Automotive Electronics--The 24V Jump Start Condition

[TerminalJack505]

Automotive circuits are expected to handle several transient over-voltage/under-voltage conditions.  Most of these conditions are short-lived and can be handled with surge suppression components such as TVSs and MOVs.

There are two particular over-voltage conditions, however, that are longer lived.  The 24V jump start condition and the failed regulator condition.

The 24V jump start condition is described in this document as follows:

Jump Start

The jump start transient results from the temporary application of an overvoltage in excess of the rated battery voltage.  The circuit power supply may be subjected to a temporary overvoltage condition due to the voltage regulator failing or it may be deliberately generated when it becomes necessary to boost start the car.  Unfortunately, under such an application, the majority of repair vehicles use 24V battery jump to start the car.  Automotive specifications call out an extreme condition of jump start overvoltage application of up to 5 minutes.

The failed regulator condition is a steady state condition (that is, it will potentially be very long-lived) and the voltage can reach +18V.

A circuit that I am designing controls two 12V solenoid-operated valves.  These valves each require about 1.5A at 12V.  Naturally, the manufacturer says nothing about what happens when the solenoid is driven at 24V.  It's probably fairly safe to assume this shouldn't be done, if it can be avoided.

My question is, what is typically done in these cases?  Ideally, I want to keep the solenoids energized during these conditions.  What is the best, most cost effective way to protect the solenoids but keep them energized?  Or is it even a problem?  Would the solenoid coils survive these conditions?

[NiHaoMike]

If you're willing to break one, just try connecting it to a 24V bench power supply.

As for a solution, you could PWM the solenoids, maybe at 2-3kHz to also cause them to make an overvoltage warning sound.

[TerminalJack505]

The PWM idea sounds like it could be an inexpensive option. 

Unfortunately, I don't have any of the valves right now so I can't test either of your ideas.  Right now a friend and I are just kicking around ideas.

If it works then it would be better than some of the ideas I came up with.  The cheapest solution I could come up with is the attached circuit.

[poorchava]

If I remember correctly ISO17650-2 standard specifies time of jumpstart condition test to 60s and regulator failure as 1 hour (for the sake of qualification).

[TerminalJack505]

If I remember correctly ISO17650-2 standard specifies time of jumpstart condition test to 60s and regulator failure as 1 hour (for the sake of qualification).

Thanks for the info.  Its sounds like the coils might be fine without any extra protection in that case.  I'll probably still add it, though.

The PWM solution would likely require no extra components.  The circuit will be MCU controlled so it would just need to detect the over-voltage condition and act accordingly.