time delay relay

Thread Starter


Joined Jul 24, 2011
Hi Guys,
I notice this thread is quite old, but I wish to try out a time delay circuit for the ignition process on my electric race bike. The original poster had issues with the circuit, so I have come up with something that *should* work (see attached).

The idea is to power on the main contactor for the high voltage system with a ~7 second delay. During this time, a high wattage pre-charge resistor, placed across the high voltage terminals of the main contactor will be closed circuit from a second contactor, precharging the giant capacitors in the motor controller. The idea is that once charged, the delay will be sufficient to stop me from belting right across to the ON position, welding the contactor across the terminals due to a ~2000 A inrush current.

Hopefully the SPST relay doing this won't have issues with 115 volts?

Any advice is much appreciated.\Cheers,



Joined Nov 25, 2009
Your post, originally found here (http://forum.allaboutcircuits.com/showthread.php?p=431333#post431333), has been moved into its own thread, where it will draw more responses.

Please refrain from "hijacking" existing threads with tangent or off-topic questions. You can create new threads for new questions using the "New Thread" button on the upper left of the page.

Can you tell us a bit more about your bike project? Is it home made?


Joined Jul 17, 2007
Consider using incandescent lamps instead of resistors to charge your capacitors. Reason is that incandescent lamps have a non-linear resistance; when hot, their filament resistance is quite high; as they cool, the resistance is quite low. This means that the initial charge rate will be high until the filament heats up, then will be more or less a straight line as the caps charge.

You might consider using a high-power SCR instead of a relay for the switch during initial charge.