Switching an inductive load from a car battery

As you might know (from my other threads), I'm building a coil gun and it's powered from a car battery. The inductive load in question is a solenoid. In one of those threads, RuggedCircuits suggested that I include some protective circuitry in my design if the battery is still connected to the car while the solenoid is being switched. What kind of components would I have to use? The inductance of my coil is about 1 - 1. mH (milli Henry). The total current flow will be about 6 A.

RuggedCircuits suggested that I include some protective circuitry in my design if the battery is still connected to the car while the solenoid is being switched.

Perhaps you can explain why this is needed first.

oh.. sorry. It's because inductlive loads (such as this solenoid) have an 'inductive kick'. In other words, they can generate high voltages when they are turned off.. I need to protect the battery (because it's still connected to the car) from these voltages and also because the protection leads to a better design..

Hi, I think you may need to protect the car's electronics if still connected to the battery, but the battery will be OK..

Car starter motors put out a bigger spike (AKA Load Dump), and cars are usually designed for this..

hm…
Thanks.
Other than that, what are the general guidelines for switching an inductive load? I’ve already put in an antiparallel diode.

The antiparallel diode will clamp the reverse voltage.

Another question is how fast do you need the coil current (and therefore magnetic field) to decrease after you 'turn off'? The reverse diode will slow the inductive decay period.

Sometimes Zener diodes or diode-resistor combinations are used to quicken the magnetic decrease at shutoff.

Google about "stepper driver time constant" etc. Fast stepper motors can not use simple reverse diodes .. too slow.

But maybe you don't care if projectile has left the area?

Thanks for the replies.

I think I'll be able to compensate for that by adjusting the timing through my arduino. If that doesn't work, I'll have to put in a resistor. I've read up a bit about this too.. basically, the system can be modeled as an inductor-resistor circuit..

I ran a quick simulation in LTspice (a circuit simulator).
With the diode in anti-parallel, the peak reverse voltage across the ends of the solenoid was about -0.9 V.
Does that sound correct? If so, it should work well no? (assuming my 1N4007 diode doesn’t blow up)

The max current in the anti-parallel diode is the same as the max current in the coil. Diode pulse ratings are usually many times the continuous current so I don't see the diode being at risk.

that's a relief.. I check the datasheet to make sure and that confirms that the diode should be OK.

Cars are loaded with indictive motors, relays, solenoids, and such. I doubt that they have individual indictive kick limiting devices.