their battery terminals while the car was running...

Alternator voltage is around 14V (depends from model number and voltage regulator inside), without battery voltage peaks is too high in car can go up to 18V-20V. Battery acts as capacitor and reduces peaks amplitude.

You can remove battery from running engine if car is older and dont have turned on FM receiver, dont have ECU, and other fine equipment. Also there is possibility of damaging ignition coil in this situation. But, I dont suggest removing batteries from running engine.

So if this is the case, then wouldn't that make TVS devices useless for protection against disconnected batteries?

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A TVS in series with the supply is typically useless for load dump protection as the energy of the load dump is huge and cannot be dissipated... You may add some current limiting device such as a resetable fuse in series so it trips when the TVS starts conducting.

A common approach used in these automotive applications are low side or high side switches (typ. mosfets) that are disconnected at a certain input voltage level. This link shows some examples: **broken link removed**

Another question I had: why are the properties of load dump transients characterized by certain peak voltages and durations? If the transient is the result of alternator inductance, then shouldn't the transient be modeled as such, i.e. a decaying current source (with a duration depending on the load) rather than a decaying voltage source? Or does some mechanism in the alternator somehow give rise to a more voltage-source-like behavior?

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You are right with the explanation, however when you design a specific component you do not know what is being connected in the 24V vehicle bus... therefore it is difficult to predict the current behavior and which voltage will be induced. The voltage model makes it much easier to test and standardize.