Have you ever wanted to own an electric car? or have you ever wanted to drive a Porsche 911? I wanted both, so I decided to build one. This blog is here to document my journey as I convert a 2002 Porsche 911/996 Carrera 2 from a gas guzzler to a completely electric vehicle - not a hybrid!

Sunday, January 6, 2013

Vaporizing relays

This weekend I wanted to finalize the electronics that turn on the new tank heaters and 12V fluid pump.  Below is a picture of the circuit I built using two cross coupled NOR gates to latch the momentary on/off switch from the cabin.  The output drives a N channel mosfet that drives a 12V relay coil which switches the high voltage & high current (~250VDC @ 12Amps) to the heating elements.

I wired up the circuit to the switch in the cabin and the relay.  The circuit latched on and latched off when the two state momentary switch was pressed on the appropriate side.  The internal light illuminated when it was in the on position, the relay also clicked into the closed position so everything seemed fine.  I then wired up the heating elements to the battery pack and inserted the relay as the on/off mechanism.  I went back into the car to test it.  I turned on the switch, the indicator light lit up.  I confirmed with my ammeter that the pack had additional 12Amps of draw, so it was on.  Then I tried to turn it off.  The momentary switch and the indicator lamp turned to the off position, but the meter still read out 12Amps.  So something was wrong.  I could smell burning under the hood.  I pulled the traction pack cable off the relay.

It turns out the relay fused in the ON position the first time I turned it on!  I picked this relay because it was supposed to be rated for 370V and 25Amps, but apparently my 250V @ 12Amps killed it on first contact.  I opened the relay to see the damage.  Here are pictures of the contacts fused together.


  1. That sure isn't a fun experience but it is a very good thing that you were monitoring it and didn't figure it was off, walk inside and later return to a dead pack.

  2. It would require a rework of your switching circuit, but it seems to me that wiring the dash LED to switch based on the presence of voltage coming out of the relay would be a good idea.

    This way the LED would indicate whether the heaters are powered, rather than the state of the output being sent through the mosfet to the relay.

    Perhaps a couple of resistors setting up a voltage divider to bring 250v down to 12v, and then a series current limiting resistor, feeding a power sense line back to the led would suffice.