# Blocking Diode?

Discussion in 'Automotive Electronics' started by Tony K, Dec 20, 2016.

1. ### Tony KMember

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Can any one help please. I am not an electronice engineer or electrician but...

I am English living in Greece and am refitting a boat.

I have a two way switch I want to use on my panel for navigation lights, Port, Starboard, Masthead and Stern on one side of the switch and I want to add two more lights both red for a special purpose on the other side of the switch. So it's either basic lights on or all.

If I connect all with a link wire to the red lights the lights will come on all together with both sides of the switch, but if I put a blocking diode in one side I beleive I can make it work. Do you see the problem and solution?

The question is for a 12 volt circuit what diode should I fit? How would I calculate what is required. With either 4 LED bulbs or 6? Small amperage I think max 5amp.

2. ### ericgibbsWell-Known MemberMost Helpful Member

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hi Tony,
Does the two way switch have a Centre OFF position.?
E

Last edited: Dec 20, 2016
3. ### Tony KMember

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Perfect Eric thank you, solved in one!

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5. ### Les JonesWell-Known Member

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Hi Tony,
The diode would need to be rated to carry the current taken by what you are calling the basic lights. These would be fed directly from the side of the switch that powered just these lights and via the diode from the side of the switch that was connected to the side of the switch for the extra lights (Which you say take a maximum of 5 amps.) The reverse voltage of the diode would need to be greater than any transient voltage you would expect on the 12 volt supply so any diode with a rating abov 50 volts (Which is probably the lowest voltage rating likely in a range of diodes.) Assuming the switch is switching the positive then the positive of the diode would go to the side of the switch with the basic lamps. I would choose a diofe with a current rating of at least 1.5 times the current taken by the basic lights. You may find it cheaper and easier to obtain a bridge rectifier and use just one of the diodes in it or two in parallel. To use one diode you would use The + output and one of the AC inputs. to use two in parallel you would use the + output and both AC inputs connected together.

Les.

6. ### Les JonesWell-Known Member

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Hi Eric,
If I understand correctly what Tony wants then you have your diode reversed.

Les.

7. ### ericgibbsWell-Known MemberMost Helpful Member

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hi Les,
'I think' Tony is saying:
When the Switch is in Main, ALL the lights are on [ including the Aux RED]
and
when in Aux, ONLY the RED lights are ON.

Perhaps Tony can clarify this point.

Eric

This is the alternative.

Last edited: Dec 20, 2016
8. ### Tony KMember

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Thanks you two, In fact I can do it either way it's just a question of reversing the diode.
Cheers

9. ### Tony KMember

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I have a list of other questions (7) which have now come up, nothing too serious, may I post them please?

Tony K

10. ### ericgibbsWell-Known MemberMost Helpful Member

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Hi T,
We await with bated breath.
Eric

11. ### Tony KMember

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OK thanks here we go......

Please can you help with the following questions, I am keen to do the work on my small boat, which I am renovating, myself.
I am quite practical but these, for me, are sort of technical questions.

The starter motor for the boat draws 1800 w @ 12v which is I believe 150 amps.
There will be two batteries one for the starting only and one for all other ancilliaries. Both will be 100AH.

1. The alternator has an output of 55amps I am fitting a Blue Sea master switch and charging relay
so that the batteries only get what they need. Is 55 amps output OK with 2 x 100AH batteries?

2. Should I fit a relay after the ignition switch for the starter solenoid? In fact normally only
a small wire? goes to the solenoid anyway.

3. Should I fuse the batteries direct on each with a 100amp fuse, is it absolutley necessary?

4. Should I fuse the ignition switch/circuit (before or after the switch?) and what size fuse please?

5. The Navigation lights will have LED bulbs and so will draw little. They like other equipment will be
wired from the domestic battery +ve bus bar to a switch panel with switches and circuit breakers.

This is how I have configured it
Switch 6, Two way 10amp CB Navigation lights x 4 or x 6
Switch 5 One way 10amp CB Anchor light
Switch 4 One way 5amp CB Panel lights (Probably LEDs inside the instruments)
Switch 3 One way 10amp CB Interior lights (LEDs)
Switch 2 One way 15amp CB Deck mount spot light (Bright LEDs 18w)
Switch 1 Push only 15amp CB Horn 5amp

I don't have a choice of Circuit breakers they came with the panel would it be a good idea to add fuses
of less amps in some cases and where?

6. The horn is only small and rated at 5 amps do you think it needs a relay?

7. The instrument panel and gauges will be wired on the ignition circuit from another +ve (ignition circuit)
bus bar should I fuse the instruments, if so where?

8. Last one.....What cable sizes should I use please. For Starter motor and to +ve and -ve bus bars,
ignition switch to solenoid, general wiring nothing much over 5 amps Is it better to have a cable the next size up?

This just about should give me a clear idea of what is required, Many thanks Tony K.

12. ### MikeMlWell-Known MemberMost Helpful Member

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Depends on what the current is through the ignition switch and how long the wire run is.
I added a 30A 12V automotive relay to my boat because the current through the starter solenoid exceeded the capability of the keyswitch, which if I recall was only 2A.

No fuse(s) in the the starter circuit. Wire Battery+ to Switch to starter solenoid with #2AWG. Wire engine block to Battery-
One big inline fuse (60A) near the batteries in the wire to the main fuse distribution block usually located in the cabin or in the driving console.

Wire diameter is chosen based on the current that the wire carries.
The end of the wire that sources current is the one that is fused.
Fuse rating is based on wire diameter (not the load requirement).

Specific to the starter circuit: Is the solenoid the type you apply 12V to get the engine to crank, or is it the type that you apply a ground (oV) to get the engine to crank?

No, but check the horn button current rating...

My boat fuses every branch circuit except the starter.

Wire diameter is based on run length, current carried, and how tightly the wire is bundled with other current-carrying wires. Use a wire table like this one to size the wires. As I stated earlier, the starting circuit (including the ground connection from engine block to negative poles on the batteries) requires a minimum of #2awg. I use #0awg for the Chev V8 in my boat.

Last edited: Dec 29, 2016
13. ### Tony KMember

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Thank you very much for that. I can go away now and start planning. I won't be wiring until probably mid February 2017. But I'm sure there will be other related questions. Thanks a lot again and Happy New Year.

14. ### MikeMlWell-Known MemberMost Helpful Member

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For your planning, add voltmeter(s) to show state of charge of the batteries. Add an ammeter to show alternator charging. Add a Perko switch to select batteries.

Add a switch which turns off the Voltage Regulator/Alternator from the pilot house.

Here is a crude diagram. I will let you refine it, and when you do, post it back here and we will comment on it...

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Last edited: Dec 29, 2016
15. ### Tony KMember

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Thanks Mike,

I have two voltmeters one for each battery and a switch for battery selection. No ammeter though I thought the volmeters would do?

Cheers T

16. ### MikeMlWell-Known MemberMost Helpful Member

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Look at post #13 again...

An ammeter (zero center meter) is very useful to see if the alternator is keeping up with the load(s). Do not pass the starter cranking current through the ammeter shunt.

17. ### Tony KMember

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Thanks for that but does not a voltmeter show if the alternator is charging or not? ie the battery is under charge or discharging?

18. ### MikeMlWell-Known MemberMost Helpful Member

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Yes, but you do not know how fast....