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The Norfolk Broads Forum / Broads Boat Owners Q & A / Rev counter dilema
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Posted By Discussion Topic: Rev counter dilema

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Bitterend
Feb-15-2014 @ 9:08 PM                           Permalink
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I am wanting to fit a rev counter to my upper helm.  I have a 12v alternator sensed gauge fitted to the lower helm.  I also have a 24v alternator.  Could i get a signal for a 2nd rev counter from the same feed already being used on the 12v alternator for the lower helm gauge,  Or would it need to be seperate.  I have a nanni 50hp, any solution ideas would be great! Ideally i would like a duplicate nanni panel but on investigation that is very costly!

The Bitter End

rustic
Feb-16-2014 @ 9:23 AM                           Permalink
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Hi Bitterend, the normal way an alternator is made is that the "W" terminal is basically an output from an extra diode wired to one of the alternator phases.
This will, in effect be a frequency output that varies with RPM,  so it shouldn't be affected by a slight increase in load.

So provided that the rev counter is the same type as the previous one, I can see no problem with piggy backing another one.

As a precaution, in case of wiring failure or faults, I would put a 1 amp fuse in line near to the alternator on the W terminal.

I will say though, I have never tried this, but I have worked with alternators and I have even fitted and modified a tachometer to even fit on a petrol outboard.

Hope it helps.



best regards, Richard.
I can't wait to be back on the Broads.

Bitterend
Feb-16-2014 @ 5:34 PM                           Permalink
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Thank you thats exactly the sort of information I was hoping for.  Do you think make of the gauge is important or aslong as its within same range as existing being 0-4000.

Many Thanks

The Bitter End

annville
Feb-16-2014 @ 6:39 PM                           Permalink
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If Rustic method dosnt work you will have to split each rev counter to get a reading you can use a indicator switch to do this, wire center terminal to alternator connection and a rev counter to each of the other terminals you then switch to each rev counter that you want to work John

rustic
Feb-16-2014 @ 8:57 PM                           Permalink
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Hi Bitterend, will any tachometer do?

The output from the "w" terminal  will have a frequency that is proportional to engine revs, but the actual frequency will also depend on the pulley ratio, on the engine, and the alternator.

So some form of analogue calibration and adjustment will be required. ( a potentiometer to turn for example).
So it depends if the tachometer has this facility.

With petrol engines, a four cylinder engine, the points will give two pulses per rev. On a 6 cylinder, 3 per rev, 8 cylinder 4 per rev, so for these types, you just need a switch on the rear of the tachometer 4,6, or 8 cylinder and calibration is sorted. However a diesel is different, there are often no signals to pick up on the older ones, although some of the later engines have a crank case position sensor, which gives a signal per rev etc.

So not all rev counters available off the shelf may be suitable.

I hope this helps you to choose a suitable unit, at least what to look for.



best regards, Richard.
I can't wait to be back on the Broads.

Bitterend
Feb-22-2014 @ 12:58 AM                           Permalink
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What i will probably do when I get the rev counter is just wire it up to test it off the existing unit downstairs to see how i get on before i start running wires to the upper helm.  ASAP have a 0-4000 12v alternator sensed unit that looks like it will do the trick.  I hadnt considered using a switch, If i run into problems having them both working together thats a very good idea! Im also thinking of adding a water temp gauge and oil/water/battery warning lights, the 3 warning lights should be easy to piggy back from downstairs panel but I think i need a dual station temp sender for the water gauge.  Im planning on powering all this via a new battery feed to an ignition fed relay.  Any ideas how many amps I will be dealing with so I can get cable size/fuse matched correctly?

The Bitter End

rustic
Feb-23-2014 @ 3:43 AM                           Permalink
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Hi Bitterend, you mention the battery warning light, if this is the indicator that normally goes out when the alternator is charging, then you need to be careful how you wire this. I will advise you later should this be the case. If it is, this bulb carries the power to the field coil of the alternator, if this is wired incorrectly, it could mean you have no alternator output.

Regarding the temperature gauge, you seem to have a good grasp on this one.
If you have a similar gauge, then you will need to switch the output to the working helm, as the temperature meter works by sending power to the sender, having two meters on line at the same time will give errors on both meters, and might damage the sender.
You could fit an additional sender, but might require drilling and tapping  the block or attaching it to  a suitabe hot part of the engine.

Rather than a switch, you could use a multi- pole 12 volt relay, with change over contacts. How many poles depends on what you have to switch. Three pole change over relays are cheap, and can be bought with 10 amp contacts, and some have an eleven pin base with screw contacts. If you only want two pole changeover, then these are called octal bases (8 pin base).  

Current wise, panel lights are only aprox 2 watts, unless you go for LED. For the gauges, you might also want back lighting, say if the navigation lights are on. So up to 2 watts each for those.

Don't forget that the negative wire could be carrrying the return current from several devices, so this wire might need to be increased in size.

Some of the panel lights might need two wires, depending on how they are wired.

Most of the gauges will take much less than an amp each, depending on their technology.

What else are you planning to put at the other helm? maybe a cigar lighter socket, to power say a sat nav, phone charger... ipad.. who knows, but this would require say cabling that could carry 20 amps on both wires.

The other important point regarding cable size also depends on the length of the cale run, otherwise you will get large voltage drops which could affect the accuracy of the meters.

Depending on how every thing is wired, each panel light could need two wires, each meter up to three, but the feed and return could be common,  you might also need a high power feed and return to a cigar lighter socket.
It might be best to run a flexible conduit to the helm, and run suitably sized wires. A cheap multicore cable for the meters and panel lights could be a couple of lengths of car trailer wiring,  each cable has 7 cores, so in total 14 wires. However a cigar lighter socket will need a much larger cable.

Fusing, on feed lines need to be below the cable current carrying capacity.
Always add a few extra wires into the loom, I guarantee you will need them later, maybe an intercom, or a horn button...

You need to plan how things will be wired, use colour coded wire, like the trailer wiring will help.

You will be amazed at how many wires you will need to do all this.



best regards, Richard.
I can't wait to be back on the Broads.

Bitterend
Feb-23-2014 @ 5:38 PM                           Permalink
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Hi Rustic thanks for the great advice.

My upper helm already has 12v socket, Speed/Depth Log, 2 horn switches, Wiper motor and a courtesy light allready wired in.  It has never had any engine instrumentation so i am going to do this with a fresh power feed.  I think the speed & depth and one of the horns is taken from the existing feed to the courtesy light which is low wattage on what cable size I dont know as I didnt fit them.  The wiper and 12v socket are on a seperate 20amp feed which I installed, thats fused at 15amps and the other horn switch is fed from there also but that just energises a relay for an air horn in the bow. The wiper and the horn are also fused so I think thats covered.

As we are only talking about low current for the power supply to the new panel could I get away with taking it from the 20amp feed do you think? The only other thing I forgot to add in my last post was the gauge lights on the rev counter and water temp gauge.

For the charge light would it be acceptable to indentify which side of the existing bulb is live when the ignition is on but the engine isnt running and piggy back a feed from there? Also should I fuse the piggy backed feeds to the new panel?

I like to get a good picture in my head of exactly what i need to do to do it properly and safely before I make a start!

The Bitter End

annville
Feb-23-2014 @ 6:06 PM                           Permalink
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You could down load Beta Marine wiring diagram for there dash board wiring curcuits this my be what you are looking for would also suggest a 4 gang fuse block and vasalen all connections John

rustic
Feb-23-2014 @ 7:09 PM                           Permalink
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Hi Bitterend, right, the charge light indicator for the alternator, as I suggested earlier, requires some thought.

Some regulators in alternators can be damaged if the current through  the charge light indicator is too large. Some Bosch units can be damaged this way, possibly others too.

This is how it works...
The ignition feed goes to one side of the charge light indicator, usually a red light, in a car it often has a pneumonic of a battery.

The bulb is normally a 12 volt 2.2 watt panel type bulb, the other end of the bulb goes to the alternator field coil, and also the internal regulator. This provides an initial low current which excites the alternator coil to enable the alternator to give an output once it goes over a certain rpm.
When the revs are high enough, the wire at the alternator field rise to approx battery voltage, thus there will be no voltage across the bulb, so it extinguishes. The field coil voltage and current is then controlled by the on board regulator.

So...
If you parallel another 2.2 watt bulb across the other bulb, for the second helm, then you will be increasing the initial field current by a factor of two, which the regulator might not like. Some might be fine, but I am pointing out the risk.

What I would do, is lower the wattage of the 12 volt bulbs to say 1.2 watts, and have these then in parallel, so a combined 2.4 watts, which should be ok.

Otherwise you will be switching out the feed to the lamp ie having some switch over, which if it failed could mean that the alternator output could stop.

If you could check the wattage of the bulb that you currently have, sometimes in older installations, they use lower voltage bulbs and a series resistor to limit the current. If this is the case, then you need to quote the voltage and wattage of the bulb, and the resistance of the resistor in ohms.

There is an option of fitting a low power LED in parallel, but this would require a bit of experimentation, and the fitting of a few other electronic components to make sure that the LED goes out at the right time, since the LED will light with a difference of only 1.2 volts (in the right direction) whereas a bulb would appear to be extinguished with only 1.2 volts across it.
Let me know if you want to go down that path.

I hope it helps.


best regards, Richard.
I can't wait to be back on the Broads.

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