Designing an alternator protector  - Page 1
Forum section: electronics / projects

author: MadScientist
author_rank: Frequent Contributor
timestamp: December 07, 2021, 09:54:32 am
content: I toying with some ideas of a precision alternator protector , ie to protect the alternator against load dumps , in particular , battery disconnects ( lithium HVC )The obvious is some big ass TVS , but the knee isn’t precise. I want to set the protection at around 18 VI was thinking of a using mosfet crowbars, driven by a precision voltage referenceAnyone any ideas or tried these types of solutions , issues , gotchas etc.

author: David Hess
author_rank: Super Contributor
timestamp: December 07, 2021, 07:01:13 pm
content: Quote from: MadScientist on December 07, 2021, 09:54:32 amHow precise does it need to be?Essentially what is required is a power shunt regulator. I have made them in the past using a power bipolar transistor and zener diode in whatever size was required. A TL431 can be used in place of the zener diode for closer regulation but transient response is not quite as good.

author: CaptDon
author_rank: Super Contributor
timestamp: December 07, 2021, 07:11:13 pm
content: You aren't dumping a load if your lithium device is charged and floating at system voltage. Is this an automotive system? The system should already be fast enough to shed loads of 20 amps or more without dangerous overshoots. Tell us more about your project. 12 volt alternators can safely output over 30 volts without diode damage when you monkey with the regulator. It is the rest of the automotive system that may get damaged.

author: MadScientist
author_rank: Frequent Contributor
timestamp: December 10, 2021, 12:22:54 am
content: It’s on a boat. HVC disconnect on the lithium leaves the alternator exposed and also any systems connected to it. I’d like to quench the overshoot to about 15-16v

author: CaptDon
author_rank: Super Contributor
timestamp: December 10, 2021, 12:55:38 am
content: Active circuits are kind of slow to respond and you don't want SCR crowbars!! Either some 50 watt 16 volt stud mount zeners in parallel on a heatsink or an active circuit using a mosfet in series with a large power resistor of perhaps 1 ohm. The active circuit should cause the mosfet to be fully saturated or completely off and not in the linear range for any length of time. I am not sure what the leakage current of several 16 voltish zeners in parallel would be at nominal battery voltage of 13.8vdc. I doubt you would want them in circuit and left unattended. I would guess the leakage current could be 10ma. or so total. Not insignificant unless your boat has trickle chargers running on shore power.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 10, 2021, 01:29:51 am
content: What's a "lithium HVC"?

author: TheMG
author_rank: Frequent Contributor
timestamp: December 10, 2021, 01:51:10 am
content: Put some supercapacitors in parallel with it. Super low impedance even compared to a battery, so they will easily soak up any temporary voltage excursions. As an added bonus you get extra cranking amps for the motor starter. Win-win.

author: MadScientist
author_rank: Frequent Contributor
timestamp: December 10, 2021, 11:08:31 pm
content: Lithium battery High Voltage Cutoff. This is the point where the BMS forcefully removes charge sources usually by a simple battery disconnect.