Diode for starter, master and aux ground power solenoids/relay

[jeffsvan]

Could someone recommend a Diode (part#) to put across the posts on the relay/solenoids that I can easily purchase. Want to reduce spark jumping back at the switches.

Can I use the same diode part# for all three (trying to keep it simple)

[Fred.Stucklen]

A 1N4001 (or higher voltage 1N4004) will work in all three locations....

Originally Posted by jeffsvan

Could someone recommend a Diode (part#) to put across the posts on the relay/solenoids that I can easily purchase. Want to reduce spark jumping back at the switches.

Can I use the same diode part# for all three (trying to keep it simple)

[az_gila]

Common parts and easy to obtain...

https://www.ebay.com/itm/10x-10pcs-I...g/252410601393

[lrgabriel]

Those diodes are significantly underrated for flyback application. I would look for something in the 3A If and 1000V Vr range, like the RGP30M-E3.

[rv7charlie]

Originally Posted by lrgabriel

Those diodes are significantly underrated for flyback application. I would look for something in the 3A If and 1000V Vr range, like the RGP30M-E3.

Meh...

Even the 1N4001 (1A 50V) is adequate, if you look at *peak* ratings. But the 3A versions are physically more robust (fatter body and heavier leads). And the higher voltage ratings don't cost that much more. When I ran an electronics repair company, we just purchased the higher rated devices to minimize our inventory.

Another number for the 3 amp 1000 volt devices is 1N5408.
https://www.ebay.com/sch/i.html?_fro...n5408&_sacat=0

The same device can be used on all the relay coils you're likely to use in an airplane.

[jeffsvan]

I found some 1N5406 in my parts inventory.
Would that work?
I'm not super keen on electronics specs.

[lrgabriel]

Originally Posted by rv7charlie

Even the 1N4001 (1A 50V) is adequate

I've seen 430V spikes from my intermittent duty starter solenoids. In fact, one such spike blew out my 400V rated flyback diode, shorted it to ground, and disabled my landing gear.

...I might be a little bit passionate about this!

[Mich48041]

The 1N5406 diode should work fine. When the controlling switch is opened, the polarity of the coil induced voltage is such that the diode will be forward biased and conduct any induced current. The induced voltage will be limited to the forward voltage drop of the diode which is about 1 volt. The 1N5406 diode is rated at 3 amps average and 200 amps peak for 8 milliseconds. Be sure to connect the banded end of the diode to the positive side of the coil and the other end of the diode to the negative side of the coil. Like Charlie said, any diode will work. The important thing is for the diode to be mechanically strong enough not to break from vibration.

[rv7charlie]

Originally Posted by lrgabriel

I've seen 430V spikes from my intermittent duty starter solenoids. In fact, one such spike blew out my 400V rated flyback diode, shorted it to ground, and disabled my landing gear.

...I might be a little bit passionate about this!

For total clarity, are we talking about diodes in the same location? I was addressing a suppressor across *the coil* of the contactor, to suppress arcing across the controlling switch. If the diode was across the *contacts* of the contactor, to suppress flyback from the starter windings, the equation would change quite a bit.

[lrgabriel]

The diode that failed was across the contactor's coil (protecting the controlling switch).

I had assumed that as long as the flyback diode's Vr rating was sufficient for bus/coil voltage, and it had a reasonably high If, it would work for flyback purposes. Based on the failure though, I think the Vr has to exceed whatever inductive spike you'd see...even if it's in the forward direction of the diode.

Edit: I'm going to get the scope out and play around with this today. I thought I had a handle on what was happening, but the more I think about it the more questions I have...