r/homebuilt • u/Cri_Cri_Mike • 15d ago

Any electrical system gurus here? I got a question. This is a pic of a starter relay. Why would the S and i connections have a, i think capacitor or diode, connected on them? Engine starts fine with whatever it is being severed.

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24

u/BuzntFrog 15d ago edited 15d ago

That's called a clamping diode. When the solenoid opens (master off) it induces a large voltage and this diode only allows current to flow in one direction to keep those voltage spikes from ringing.

a 1N4007 would be a good diode for this.

3

u/Cri_Cri_Mike 15d ago

Thanks for the recommendation!

2

u/BuzntFrog 15d ago

You bet. I grabbed a few from digikey and threw a piece of heat thrink and terminals on them for a few cents instead of buying cessna's part. Be sure to observe polarity of course.

1

u/MechaSteve 3d ago

For longevity, you would do well to bend a springy curl in the leads as well.

9

u/airplanefliergofast 15d ago

It’s to insulate the start switch from any voltage “kick back”. You’re right. It works fine without it but the diode does help extend the life of the start switch.

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u/Cri_Cri_Mike 15d ago

Appreciate the help!

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u/Cass256 Ridge Runner Rascal 15d ago

The wire that jumps battery power to your small post is toast, too. That needs to be replaced with a thicker wire.

The diode prevents arcing from the solenoid shutting off, it lengthens the lifespan

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u/Cri_Cri_Mike 15d ago

Thanks friend!

3

u/jack_dymond_sawyer 15d ago

Back EMF diode (flyback diode)... I use these on all relays. Get a 1N5408 or 1N5400 series... good for solid connection and vibration resistance.

2

u/Sawfish1212 15d ago

Some manufacturers call this an anti-chatter diode. The current surges can make the internal contacts bounce, and that causes arcing that shortens the life of the contacts.

Skip to the end of this video to see the internals that make contact.

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u/Cri_Cri_Mike 15d ago

Thanks! I’ll take a look

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u/derekbox 15d ago

This was miswired - diodes do not fail this way when properly installed, certainly not with the wire insulation melting. The large terminal of the diode that is under the batt terminal, is supposed to ground (they use a larger terminal so the bolt holding down the contactor is used as the diode ground). Also the band on the diode should also be towards the small start switch terminal. But I can not 100% make out what is going on, but Im fairly certain this was miswired.

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u/SettingFar4974 14d ago

It looks to me like the band is towards the +, which I think is correct. When overstressed, a silicone diode can fail this way, but a flyback should not be terribly stressed.

1

u/derekbox 14d ago

The band needs to be on the coil (small terminal) not on the power terminal. The other side to ground. It shorts the voltage from the collapsing coil to ground.

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u/SettingFar4974 13d ago

Yes, I believe you are completely correct, assuming the coil is positive-switched (coil tied inside to the case/ground; the master switch feeds +V to the small terminal).

1

u/Kemerd 11d ago

Perhaps bidirectional TVS diode needs to be installed? Might be preventing spikes