Car transient/overvoltage protection - Page 1
Forum section: electronics / beginners

author: cs.dk
author_rank: Unknown
timestamp: December 19, 2015, 09:22:19 pm
content: Just had a discussion with a mate.Whats inside those cheap ass overvoltage/transient protectors? (Like this onehttp://www.conrad.com/ce/en/product/377175/Voltcraft-12V-Overvoltage-Protection) - They claim to extend bulb-life, etc.. I really don't believe they actually are worth the money. A leadacid is a good "slow" capacitor, and will handle spikes well.Any clues?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2015, 09:30:29 pm
content: Doesn't look like anything that would withstand a full series of automotive transients, not more than once.Tim

author: Simon
author_rank: Unknown
timestamp: December 19, 2015, 09:32:01 pm
content: probably a nice set of 600W TVS diodes that cost around £0.3 each! You can get spikes in automotive but yes to put on one the battery is pointless. Spikes travel around a system very fast and sticking a TVS at one end of the circuit and expecting it to protect something at the other is naive at best. Yes it can be useful if applied to the device that needs protecting.I once investigated the reason for a comervial thermostat circuit being blown up on an air conditioning system and found that back EMF from fans having power removed reached 400V, despite it being battery powered, the spike move faster than the batter can absorb it assumin the battery comes before the sensitive thing that will get zapped.It's hard to visualize how you can have a 24V DC system with -400V spikes running around it as well..... but it happens, didn't I half get a shock off it too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2015, 09:50:19 pm
content: Quote from: Simon on December 19, 2015, 09:32:01 pmWell, in transmission line terms, what's happening at one end knows nothing of what's connected at the other, until the pulse has had time to reach and reflect off that end. And even once it has, it takes many repetitions of this to 'negotiate' a large impedance ratio, whether the line is effectively open and charging to a high voltage (can happen with loose cables after handling -- plugging in a charged cable delivers a flat topped pulse directly into the circuitry on the pins!), or effectively short (whether by low impedance, like a battery, or by low transient impedance, like a TVS or MOV or spark gap clamping device), charging current.Spikes of 300V are common in automotive applications, and spikes of 1000V+ are common in mains and telecom applications. Typical pulse widths range from 50ns (EFT, ESD) to 20 or 1000us surges (if applicable). Automotive also has the white elephant of load dump: supply voltage swells of ~5x nominal with around an ohm source impedance, for a risetime of about 3ms and duration of 300ms. This is why most "automotive" parts are rated for 60V or more, rather than 30 or 40V like conventional linear ICs.Tim

author: TorqueRanger
author_rank: Frequent Contributor
timestamp: December 19, 2015, 09:55:29 pm
content: Couldn't you ferrite beads to help with the spikes like in a power strip ??

author: Simon
author_rank: Unknown
timestamp: December 19, 2015, 09:57:52 pm
content: Quote from: TorqueRanger on December 19, 2015, 09:55:29 pmagain if you put them in proximity of what you want to protect maybe, but nothing beats a TVS. If you want to get medeival on it then a TVS with a capacitor in parallel and an inductor in series.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2015, 10:11:13 pm
content: Quote from: TorqueRanger on December 19, 2015, 09:55:29 pmNo -- even the shortest pulses contain too much flux for a core to absorb, and the impedance of a core is too low anyway (a common mode core works against the common mode impedance of the cabling, typically 150 ohms already). Surges will completely saturate a core.The only thing you can do is absorb or shunt it (with a TVS/MOV/GDT), or ride it out (with enough voltage range, or in the case of common mode, with contiguous grounding to make a Faraday cage).Tim

author: cs.dk
author_rank: Unknown
timestamp: December 19, 2015, 11:12:18 pm
content: Gotta buy one, and tear it apart. But they are probably gunked upI would say, waste of money, or can they actually do some?

author: SeanB
author_rank: Super Contributor
timestamp: December 20, 2015, 07:30:05 am
content: Car electrics are really noisy, and will have horrid spikes, surges and noise on them at all times when the engine is running. Even worse if the electrics are ageing, and truly horrid if made by the Prince of Darkness, AKA as Lucas. 400V spikes on the battery every plug firing, 30V pulses from the alternator and more as the ground wires typically are daisy chained to many devices inside the loom so they only need a single chassis point to connect during assembly.Thus noisy, but putting something at the battery will do nothing for that, you need it on each device that is sensitive, to act on both the supply and ground wiring, which is why a radio set up is often supplied direct from the battery and has it;s own dedicatedfilter on the supply lead along with a short separate chassis connection to ground it, and often the higher power units have a separate battery for themselves only, fed from a separate alternator or from a charge controller on the regular alternator to split them electrically from each other.