Varistor overvoltage protection in tail lights

Thread Starter

tomk825

Joined Dec 9, 2018
11
Hi everyone, I'm designing a few tail light circuits, one for brake lights and one for turn signals. Keep in mind these circuits will be completely sealed in the tail lights so I won't be able to easily adjust components or replace broken components like fuses.

(1) Do I need to worry about lightning-induced overvoltage in automotive applications? The only concerns mentioned in articles on this topic are electronic discharge and load dump from the alternator. The wire used for my harness is about 20 AWG and the rest of the circuit is on a PCB.

(2) The turn signal is powered by a 12V square wave coming from the central electronic control module (CECM) in the car. Would this CECM typically have a built-in overvoltage protection circuit? Or would I still need a voltage protection circuit in my tail lights anyway? I can't find any information on the topic. The car is a 2011 VW Golf. Thanks.
 

wayneh

Joined Sep 9, 2010
16,400
Hi everyone, I'm designing a few tail light circuits, one for brake lights and one for turn signals. Keep in mind these circuits will be completely sealed in the tail lights so I won't be able to easily adjust components or replace broken components like fuses.

(1) Do I need to worry about lightning-induced overvoltage in automotive applications? The only concerns mentioned in articles on this topic are electronic discharge and load dump from the alternator. The wire used for my harness is about 20 AWG and the rest of the circuit is on a PCB.

(2) The turn signal is powered by a 12V square wave coming from the central electronic control module (CECM) in the car. Would this CECM typically have a built-in overvoltage protection circuit? Or would I still need a voltage protection circuit in my tail lights anyway? I can't find any information on the topic. The car is a 2011 VW Golf. Thanks.
I can't address #1, but one partial answer to #2 is to determine whether the turn signal is switching the high-side (supplying and removing power) or the low-side (supplying and removing a return to ground with +12V supplied continuously). I'd put money on it being the latter, but I do believe there are different systems out there. In the low side switching scenario, the light sees full system voltage minus just the tiny drop across the switching transistor. This would be true of high side switching as well unless the ECM actually supplied regulated voltage for the lights, and I very much doubt that.

Are you certain that the power for the blinkers is really switched by the ECM? Most (older?) systems use relays or flasher modules.
 

Thread Starter

tomk825

Joined Dec 9, 2018
11
I can't address #1, but one partial answer to #2 is to determine whether the turn signal is switching the high-side (supplying and removing power) or the low-side (supplying and removing a return to ground with +12V supplied continuously). I'd put money on it being the latter, but I do believe there are different systems out there. In the low side switching scenario, the light sees full system voltage minus just the tiny drop across the switching transistor. This would be true of high side switching as well unless the ECM actually supplied regulated voltage for the lights, and I very much doubt that.

Are you certain that the power for the blinkers is really switched by the ECM? Most (older?) systems use relays or flasher modules.
I confirmed with a voltmeter that the turn signal is switching the high-side. The power wire for the turn signal comes out of the CECM.

Do you have any advice on a simple overvoltage protection method? The maximum voltage and current my circuit can withstand are about 18V and 2.5A.
 

wayneh

Joined Sep 9, 2010
16,400
Well your system will have other problems if you have 18V coming out of the CECM for more than a transient. I wouldn't worry about an average voltage over 15V. But transients are a whole different thing that I'm not qualified to address. Somewhere around here is a classic paper on the nature of automotive transients and how to design for them. Maybe authored by Pease? Not sure but that's where I'd look for guidance.
 

bwilliams60

Joined Nov 18, 2012
1,350
Most automotive ECMs are designed to handle up to 32VDC without any problems. They are also designed for reverse voltage protection but your best bet is to check with the OEM for more accurate statistics on your particular vehicle.
As for transient voltage spikes, most relays now have resistors or diodes to suppress these and the battery itself also assists in suppressing spikes.
 

ian field

Joined Oct 27, 2012
6,539
Hi everyone, I'm designing a few tail light circuits, one for brake lights and one for turn signals. Keep in mind these circuits will be completely sealed in the tail lights so I won't be able to easily adjust components or replace broken components like fuses.

(1) Do I need to worry about lightning-induced overvoltage in automotive applications? The only concerns mentioned in articles on this topic are electronic discharge and load dump from the alternator. The wire used for my harness is about 20 AWG and the rest of the circuit is on a PCB.

(2) The turn signal is powered by a 12V square wave coming from the central electronic control module (CECM) in the car. Would this CECM typically have a built-in overvoltage protection circuit? Or would I still need a voltage protection circuit in my tail lights anyway? I can't find any information on the topic. The car is a 2011 VW Golf. Thanks.

Vaguely remember GE offering a range of transient suppressors for automotive - I'd imagine there's application notes too.
 
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