3 Diodes, Relay and Alternator

[Vladis]

In my Beneteau Oceanis 40 I have 3 black wires, each one
connecting to the anode of a rectifier diode and the 3
cathodes joined together. From the cathodes join goes another single
black wire to one of the contacts of a relay. The other contact
of the relay is connected to ground. The relay is activated with a
signal coming from the alternator. As far as I know this scheme
is used in many Beneteaus and Jeanneaus. Returning to the
3 black wires, one comes from the anchor winch switch and the
other two come from the engine exhausting fans.

The reason is to guarantee that both the anchor winch and the fans
only work when the engine (alternator) is running. As I have
a 100A reset-able switch breaker for the anchor winch I removed
the corresponding black wire (together with its diode) from the
relay and connected it directly to ground.

My question is that I do not understand the purpose of these diodes.
I would be grateful if anyone could give me an explanation!

 

[Lon nan Gruagach]

Diodes like this are to prevent power appearing in strange places...
Consider the situation when the engine (alternator) is stopped and for whatever reason one of those circuits is energised, say, the exhaust fans... +ve would pass through the fans, through the common point, back (the wrong way) through the anchor winch and appear at the wrong side of whatever controls it.
With diodes at the common point that cant happen.
Not quite an intuitive design, I would have gone for a 3 pole relay, but they probably have a very good reason for that since running an anchor winch through a diode wastes oodles of power.

 

[ithet]

I recently noticed a similar thing on my Beneteau switch panel, wires from the main nav light switch and the tricolour switch going to diodes connected to a common wire. Confused me for a while but now I assume (but have not checked) that the common wire goes to the compass light.

 

[PaulRainbow]

Diodes like this are to prevent power appearing in strange places...
Consider the situation when the engine (alternator) is stopped and for whatever reason one of those circuits is energised, say, the exhaust fans... +ve would pass through the fans, through the common point, back (the wrong way) through the anchor winch and appear at the wrong side of whatever controls it.
With diodes at the common point that cant happen.
Not quite an intuitive design, I would have gone for a 3 pole relay, but they probably have a very good reason for that since running an anchor winch through a diode wastes oodles of power.

+1 to most of that, but i doubt they are running the windlass through the diodes, more likely to be the windlass switching.

 

[Kinsale373]

View attachment Volvo P Control Panel. (Early Version ).pdf

I think the diodes you are referring to are similar to the three at the bottom of this old Volvo Control Panel schmatic. Here they are used so that either of the three signal lines, Oil temp, Charge or Water temp can activate the Sounder. If the diodes were replaced with a wire the circuit wouldnt work as the + Ve feed coming through the indicator lamps would reverse bias the Sounder and it would never come on. Sounds like they are doing the a similar trick to hold off a number of power hungry circuits if your Engine is not running.

I have a similar but newer Volvo control panel on my Bene 373 and i think the bias diodes are fitted on control panel.

Regards, Kinsale 373