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Old 03-27-2011, 02:22 AM   #1 (permalink)
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Precharging and Closing solenoid using RFID ignition system

So I'm almost finished converting my motorcycle to all electric. I am using a DC solenoid in between the controller (an Alltrax AXE 4844) and the battery ground. I already have written up an RFID recognition program for the Arduino. So when i swipe the correct card across the antenna the Arduino attempts to recognize whether or not the it is the correct card, and then it blinks a green or red LED accordingly. Once the correct card is swiped I have a digital I/O pin write high to switch on an N type mosfet, similar to the TIP220. When the mosfet closes it completes the 12v circuit across the coil of the solenoid causing the solenoid to close and the full 48v to run across the main contacts and straight into the controller.

Now, across these main contacts i have a precharge resistor. its purpose is to precharge the large capacitors in the controller and prevent them from being damaged by the large influx of current and voltage when the solenoid closes. But this resistor is always allowing (a relatively small) voltage across it, therefore draining the battery. In order to prolong battery life I want to be able to turn off this resistor when the bike is parked. I have tried doing it by running it through another transistor. And changed the code to turn that precharge transistor on first, wait a certain amount of time, and then power up the other main transistor. But when the precharge resistor should be on, nothing happens. Is it because the precharge resistor has no specific polarity and therefore isn't ever in contact with the positive wire? Or am i going to have the transistor turn on another, smaller, relay? Help would be greatly appreciated.

Check out the rest of the build at:

Last edited by majorpayne; 03-27-2011 at 02:28 AM..
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