Dino Starter Solenoid Flyback Diode

Apply 12v to the solenoïd, see if it goes in the appropriate direction, if not you know the voltage was reversed...

From then on (when the polarity is properly idendified), you just install a diode with the (-) of the diode toward the (+) of the solénoïd and vice versa ( + to -). This way the flyback current from the solenoid winding is captured before it gets back to your electric system.

 

I have never heard of this before for a starter application. However, in electrical enginnering it is very common to add a diode to eliminate the voltage spike when a magnetic field is deenergized. The diode is connected to the tab connector on the solenoid and goes to the chassis ground. See https://en.wikipedia.org/wiki/Flyback_diode for the theory behind this application.

 

I am more interested in the why. I understand the why with alternators but a starter? I am ready to learn so new stuff.

 

Here is a picture/diagram to put a figure to raemin's words:
Image Unavailable, Please Login

But I'd also ask "why" with Wade -- there are literally billions of ignition switches and starter solenoids working OK for a long lifetime without a flyback diode. Are you having a specific problem with your ignition switch life or just looking for some "insurance"?

 

In the alternator this is another story, diodes are placed by group of three in order to make a rectifier bridge, i.e a device that convert alternative current to continuous.

As far as the starter is concerned I never had to do it in any of my car, but some other windings have been troublesome in my car (power mirror solenoid being the most nasty one in my case). Maybe @racerboy9 wants to cure such an issue? The concept is quite easy : the rise and the fall of a magnetic field in a winding generate some current. When you switch off the solenoïd, there is a rapid fall of magnetic field, which produces a "negative" current (current spike) that gets back to the electric system. The starter is so close to the battery (unless the spike returns to the neiman first?) that this should not be an issue/ In any case the spike will only occur once (when starting the car), and there is so much noise at this specific moment that you may not notice the interference in your radio...

 

If you check some of the electrical schematics for modern cars, you will see a flyback diode for the starter in the schematic. I use a flyback diode in my 308 and 1750 Spider Roundtail to prevent damage to the expensive Bosch contact block in my ignition switch.
We use diodes in some of our electrical systems at my nuclear plant to protect our electronic assemblies from the coil discharging when continuity is broken.
I connected my diodes to the ignition switch terminal and ground.

 

Ignition switch to ground would work too, however not really necessary unless driving the starter directly by MOSFET or some other semiconductor sensitive to voltage spikes. No harm done adding it though.

 

Thanks guys. I love learning new stuff and electrical theory is not my strong point. At least I am glad I understand what is going on and why the diode is added now.

 

The alternative approach is to capture the spike with a "varistor" instead of a diode. If memory serves me well, there is a slight voltage drop induced by the diode as well as a higher delay.

 

No voltage drop or current flow across a diode when reverse biased by having 12v positive on the cathode. When ignition switch is released, coil field collapses and changes polarity across the coil. The current goes to ground via diode cathode. No arc across opening contacts of ignition switch.

 

The forward drop of the diode somehow determines the amount of current that will leak, you can reduce this with 0.2v forward drop diodes (a.k.a Schottky) diodes or a varistor. Also the diode will keep the coil energised, so the solenoid will take longer to return to its resting position. Probably not a big issue on a starter (this is not a fast switching device).

For the starter a Schottky diode is a good idea: these are easy to source and just as easy to install as a regular diode. For other windings that are more frequently used I would try something more elaborate.

 

Yes, that wiring is correct, but you might want to use a higher-current capable diode (the diode in those pics is a form factor typically limited to 1A max without blowing up).

 

That's better. It's not an easy thing to judge because the current rating is for a continuous current and this is a very quick, transient current thing (so going over the current rating for only a brief instant may be more survivable). One thing you could do is measure the conduction of the diode in the direction that it should conduct occasionally after some use just to make sure it is surviving. Otherwise, it's your homework to do some internet searching for what diode current rating is best for a typical starter solenoid application .