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2020 Issue-2

Protection Of Device Against Automotive Transients, Reverse Voltage

I was wondering what would be suggested to properly provide protection of a device with a 5V low power regulator and LED driver against automotive transients, and reverse voltage that is connected to an automotive starting battery.

I presume a TVS diode could be used, but should it be bi-directional or uni-directional? A Schottky diode could be used for low voltage drop, first in line for reverse voltage protection, but it would have to withstand the clamping current of the TVS diode.

Would it be be necessary to include a resettable fuse if the transient lasted too long and exceeds the power rating of the TVS diode?

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Wayne Carpenter
Omak, WA

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Answers

Using a uni-directional TVS diode and series Schottky diode is an excellent start. This will take care of garden variety transients caused by relay switched inductive loads. Cars are full of those. Some capacitors on the input will help too. In addition, there are a couple of unique automotive transients that require attention.

The first is called double battery jump start and happens when the tow truck guy jump starts your car and has a second battery in series with his truck battery hooked to his jumper cables. This results in about 26 volts and a typical automotive device must withstand this for one minute.

The second transient is called load dump and occurs when the battery becomes disconnected while being charged. The alternator regulator cannot react instantly and will generate a transient that can reach 60 volts peak and then slowly return to normal charge voltage over about 400mS.

I solved these problems by using regulators that would stand the 26 volt jump start. You will need a bit more heat sink but it only lasts one minute. There are some automotive regulators that will stand the 60 volt load dump as well. An example is the LM2931. However, it is low current (100mA).

Another way to deal with load dump is use a resettable fuse (PPTC) in series with the input diode and use a large enough TVS to blow the fuse. In case you have not dealt with automotive 12 volt systems before, the normal run voltage is actually 9-16 volts with nominal about 14 volts.

Jim McGrew
Saline, MI