Correct, I have seen 60V transistors blow up when releasing the current in a 12V coil, so in order to still *quickly* release the energy from the coil, use some kind of voltage limiter (Zener, TVS, transzorb, MOV, pick whatever your preference to limit the peak voltage to below what the switching transistor can handle. Indeed the "reverse diode" works, but that means that the voltage that is driving the coil to reduce its current and release, is only 0.7V so the time it takes for the current to decay is an order of magnitude longer than the time you took with the 12V drive to close that contactor. Since especially DC contacts opening are susceptible to sustaining an arc, opening slow under fault condition (carrying drive current) will literally burn away the contacts quickly with the long sustained arc at full current... It is like you *want* those contacts to disappear into a metal vapor mist... Cor.
On Fri, Aug 16, 2024 at 6:29 PM Bill Dube via EV <ev@lists.evdl.org> wrote: > > As an aside, I would like to note that the reverse voltage spike that > occurs when you switch off power to coils, motors, etc. on the 12 volt > side of things must be dealt with. The magnetic field in these devices > collapses when you switch off the current flow, and then create HUGE > reverse voltage spikes which will find some component somewhere in the > 12 volt system. The energy of the spike _must_ find a home and will rise > in voltage until it finds some path to discharge itself. > > Typically, you put a "transorb" across the coil (or motor, etc.) to > absorb the reverse voltage spike at the source. It is also wise to also > put reverse voltage protection on other devices in the 12 volt system. > You can use a transorb in parallel, or a diode in series with the input > for protection. The spike will indeed find a home. You can either > provide it, or it will find a path on its own. > > I have seen all sorts of "mysterious" failures of all sorts of 12 > volt devices that "mysteriously" vanished when these reverse voltage > spikes on coils were properly addressed. > > Bill D. > > On 8/17/2024 3:47 AM, (-Phil-) via EV wrote: > > Yes, all EV contractors of any size need an economizer. Tesla uses a > > proper 2-stage driver instead of a freewheel diode so they can pull the > > contactor in FAST and transition to a PWM drive to keep the hold power low, > > then disconnect the recirculating MOSFET so they can discharge the coil > > FAST. You don't want fixed freewheeling diodes across your coil if you > > ever expect to break a load, as it will cause the coil to release slowly. > > > > Yes, you absolutely cannot just connect the coils to 12v, they will burn up. > > > > On Thu, Aug 15, 2024 at 5:06 PM Alan Arrison via EV <ev@lists.evdl.org> > > wrote: > > > >> I was wondering if most EV's keep chargers connected to pack full time. > >> > >> I want mine disconnected unless charging so that there is no pack > >> voltage outside the battery box (charger is in the cab). > >> > >> I have the DC fast charge contactors from a Tesla Model 3. They seem to > >> be custom made by Gigavac. > >> > >> They have isolated auxiliary contacts which I can use to enable the > >> charger only when the pack is connected. > >> > >> However, the coil resistance is only 1.2 ohms! At 12 volts each coil > >> would draw 10A and dissipate 120W! What is the deal here? > >> > >> Why such a low resistance? They must be using an "economizer" circuit > >> right? > >> > >> Thanks, Al > >> > >> _______________________________________________ > >> Address messages to ev@lists.evdl.org > >> No other addresses in TO and CC fields > >> HELP: http://www.evdl.org/help/ > >> > >> > > -------------- next part -------------- > > An HTML attachment was scrubbed... > > URL: > > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20240816/6c554238/attachment.htm> > > _______________________________________________ > > Address messages to ev@lists.evdl.org > > No other addresses in TO and CC fields > > HELP: http://www.evdl.org/help/ > > > > _______________________________________________ > Address messages to ev@lists.evdl.org > No other addresses in TO and CC fields > HELP: http://www.evdl.org/help/ > _______________________________________________ Address messages to ev@lists.evdl.org No other addresses in TO and CC fields HELP: http://www.evdl.org/help/
