[Bill Harris]

As an electrical engineer with 30+ years of experience and WAY too much time on my hands, I'll add my 2000 cents.

The concept behind the device is fairly reasonable, although the execution and price might not be.

A solenoid is nothing more than a hollow electromagnet with a movable element in its axis. When a solenoid is energized there is a strong magnetic field created which is especially concentrated in the hollow center. It is this magnetic force that moves the hydraulic valve controlling the trans-brake. When the solenoid is de-activated the magnetic field collapses on the windings which created the field to begin with. A changing magnetic flux induces a current in any electrical conductor which crosses the magnetic field lines as discovered by Faraday in 1831. This is the exact same phenomenon that is at work in an ignition coil (which is actually a transformer which has two electrical windings as opposed to the one in a solenoid). So the collapsing magnetic field induces a current in the solenoid windings even though the trans-brake switch is open.

If a current is induced in an electrical circuit, then it has to flow, regardless of how much resistance there is to that flow. Ohms law says that voltage = current times resistance (V=I*R). Since the trans-brake switch is open, it's resistance is essentially infinite (but not really) and the current is greater than zero, so that means that the voltage also has to rise toward infinity. Of course the voltage won't reach infinity since something in the circuit is going to be unable to withstand the rising voltage and will electrically break down, allowing the induced current to flow. In an ignition system, what gives is the resistance across the spark plug gap when the fuel/air mixture ionizes causing the current to flow very radidly in the form of a spark (unless you have a "bad" plug wire and it breaks down first, causing a miss in that cylinder).

The trans-brake circuit is part of the car's normally 12 volt electrical system. Since it is a low voltage system, there is usually something in the circuit which cannot withstand the voltage rising beyond 60 volts or so and whatever that something is will break down and conduct, allowing the induced current to flow.

Automotive electrical systems are some of the nastiest systems that engineers have ever had to deal with. There are all sorts of faults which must be tolerated, such as alternator load dumps, reversed batteries and horrendous EMI from the ignition system. Any properly design automotive electrical (and especially electronic) device will have circuitry whose purpose is to protect the device from excessive voltage. In almost every case, this protection circuitry will begin to conduct when the 12V input rises above a safe level for the device. It is this (these) path(s) where the current induced by the solenoid finds it's way to ground.

So, the graph shown in an earlier post is accurate in that it illustrates the voltage spike on the +12V circuit which can occur when a sufficiently large energized solenoid is de-energized.

So what? The fact of the matter is that the spike in voltage occurs AFTER the solenoid is de-energized, i.e., AFTER the trans-brake is released. The voltage spike is not going to re-energize the solenoid since the voltage is going the wrong direction for that to happen. The spike MIGHT affect other electrical (or electronic) devices that are on the 12V circuit, such as a delay box or ignition box, but these devices should have their own over-voltage protection. If they are properly designed they won't give a hoot about the voltage spike. If they ARE affected then THEY should find their way into the lake at E-town since they are not up to the task of operating in the inevitably nasty electrical system of a racecar.

So what is inside this "stop it" thingy? One post said it was a resistor and capacitor in parallel, then later said it was a resistor and a "diode so that the voltage wouldn't feed back". I have not been inside one of these things so I'm only speculating, but from an engineering perspective it makes no sense at all to have any capacitors in that box. A capacitor and resistor combination would cause an electrical delay which would likely vary with temperature and do nothing much to suppress the voltage spike, although it could spread it out over time depending on the values of the resistor and capacitor. An RC circuit would also act differently with different solenoids since their inductances are not all the same. More likely there is a low ohm resistor and a semiconductor known as a "transzorb" or "varistor" in series which makes the most sense electrically.

A "transzorb", a.k.a. "transient voltage suppressor" is a semiconductor which is specifically designed to limit the transient (i.e. short-term) voltage on an electrical circuit such as an 12V automotive supply. Physically it looks like a diode and electrically it sort of operates like a specific type of diode known as a "zener diode", but it is not the same thing. A transzorb reacts very quickly to voltages that exceed a particular level and is designed to absorb large amounts of power for very short periods of time, which is exactly the situation created when a large solenoid is de-energized. It's function is to "clamp" the power rail at a set voltage level (usually 18 to 20 volts for automotive use) and absorb any excess transient power that is on the rail. Putting a low value resistor in series with the transzorb would make sense to limit the current through the transzorb under extreme conditions.

So, my guess is that it what is in the box. If you have quality electronics on your car, they will be designed to withstand any voltage transients (probably with transzorbs of their own). You can buy a transzorb and resistor that would do the same thing as the box for less than a dollar, it just wouldn't be as pretty.

Damn, am I bored.... when do we start racing again?