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Old 08-17-2019, 02:40 PM   #1
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Diode on starter button

1974 Grand Banks with twin Lehman 120
Start is via push button
Each button has a wire off incoming pole that goes through a diode and to the ground post of a blue sea acr charge regulator.
Can't figure out what this does ?!
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Old 08-17-2019, 03:50 PM   #2
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Without the diodes, pushing either button would provide power to the other.
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Old 08-17-2019, 04:01 PM   #3
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EDIT: Now that I reread the post, the below doesn't fit. The OP said the diodes are on the "incoming" side, not starter side.


Can you provide a drawing of the circuit and/or a picture or the diode?

Just guessing here....

That diode sounds like it is in parallel with the start solenoid?

If so, it could be to bleed off a CEMF across the coil when one comes off of the starter. The CEMF could lead to a very short-lived reversal of voltage and current flow.

Although there probably isn't much current, I can imagine a situation where the voltage is high enough to arc back across the switch to the DC loads and supply on the other side.

In other words, it may be there as a surge supressor to clean up interference from the starter solenoid on the circuit on the supply side of that switch.

Having said that, I'd expect such a diode to be all the way on the other side, right across the solenoid's terminals. It would make more sense to be to do it there than on the other side of the wire.

Again, just a wild guess based upon what little I know of the actual circuit.

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Old 08-17-2019, 05:34 PM   #4
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Since the diode is connected to the regulator, I'd assume it provides the initial field voltage for the alternator(s). As the alternator comes online the diode prevents voltage to feedback into the circuit preventing the starter from staying energized.
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Old 08-17-2019, 06:28 PM   #5
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He says the diode goes to the ground terminal on a Blue Seas ACR (automatic charging relay) which combines two battery banks when the voltage rises above a certain set point. I don’t see the reason behind this. I have a switch on my ACR ground to disable the ACRs when we are plugged into shore power so the banks will charge individually.
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Old 08-18-2019, 06:39 AM   #6
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Can't figure out what this does ?!


Any DC motor is also a fine generator.

As the starter is disconnected by the Bendex the starter is spun rapidly by the engine to force the disconnect.

So for a part of a second your starter will be generating voltage , high enough to knockout lights and some electronics.

In autos the ACC terminal is not connected during starting , but on boats with push buttons this is not common.
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Old 08-18-2019, 02:10 PM   #7
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FF,

Makes sense. But isn't the start button circuit a relatively low current circuit which controls a starter crank relay (or contactor or solenoid), which in turn magnetically switches the isolated high current circuit to the starter motor?

In other words, isn't the starter button isolated from the high current of the starter motor by the starter crank relay (or contactor or solenoid)? Otherwise, the wires to the starter button would need to be really thick to carry the starting current that long distance?

Also, in a car, there is really just one ACC circuit. But, in a boat, we often have many circuits on a house panel that are really somewhat ACC circuits and aren't on that key switch?

I guess what I'm asking is, wouldn't that diode need to be placed back by the relay (or contactor or solenoid) or the starter vs at the key switch?

Cheers!
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Old 08-19-2019, 03:30 AM   #8
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I am 90% certain that diode is there to prevent arcing on the push button switch contacts as the button is released. This makes the switch last MUCH longer than it would normally. There's probably another one installed the same way on the relay contacts that the push button controls. It's a pretty common thing to do on high-quality DC circuits that are designed to last a long time. The arcing is probably caused by an induced voltage in the long wires between the push button and the relay it controls when the magnetic field around the wire collapses as the current stops flowing.


There is a downside though, I have seen diodes fail in such a way that they become a dead short (to ground in this case) and I have never seen one of these installed with a fuse from the factory.


I recently repaired a vaporized land on a circuit board on my slip neighbor's gen set that was damaged by a shorted arc-prevention diode. When the diode shorted, +12 VDC went to ground through the circuit board with almost no resistance and the circuit board land became the fuse. The genset would not turn over after that. After I removed the diode from the circuit and soldered a wire on the board to replace the land, it turned over normally. I put a new diode in place with a 10 amp fuse and it's been fine since.


I'm not saying you should go around installing a bunch of fuses, but if you do have an issue with a circuit that does not make any sense, check the diode to see if it's shorted and then see if said short damaged the part that the switch or relay controls.
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Old 08-19-2019, 08:30 AM   #9
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Quote:
Originally Posted by Mischief Managed View Post
I am 90% certain that diode is there to prevent arcing on the push button switch contacts as the button is released.
If arcing is the problem, why a diode? Wouldn't a capacitor do a better job? Or perhaps even a resistor?
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Old 08-19-2019, 08:39 AM   #10
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If arcing is the problem, why a diode? Wouldn't a capacitor do a better job? Or perhaps even a resistor?

Diodes are effective, reliable, and super inexpensive. A capacitor would be effective too, but they are a lot more money and tend to get weaker with time. Not sure about how effective a resistor would be.
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