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I need to connect some things (LEDs and other misc.) to the 12V rail in a car. I understand that this rail gets a beefy ~100V spike on ignition. I have also heard that the battery soaks this spike (like a big capacitor). In any case, should I add circuitry to prevent the spike from hitting my LEDs?

Let me know if you need clarification. Thanks in advance.

Edit: I am also interested in keeping MOSFETs and microcontrollers safe. Would hiding it behind an LM7812/LM317 offer protection?

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migrated from mechanics.stackexchange.com Jun 21 '17 at 11:40

This question came from our site for mechanics and DIY enthusiast owners of cars, trucks, and motorcycles.

  • \$\begingroup\$ Where did you read about the 100v spike? \$\endgroup\$ – Moab Jun 21 '17 at 0:33
  • \$\begingroup\$ An engineer told me. I assumed he had measured it. Or he was perhaps trolling. In any case, it makes perfect sense that the battery would soak this spike. Have you connected sensotive circuitry (with MOSFETs, microcontroller baboonery) to this 12V rail without protecting it? \$\endgroup\$ – user2497 Jun 21 '17 at 0:35
  • \$\begingroup\$ "sensotive circuitry (with MOSFETs, microcontroller baboonery" Maybe this info should have been in your question. \$\endgroup\$ – Moab Jun 21 '17 at 0:45
  • \$\begingroup\$ @Nasha Wow. Thanks, this is immensely useful information. \$\endgroup\$ – user2497 Jun 21 '17 at 12:46
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    \$\begingroup\$ @user2497 now you are with rocket scientists! \$\endgroup\$ – Moab Jun 22 '17 at 0:02
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It is not uncommon indeed to see voltage spikes as high as 100V or even 120V. Generally spikes are clamped. To what voltage first depends on country/local regulations and standards (when there are). The lowest clamp I've read about (sorry, I cannot get my hands on that document anymore) is about 40 volts, which is still too high in almost all cases. You will need some form of protection as soon as non-passive, non-resilient circuitry is involved. But don't take my word for it as it's a condensed version of what I remember. This is just an introduction to what the automotive environment can be.

You can search for terms "engine crank and load dump" to get an idea. This is what I did when I started my automotive projects. You might be interested as well in a dedicated application note from Texas Instruments. There's also a "simple" protection against load dump overvoltage from the same manufacturer.

As a final note be sure to select automotive components when designing your projects. Not that it won't work with ordinary parts but the automotive qualification guarantees that those parts are designed for the harsh automotive environment, which is not limited to just electrical "goodies".

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  • \$\begingroup\$ What aboot decoupling capacitors? If I add e.g. a 150V 100uF cap (should be enough for 400ms?), will I soften the impact of the spike? \$\endgroup\$ – user2497 Jun 21 '17 at 13:25
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    \$\begingroup\$ Don't use caps as means to protect against spikes because it's all about energy. A capacitor will absorb energy but will release it to the circuitry after the spike has gone, hence actually lengthening the spike instead of killing it. It is best to use clamping TVS instead or an active transient suppressor as described in the application note. \$\endgroup\$ – user59864 Jun 21 '17 at 13:40
  • \$\begingroup\$ You are a font of knowledge. \$\endgroup\$ – user2497 Jun 21 '17 at 13:57
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    \$\begingroup\$ @user2497 Well, I don't view myself as such (been a n00b myself before) but thanks for the compliment :) . \$\endgroup\$ – user59864 Jun 21 '17 at 14:06
  • \$\begingroup\$ I've been looking for a verbose answer to this question for months. I have no oscilloscope. Thanks! \$\endgroup\$ – user2497 Jun 21 '17 at 14:59
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The biggest inductive spike is normally from the starter motor, especially on colder days.

The battery does absorb the spike.

Normally, your LEDs would not require overvoltage protection, and many LEDs and other appliances used in cars and trucks, have some kind of overvoltage and spike protection already integrated.

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  • \$\begingroup\$ Well done, thank you. So if I add a 12V regulator (and accept the perhaps 1.25V drop), that should be good enough? What about a decoupling cap? \$\endgroup\$ – user2497 Jun 21 '17 at 0:37
  • \$\begingroup\$ This integrated overvoltage protection covers every part of the 12V rail? Or just audio system and cigarette lighter port? \$\endgroup\$ – user2497 Jun 21 '17 at 0:39
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    \$\begingroup\$ In general, there is no protection for cig lighter adapter ports. And in general audio devices and electronics for the car has integrated (internal) protection. This is often some diodes, zeners or MOV (metal oxide varistor). Why add a 12V regulator on a 12V car? Automotive LEDs are normally designed for a 14V supply (given the alternator is running and charging the battery.) \$\endgroup\$ – mongo Jun 21 '17 at 0:49
  • \$\begingroup\$ These are rated for 12V, though I expect they use a lot less. My brutish logic is simply that a spike would not pass e.g. a LM317 regulator, and in the worst case just break that. I have an inverter, and its manual explicitly states that I must not have it connected when turning the ignition. FUD? \$\endgroup\$ – user2497 Jun 21 '17 at 0:51
  • \$\begingroup\$ If your devices are not just LEDs and are voltage sensitive and not rated for automotive environments, then greater protection might be in order. The inductive spike does not come from turning the ignition on, but rather from engaging the starter motor, to be clear. \$\endgroup\$ – mongo Jun 21 '17 at 0:56

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