Precharging is certainly an important safety topic.
Leaving the controller energized can be a safety issue, as well as a fire hazard in case there is a failure when away from the car. I have heard of more than one Curtis catching on fire while charging (granted, the charging voltage was too high) when they had the precharge resistor connected all the time.
The other important reason for precharge is to insure that the controller has not failed "full on" while cooling off from being overheated (relatively common). If that happens and the precharge circuit turns the controller on despite it, then the car takes off uncontrolled.
Fifteen years ago I made a simple precharge controller for use with Curtis style controllers. You can see a schematic of it here:
http://cafeelectric.com/downloads/precharger_1.1.JPG
This circuit is very old, but it is where I started when I developed the integrated precharge for the Zilla so it is simple and lacks things.
For one, it has no way of insuring that your main disconnect is on before turning on the main contactor. Most people respond to that situation like this:
Hmm, car did not go but the contactor turned on...
Oh! Breaker is off!
Turns on breaker.
Bang! Damage is done to breaker contacts, possibly to controller as well.
If the breaker was tripped due to a controller failure you may find yourself dislocating the garage wall. A friend had the unfortunate luck of doing that when his Auburn controller blew on startup, I won't name him since I think he has still not told his wife what really happened.
Marginal controllers can be damaged when turned on without precharge, a Curtis engineer once told me it can damage their controllers, I've never seen it damage one of mine but since Y2K my designs won't start without precharge.
For those reasons it would be good to have a circuit that keeps the contactor from engaging if the HV pack is disconnected.
If anyone wants to play with this circuit, beware that I provide it only for entertainment use and needs many changes before it is safe. I never produced it since when I started to try to account for all the possible safety issues, I decided it needed to be integrated into the controller with the microcontrollers that watch each other for safety.
Also, as suggested by Lee Hart on the EVDL, put a 10 ohm 1/2 W Carbon comp resistor in series with the 400V cap. It helps protect the contacts at precharge initiation while still being low enough to allow the cap to snub 150V turn off.
As to what the circuit does:
HV inputs 1 and 2 are on the high voltage terminals of the contactor. As long as the voltage on those is over about 6 Volts (indicating the controller is not precharged, and sensed by the series circuit of the bridge, current source Jfet and optocoupler) then the fet that drives the contactor is held off.
Turning on the Key input turns on the precharge relay, which turns on the precharge resistor which is a PTC fuse. This allows for a fast precharge while also avoiding a blown precharge resistor in case of a controller fault.
If the controller has a fault, the PTC goes high resistance and the voltage stays on the main contactor contacts keeping the main contactor disabled.
D4 is just a contactor coil snubber diode.
This post is long enough, for my views on contactor safety you could visit my FAQ page at
http://cafeelectric.com/Sfaq.php and click on the link titled "How do I wire two contactors for higher voltage use?". Sorry for the funky use of a blog for the FAQ, it seemed the easy way at the time.
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