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
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