I'm not convinced that a 3 Ohm resistor will do enough to stop a surge. The voltage surge with one of those TVS diodes fitted can still be quite big. The other electronics in the car can be damaged as well as the alternator. Some alternators now include something similar. Something like this:- seems to be designed to deal with the load dump. The regulator will reduce the field current if the voltage is too large, but it will take time, maybe 200 - 500 ms. ![]() Therefore you may need to handle the full current of the alternator until the field winding current has reduced. ![]() The problem is that the alternator is rotating, and there is a magnetic field from the field windings, so it continues to generate current with nowhere for that current to go. Is there are separate connection to the cutoff switch that stops the engine? If the alternator continues to generate enough power to keep the systems running, the engine will continue to run. Also, it's possible that the mystery-box regulator has this function already built-in and I'm wasting my time, but I don't know - better to be safe than to burn through alternators.ĭisconnecting the battery won't always stop the engine. Sorry, I don't know how much energy the diode will need to dissipate. I was thinking that I would just get a diode with breakdown voltage around 18 to 20 V, but when I searched I came up with $40 diodes on DigiKey, so I think I made some incorrect assumptions. I think I need a diode that will allow the field energy in the alternator to dissipate to ground when the cutoff switch is flipped and the alternator is simultaneously disconnected from the battery and all loads. (2 and 3 weren't connected to anything when I joined the team, and the regulator still works acceptably.) The alternator doesn't have separate inputs for the field coil it only has 1) a connection for the battery, 2) a connection to sense voltage elsewhere in the car, and 3) an output to a warning LED. The motor is a Subaru EJ25 from around 2011. For simplicity, the team regularly uses the cutoff switch to turn off the motor. The alternator is connected to one output terminal, and all loads are connected to the second output terminal. The battery is connected to both input terminals. ![]() I have an endurance racecar with a DPST safety cutoff switch. ![]() To learn more about how and for what purposes Amazon uses personal information (such as Amazon Store order history), please visit our Privacy Notice.TL DR: I need to select a diode that will protect the alternator from a voltage spike when the battery is suddenly removed. You can change your choices at any time by visiting Cookie Preferences, as described in the Cookie Notice. Click ‘Customise Cookies’ to decline these cookies, make more detailed choices, or learn more. Third parties use cookies for their purposes of displaying and measuring personalised ads, generating audience insights, and developing and improving products. This includes using first- and third-party cookies, which store or access standard device information such as a unique identifier. If you agree, we’ll also use cookies to complement your shopping experience across the Amazon stores as described in our Cookie Notice. We also use these cookies to understand how customers use our services (for example, by measuring site visits) so we can make improvements. We use cookies and similar tools that are necessary to enable you to make purchases, to enhance your shopping experiences and to provide our services, as detailed in our Cookie Notice.
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