Protect 12/24V leds in a car/truck from over voltage alternator burst? - Page 1
Forum section: electronics / beginners

author: nardev
author_rank: Unknown
timestamp: July 18, 2020, 12:28:59 pm
content: Hey, guys, what would be the cheapest yet good enough protection for 12/24V led strip.Sometimes, the LEDs and the RGB controller simply gets killed while starting the truck, I want to protect that but I need small and cheap as possible solution?Is the fuse+diode in line good enough? How would you do that?Thnx

author: helius
author_rank: Super Contributor
timestamp: July 18, 2020, 01:40:48 pm
content: Search for application notes for TVS load dump protectors

author: nardev
author_rank: Unknown
timestamp: July 18, 2020, 10:55:51 pm
content: I would need something pretty dumb. I just don't know if single fuse and single zener diode would do the job?

author: nardev
author_rank: Unknown
timestamp: July 20, 2020, 03:06:33 pm
content: Anyone?

author: Ian.M
author_rank: Super Contributor
timestamp: July 20, 2020, 06:49:01 pm
content: Edit: See Nardev's *original* diagram belowThe problem is, from just the above, you come over as someone who knows just enough to be dangerous (not to yourself as that's quite hard to do with only 12V) but definitely to your LED strips and even your vehicle.I *ASS*U*ME* you know that the cathode of a diode is normally marked with a stripe or band, and that Zeners operate by controlled reverse breakdown at or near their specified voltage, in a circuit that limits the current through them sufficiently to avoid their destruction by excessive power dissipation, and that fuses are typically at least an order of magnitude too slow to protect a semiconductor device from destruction.You've pictured the diode in parallel with the fuse. Depending on which side of the fuse is the supply and the load it will either render the fuse ineffective, or has a significant risk of failing shorted (or even exploding) as soon as the fuse blows when the circuit's self-inductance causes the voltage across the Zener to reach its breakdown voltage and its forced to carry the load current that just blew the fuse!If however we assume your diagram is wrong, and at the top, the 12V feed is on the left and the load on the right, and the vertical black line on the left is *NOT* present, and the bottom black line is connected to ground/chassis, its still a disaster, because the Zener diode would then be forward biassed, would draw as much current from the vehicle battery as the wiring loom resistance permitted, and destroy itself, blowing the fuse in the process, and depending on the magnitude of the fault current, either end up shorted, open circuit, or if there's enough energy, having blown itself to smithereens.You aren't a newbie, (member for eight years, and participant in some fairly technical threads): Are you trolling us or are you really that clueless with basic circuits?If the latter, we can work with that, *IF* you are willing to learn, although *PLEASE* don't be offended when you get directed to really basic learning resources, possibly even ones targeting students just starting high school physics.

author: nardev
author_rank: Unknown
timestamp: July 20, 2020, 06:55:30 pm
content: Sorry, this was the schematics that i saw elsewhere, just wanted to see what other ppl think. I simply need to make it as stupid as simple as possible.I read this too, but i don't have confidence so i'm actually asking for experience of others. I believe this could be pretty common thing, so, why not ask and see what other do in this situation.https://www.maximintegrated.com/en/design/technical-documents/app-notes/7/7084.html

author: Ian.M
author_rank: Super Contributor
timestamp: July 20, 2020, 07:27:43 pm
content: For the record, Nardev's original reply #2:

author: Ian.M
author_rank: Super Contributor
timestamp: July 20, 2020, 07:31:43 pm
content: The Zener probably wont handle the peak power in the transients commonly found on an automotive 12V system very well. For that circuit to work, you need a beefy unidirectional TVS diode, (basically a Zener on steroids intended for surge clamping applications!) with a working voltage of min. 18V in a 12V system or 36V in a 24V system so it doesn't conduct all the time and destroy itself if the alternator output goes a bit high.Don't get a bidirectional TVS diode as they are like a series pair of back-to-back Zeners, and you specifically need the TVS diode to clamp any negative going spikes on your supply, as I suspect that's what's killing your LEDs.To give you better suggestions, we'd need to know the current your LED strip(s) draw at 12V and at your actual running battery oltage, which is probably up around 14V, and their minimum operatingvoltage

author: Renate
author_rank: Super Contributor
timestamp: July 20, 2020, 07:43:26 pm
content: Let's start with the basics.Most crappy 12V LED stuff that you buy just uses a resistor to limit the current.For example, all those 24 or 27 (or whatever) LED bulbs use 3 LEDs in series and throws in a resistor for some sort of nominal voltage.In the real world vehicle voltage goes anywhere from 10 V (when cranking) to 14.8 V (when equalizing).(But in point of fact, most alternators only generate a high voltage when first started and cold.Since they are using the alternator temperature as the battery temperature they turn down the voltage pretty quickly.)No ignorant resistor is going to maintain the same current over this range.What you need are LEDs with a constant current driver or a voltage regulator for your LEDs.

author: Ian.M
author_rank: Super Contributor
timestamp: July 20, 2020, 07:56:15 pm
content: Yes. Assuming white or blue LEDs with a per LED Vf close to 3V, they'll be arranged in series strings of three for a total Vf of 9V. For a 12V supply, the difference is made up by the voltage drop across a series current limiting resistor, which will be chosen for the intended operating current and the supply voltage. Apply 14V instead of12V and the LEDs will get 66% extra current!If its a 'dumb' strip without a dimmer or flashing controller, you could try a string of three silicon diodes in series with the supply to the strip to drop approx 2V. Choose diodes rated to handle the total strip current.

author: nali
author_rank: Frequent Contributor
timestamp: July 20, 2020, 07:56:34 pm
content: OP mentions a RGB controller, so should be more than resistive regulation.Load dumps are pretty horrible very low impedance surges so a simple fuse/ zener (or better, TVS) would result in plenty of blown fuses, so a load disconnect would be better something likehttps://atmelcorporation.files.wordpress.com/2013/10/overvoltage-cut-out-circuit.gifIf OP doesn't want to get his hands dirty just buy an automotive fit-for-purpose DC/DC converter and run the LEDs from that - anything "E-Mark" certified would do the job.

author: Ian.M
author_rank: Super Contributor
timestamp: July 20, 2020, 08:04:40 pm
content: I hadn't spotted that. Yes, a RGB strip with a controller is going to need a fairly well regulated supply, and a diode dropper (without even a load resistor to keep the max voltage down) probably isn't going to be good enough. While I might be capable of DIYing a reliable low dropout automotive regulator and power conditioner, its far harder than it first appears to be, and unless I was being paid to develop it I'd probably also recommend getting an off-the-shelf automotive DC-DC converter intended to power 12V multimedia entertainment systems.

author: nardev
author_rank: Unknown
timestamp: July 20, 2020, 08:58:10 pm
content: Ok, thank you all for your generous effort.I can tell you what is happening in practice. In most cases, just wiring the led strip directly to truck's 24V line, works fine for a while. Although, sometimes, like in 2% cases, after a while, it stop working.Also, some trucks provide 12V line.My initial idea was to switch to 5V leds and DC-DC buck converter which would provide us kind of isolation. It would also simplify things as i would only have 5V diodes.I think that was it. I thought that some clamping could do the job but even i can see that it's not great idea.Also, i didn't get the chance to check the truck when such things happened, but i also assume that it could be something like this:"In automotive systems, a load-dump transient occurs when an alternator is delivering current to a battery and the battery is abruptly removed. This can happen when a discharged battery loses connectivity while the alternator is generating charging current and other loads are connected to the alternator circuit, as shown in Figure 2."here:https://www.maximintegrated.com/en/design/technical-documents/app-notes/7/7084.html