Relays and diodes

[Harry in Ky]

I've been studying the circuit diagrams and operation of various electrical functions in the service manual for the Kioti DK 35 45 50 tractor.

I see a number of relays (not all, but some, including the start relay) that have a diode placed in a loop around/between the terminals for power and ground on the coil circuit. Having dealt with this style relay for years, I don't understand the need for a diode in this application. Maybe someone can give me a "nutshell" explanation for why that might be needed?

I'm not looking for long winded breakdown on the theory of diodes/rectifiers, atoms and molecules, protons and neutrons or any of that. I know what a diode does. I just don't see why there would be one here.

 

[repete]

Can you post a circuit diagram?

 

[Harry in Ky]

Not really. I don't have anything I can post and am not computer literate enough to make one up.

 

[Agvg]

Diode Connected in Parallel to a Relay Coil – Relay diode - Electronics Area

The purpose of a diode connected in parallel to a relay coil (flywheel diode or freewheeling diode) is to avoid damaging some nearby components sensitive to high voltage.

electronicsarea.com

 

[5030]

Diodes only allow one way passage of power. My Kubota's are the same way actually.

 

[Harry in Ky]

From that I'm picturing something on the order of induced voltage as in an automotive ignition coil. Points open and close, circuit in primary coil winding generates voltage in secondary winding, that sort of thing. Maybe not precisely the same, but some of the principles overlap a bit.

 

[TractorTech]

An extra layer of protection

 

[etpm]

From that I'm picturing something on the order of induced voltage as in an automotive ignition coil. Points open and close, circuit in primary coil winding generates voltage in secondary winding, that sort of thing. Maybe not precisely the same, but some of the principles overlap a bit.

The voltage is induced the same way as an ignition coil. When power is removed from the relay coil the magnetic field collapses just like in an igniton coi.
Ericl

 

[WranglerX]

If your tractor has a computer the diode is there to protect computer from a voltage spike as magnetic field collapse when relay is deenergizes....

 

[Harry in Ky]

How about that? I've installed more of those relays than I can count, on all sorts of tractors, for all sorts of reasons. A good many with computers. No one has ever accused me of damaging their computer because of a diodeless relay. You learn something new every day, whether you want to or not.

 

[Fuddyduddy1952]

Yes that's what the diode across relay coil is for, to dampen reverse or back emf. No diode means noise or a voltage spike happens along the supply line which can cause damage.
Sometimes you get lucky without it, but better safe than sorry.
I'm a retired electronic technician 43 years.

 

[Harry in Ky]

And you have seen how many tractors damaged by improperly wired relays? I've only been working on tractors since 1972 and haven't found one yet. Go figure.

 

[Fuddyduddy1952]

I'm replying to your original post..."What's the diode for". I and others gave you the answer, it's not an argument. People do things for years that "work", it doesn't mean it's the correct way or should be done that way.
Reverse EMF is purpose of the diode.

 

[Harry in Ky]

I get the picture, and yes, my question was answered.

Thanks to one and all.

 

[WranglerX]

Keep in mind some applications have external diodes across relay socket, some applications have diode internally inside the relay case..... When replacing a relay do it by part number(style)...

 

[etpm]

And you have seen how many tractors damaged by improperly wired relays? I've only been working on tractors since 1972 and haven't found one yet. Go figure.

Things change and so a diode may be necessary now where it didnt used to be. I can give you a case in point. My old 2000 something Lexus lost the left low beam headlight function. I have been trying to find the fault. The fault is in the positive wire, not the ground. So far no luck. But since I need a left low beam I wired in a relay. So when the right low beam turns on the relay I added is energized and it supplies power to the left low beam. But when I turned off the headlights the voltage spike burned out the right low beam. The headlight was old but still... So I added a diode across the relay coil. No more burned out headlights. More important is no more voltage spikes that might damage other electronics. It could be the same for your tractor.
Eric

 

[dobekeeper]

Your relays are equipped with "Kickoff Diodes" A relay is basically a coil that stores energy in the circuit like pushing down with weight on a spring. When the voltage is removed from the coil currents rushes back in the circuit in the opposite direction.
Like releasing a coiled spring, this current shorts across the diode resulting in a brief pulse of heat (like dropping voltage across a resister), this short across the diode makes the coil switch off quick that making the contacts on relay switch off fast.

 

[ttouring]

I've been studying the circuit diagrams and operation of various electrical functions in the service manual for the Kioti DK 35 45 50 tractor.

I see a number of relays (not all, but some, including the start relay) that have a diode placed in a loop around/between the terminals for power and ground on the coil circuit. Having dealt with this style relay for years, I don't understand the need for a diode in this application. Maybe someone can give me a "nutshell" explanation for why that might be needed?

I'm not looking for long winded breakdown on the theory of diodes/rectifiers, atoms and molecules, protons and neutrons or any of that. I know what a diode does. I just don't see why there would be one here.

Diodes make sure there is no feed back on a circuit .

 

[dkmc]

Snubber diode, flyback diode, the names continue. There are pre-made cubes called "arc-killers" that contain various combinations of diodes, resistors, and capacitors that all help quell the breakdown voltage that is produced when the coil is de-energized. A relay coil or a solenoid coil can have this phenomena. Without them voltage spikes can occur, and/or relay contact points can be eroded.

 

[JTERWALS]

I've been studying the circuit diagrams and operation of various electrical functions in the service manual for the Kioti DK 35 45 50 tractor.

I see a number of relays (not all, but some, including the start relay) that have a diode placed in a loop around/between the terminals for power and ground on the coil circuit. Having dealt with this style relay for years, I don't understand the need for a diode in this application. Maybe someone can give me a "nutshell" explanation for why that might be needed?

I'm not looking for long winded breakdown on the theory of diodes/rectifiers, atoms and molecules, protons and neutrons or any of that. I know what a diode does. I just don't see why there would be one here.

As others have said, when voltage is removed from the coil, the magnetic field generated by the coil collapses, generating a voltage spike in the circuit. To protect sensitive components from the spike, a diode is used to "bleed off" the spike.