Project Lexus SC300 Road Racer: Part 6 - Practical Panels

by Erik Jacobs

When we left our project SC300 we had just installed the XS Power S925 battery. As we said during the parts selection, the battery is a critical component to ensuring solid starting and good electronic component performance overall. For safety reasons, every race car requires some kind of cut-off mechanism to make certain the car dies when the lever is thrown. This helps completely kill the car in an emergency. In many cases, the easiest way to accomplish this is to ensure that battery stops being a part of the party at the right time.

When you start adding up all of the current (amperage) that flows through all of the electronics in a race car, and remember that all of it essentially comes from the battery, you’ll quickly realize that you need quite a big, heavy-duty switch to be able to just cut off this current. And, in fact, many race cars employ just that -- a very large heavy-duty rotary switch that has big heavy battery cables running to it to help disconnect things.

Some of you might be thinking "what about the alternator?" That's a very good thought. In an over-simplification, when the engine is spinning, the alternator is generating voltage and current to recharge the car’s battery. It is part of the charging circuit. Even without a battery, if the engine is spinning, the alternator can generate sufficient voltage and current to power the vehicle. If you started your car and disconnected the battery, it could likely keep running depending on how the car was wired. Many poorly wired race cars have a battery cut off that doesn’t isolate the car’s electrical system from the alternator and the battery. This results in a car that doesn’t go off when the kill switch is thrown. That, my friends, is no good. How can we ensure that the car is properly killed? Read on.

Since we are going high-end on this project, we decided to use something a little bit fancier than a big honkin’ switch: a contactor. According to the universal source of knowledge, Wikipedia, “A contactor is an electrically controlled switch used for switching an electrical power circuit, similar to a relay except with higher current ratings. A contactor is controlled by a circuit which has a much lower power level than the switched circuit.”

This sounds like the perfect thing for our needs. We want to be able to use a very low-power switch to turn a big heavy-duty circuit on and off. If you look around for race car cut off switches, you may see them (somewhat improperly) referred to as solenoids. While the contactor does have a coil, and a relay uses electromagnetism to move a contact similar to how a solenoid can convert electrical energy into motion, it’s not really a solenoid in the classical sense.

So, where does one find such a contactor? This is where some time and research pays off. In many cases every-day kinds of things have “race car” slapped on them and their price is doubled. A bunch of spelunking on the internet led me to find the Trombetta Defender family of contactors. This is the exact same unit that some racing equipment places sell for twice the price. It is rated for harsh conditions, has a nice Deutsch form-factor environmentally sealed connector for its controls, and will support switching up to 200 amps, which should be more than enough for our race car.


So, what do you do once you have your contactor? You drill giant holes in your car, of course. And, as Tom says, “there’s nothing I like more than drilling big holes in perfectly good cars.” Thanks, Tom. At least there was very little perfectly good about this car before we started drilling holes in it. Anyway!
The contactor ideally is located close to the battery, hence installing it on the transmission tunnel. Our good friend the nutsert (or rivnut, depending on how you want to describe it) comes to the rescue and provides a convenient threaded hole for a bolt to go through.
The final product is clean and affords good clearance to the transmission itself. It also means you don’t have to find your way underneath the car to hold a nut when installing the contactor, and also is very unlikely to back out since it’s affixed to the sheet metal like a rivet.
Here’s the result. The positive battery terminal will attach with 0-gauge XS Power battery cable to the contactor, and then the switched side of the contactor will continue on to supply the rest of the car. One side of the Deutsch form-factor connector will attach to battery positive, and through the kill switch circuit (detailed later in the series) will connect to ground, which will energize the contactor.
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Tuesday, November 15, 2016 8:49 AM
Good article,

How much did you derate that contactor?
Are you worried at all that the contacts might weld together?
I know there isn't much of an inductive load in the car but you might want to get some protection across that thing anyway, the ones in some welding machines are rated up to 300A.
Tuesday, November 15, 2016 8:51 AM
I meant some of the protection diodes in welding machines are rated to 300A.
Tuesday, November 15, 2016 10:25 AM
I feel like a contactor will not be accepted as a battery shutoff by sanctioning body. The point of a battery shutoff is to have a switch that can be mechanically and positively shut off by the driver or track worker in case of a crash (as I am sure you know) and is easily accessible from both inside and outside of the car. I think the contactor will not be considered a positive enough system for those grizzled tech men.
Spectrum (WI)
Spectrum (WI)link
Wednesday, November 16, 2016 7:18 PM
Instead of using that covered cutoff switch use a an E-stop button. I use these at work when I design assembly machine electrical controls. www.automationdirect.com is a good stite for the switch. For the guard around the button Google "e stop button guard" and search for your guard of choice. I kinda like the ones that have emergency stop etched around the guard. The nice thing about the switch if you are not familiar with them is that you can hook up multiple contact blocks in a wide variety of configurations (one for battery, another for alternator, etc.).

Personal preference about that switch manifold. Why not just mount each switch on the panel itself and ditch the that rectangular box thingy. Borrow a vinyl printer and print out a switch layout straight on the panel labeling everything nice and neat. If its a can bus thing I guess short term it would be easier to just use something already made.

I noticed you are still using a key to start the race car. Maybe use a push button start so you can delete that whole lock mechanism. I believe that would go well with your theme of simplifying wiring. If you don't want people starting the car use a keyed push button (again refer to the automation website). Just keep the spare key in your trailer in the rare event you want to lock out the vehicle.

What software program did you use to plan out the wiring harness?
Thursday, November 17, 2016 4:22 AM
The Ag OEM I used to work for loved using those Trombetta Defenders as main power relays. Which begs the question... are you building a racecar with tractor parts or were we building tractors with racecar parts?!? In all seriousness, we had a lot of problems with those trombettas when it came to real world abuse.
Thursday, November 17, 2016 6:56 PM
@dstroy and @dieseltech - I didn't spend much time deliberating on the contactor for a few reasons:

1) These are identical to the contactors that another high-end racing equipment vendor is selling
2) These are identical to the contactors used on the Robertson Racing Ford GTs (http://www.motoiq.com/MagazineArticles/ID/3712/The-Ford-GTs-of-Robertson-Racing.aspx)

As far as welding the terminals inside the contactor together, I'm kind of thinking that we'll be OK. There are a number of things at play here:

The contactor will not be switching the full current capacity on or off. While it is rated for 200A, it's unlikely to see near that current during standard operation. Two fuel pumps at 15A (overestimate), horns (yes my race car will have horns), every lamp (which will all be LEDs), and the rest of the electronics probably only add up to 125 amps (overestimate).

The Smartwire itself is capable of switching 380 amps (22x10 and 8x20) but we won't ever get anywhere near that much current.

As far as circuit protection goes, most people that do it on race cars end up doing it wrong. For example, the SC300 had a big lovely honkin' fuse in the engine bay, and about 13 feet of zero gauge wire to the battery in the back. That would have been great if the overcurrent event occurred in the engine bay. But if the battery cable got shorted in the cabin, the car would have burned down.

In the case of the Ford GTs, there is no circuit protection between the battery and the power distribution system. There's just the contactor.

The only current capacity concern might be driving the starter amperage through the Defender, but I again look at the Ford GTs as my example. A bigger, higher compression motor is able to be started using the same "little" contactor. I think I'll be OK!

Lastly, as far as real world abuse goes, I have a feeling that agricultural equipment probably has it worse than a race car! I'm willing to give it a try.

@dieseltech did the Ag OEM ever switch to a different contactor?

@SM_Clay72 a sanctioning body only cares about one thing - that the vehicle is properly shut off. The Ford GTs passed FIA tech with a contactor, so I have a feeling that NASA will approve as well. There is no requirement for mechanical or non-mechanical disconnect - only that activating the disconnect mechanism actually does the job. In the case of a contactor, which is a DC switching device that is essentially a normally-open relay, suspending the current flow through the "solenoid" side will effectively open the circuit, cutting the power.

Additionally, when we get to the wiring planning article segments, you'll see that both the internal and external cowl-mounted kill switches will disable not only the contactor (cutting complete power to the vehicle) but will also engage the power-off features of the Smartwire.

Directly from the NASA pro racing Club Codes and Regulations (https://nasa-assets.s3.amazonaws.com/document/document/282/2016.9.pdf):

"An electrical master switch is recommended on all cars, and required on some, as listed in the class rules. It should be mounted so that it is easily accessible from the outside. If mounted outside the cockpit, it should be mounted in an area where it is least likely to be damaged (e.g. cowling near wipers). The switch should shut off the motor and cut all power except to the on-board fire system and any other life support / medical device. The switch location must be clearly marked. "

The IMSA Continental Tire Sportscar Challenge rules (http://competitors.imsa.com/sites/default/files/2016_ictsc_technical_regulations_march_2_blackline_.pdf):

"8-7.2 A master electrical disconnect switch is required within reach of the Driver normally seated and belted. "

And, in the safety appendix:

"7.1 The exterior switches for fire suppression system activation and electrical circuit breaker, whether separate or combined, must be located for easy identification and activation by emergency responders. Exterior safety switches must not be located under the engine cowl, wipers, behind bodywork or hidden in any way that may impede activation by emergency responders.

7.2 Activation of the exterior safety switches by a gloved finger or hook using a single action by the emergency responder is required. The installation must not require the responder to perform multiple actions to activate the switch. The safety switches may be secured only during scrutineering, in Paddock or when the Car is located near large crowds (during Fan Walk) but must be removed prior to on-track activity."

You'll notice that both series make no specification regarding mechanical, electro/mechanical, or electric safety or cut-off switches.

There is actually a completely solid-state cut off switch that I originally looked into. While it carried an FIA certification, the way that it required to be wired seemed a little silly, so I went with this much simpler contactor setup.

@Spectrum (WI) - The covered cut-off switch is paired with an exterior safety switch. The wiring planning segments will help this decision make more sense.

If you refer back to the wiring planning article (http://www.motoiq.com/MagazineArticles/ID/4330/Project-Lexus-SC300-Road-Racer-Part-4--Beginning-the-Epic-Rewire.aspx) the Racepak Smartwire Switchpanel assembly is internally wired and set up, and only requires a single cable between it and the main Smartwire unit. As you suggest, it does use a digital communications bus (CAN with a proprietary protocol), which also makes it easy to add more switch panels, in addition to sensors and other things. I also just learned that Racepak is going to be debuting a membrane switch panel that provides much more feedback on the controlled circuits.

We simply have yet to remove the factory key/ignition/steering lock assembly. All in good time! The Smartwire Switchpanel has a start(er) momentary pushbutton on it that will be used to engage the starter.

For harness design, we'll be talking about Arcadia from Cadonix, a web-based software as a service (SaaS) harness design suite with a lot of really cool features. More on that in the future.
Friday, November 18, 2016 6:35 AM

As long as you're comfortable it'll keep you safe and it's already proven.

I know for a fact that protection diodes at least across the coil are extremely common, and that's not something you want to stop working while you're ripping around a track.

For example: They're used in milling machines, electric forklifts, and almost all smaller relays inside of OEM cars have some form of protection across the relay coil.

Anything that gets toggled a lot because the voltage spike that happens when the coil's magnetic field collapses is enormous comparatively and that can cause all kinds of stuff wear out prematurely without a reverse biased diode and resistor wired in parallel.

Friday, November 18, 2016 2:33 PM

Not sure I'm following you entirely. The coil-side is the low voltage/current side. Granted, the spike may be large, but this is not a device that gets toggled "a lot". If you figure a driving school weekend has four sessions per day, that's 8 "off" toggles a weekend. You would have to do 12 weekends to get to 100 toggles. Would you consider that "a lot"?

According to the Trombetta Defender datasheet ( http://www.trombetta.com/images/pdfs/dccontactors/defender/defender.pdf )

"With coil Transient Volt Suppression (TVS), the Defender protects sensitive components in your system."

That sounds a little odd, though, because the coil is the switched side, not the actual "contacts" or high-current side.

You've piqued my interest, though, so I'll send Trombetta an email and see what they say and report back. Thanks for the great feedback!

Sunday, November 20, 2016 10:49 AM
If I remember correctly, there were no plans to make any changes. I think it was mostly our self propelled sprayers that had issues. They're running through fields at quite a bit higher speeds then say, a tractor. Not to mention jumping terraces and drainage ditches. Lots of vibration. If it's already racecar proven, should be a slick setup.
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