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No sure this belongs here. If not, can you suggest which forum to post to?
I have a 12vdc circuit that feeds 12v+ to a 12 volt device that would shut itself down if it sees a spike or a pulse or sustained input of over 20 volts. (BTW: The max amps in the input would never exceed 3 amps.)
Although the voltage source is highly unlikely to ever exceed 14 volts a shutdown in this application would be highly undesirable.
So I’m looking for a module I could put in series on the 12 volt input that would clamp/limit voltage below 20 volts…. Say 17 or 18.
Does Digi have such a module? Is so, which?
If not, could you point me to a source?
A person needs to define normal and put some boundaries on expected fault sources in order to select a suitable overvoltage protection solution. For static discharge or the like something like a TVS diode might be suitable. If one’s contemplating something like a load dump scenario, there are purpose-built ICs such as the MAX16127TC+T made for that. (Eval board here.)
In terms of prepackaged/modular solutions the landscape is a bit thin, since it’s hard to give an answer to a question that’s not yet been asked, and such functions tend to be built into appliances that require them, rather than being a user-supplied afterthought.
One should also weigh the balance of “highly unlikely” vs. “highly undesirable.” It’s not uncommon for folks to over-think or mis-prioritize a problem and introduce unneeded complexity that ultimately makes for a less-robust solution.
The voltage source is an automotive type 12 volt battery that’s in parallel with an automotive type alternator with it’s own automotive type voltage regulator. Normally, before startup of the engine there is no risk of overvoltage. Once running the voltage regulator would hold voltage between 13.8 to 14.3 volts. The minor problem scenario (not the one I asked for a fix about) would be an alternator failure and voltage dropping below 12 volts as battery discharges slowly. So separate question would be a warning light or buzzer for undervoltage condition.
The rare but dangerous scenario, one I would like to mitigate would be a spike over 20 volts, or a failure of the voltage regulator, letting the alternator output excessive voltage over 20 volts.
I have no way of evaluating what the probability of that is other than “very low.”
The device that will shut itself down (until rebooted) if it sees an input over 20 volts is a CDI ignition module in an experimental small aircraft. So in a worse case scenario it could result in death. That’s what I meant by “serious.”
So it would be of comfort to have a 17 or 18 volt limiter on the line feeding it.
The additional context is enlightening, and further response would follow a few different threads.
First, the MAX16127 device would appear to be an appropriate basis for a system of the type requested. However, it’s not a finished device unto itself; one would have to plant it on a board along with other components in order to make use of it. That implies a multiplicity of solder joints, wire connections, and other potential points of failure, which without a lot of testing and quality control would probably decrease reliability rather than improve it.
Second, the big ugly in terms of “automotive” over-voltage transients is a phenomenon known as load dump, usually caused by abrupt disconnection of the battery while the alternator is dialed up to “bake”. So long as it stays connected, an SLI (starter/lights/ignition) battery is a pretty effective transient suppressor, capable of sourcing or sinking hundreds of amps in order to keep the bus voltage in that 12-ish window. If you get a regulator failure to a high-output state, that failure has a strong likelihood of being temporary, since short-circuit failures in semiconductors usually turn to open-circuit failures once the semiconductor device has been fully converted to smoke. An intact and connected battery would likely keep the bus voltage within reasonable limits for the duration of that process.
Third, this load dump phenomenon requires that something be spinning the alternator, which would in short order not be the case if it precipitates an ignition cutoff… Self-correcting on that front, though still pucker-inducing from the perspective that the big fan is shut off and the starter battery is now implicitly disconnected. There are reasons that so many aircraft engines run a stone-age dual-magneto ignition; more modern systems do have things to offer, but they are not without tradeoff.
Were it my choice I’d skip the added complexity, secure my battery, cables, and connections well, and remind myself that the “experimental” designation is not a mere regulatory formality.
