# Running a nav light from a Tohatsu 18 hp outboard



## TQA (Apr 4, 2009)

I have fitted a 12 volt nav light to my Tohatsu 18hp outboard and wired it up to the charging circuit. The bulb promptly blew the first time I revved the engine. [Never had this problem with my old Johnson.]

OK I understand that I have an unregulated AC output from the lighting coil. The Tohatsu manual suggests that I use an 12v 80w bulb. This is car headlight size.

Is there an easy fix eg

what about a 24 volt bulb?

what about an inline resistor say 33 ohms?

PS I don't need to worry about a picky coastguard measuring my light output in lumens where I use my dink.


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## zz4gta (Aug 15, 2007)

And if you're motor dies and you're dead in the water, with no lights, what do you do then? 

They make portable lights that run on batteries, pretty easy solution.


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## tommays (Sep 9, 2008)

No battery to keep things in control ?


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## asdf38 (Jul 7, 2010)

TQA said:


> I have fitted a 12 volt nav light to my Tohatsu 18hp outboard and wired it up to the charging circuit. The bulb promptly blew the first time I revved the engine. [Never had this problem with my old Johnson.]
> 
> OK I understand that I have an unregulated AC output from the lighting coil. The Tohatsu manual suggests that I use an 12v 80w bulb. This is car headlight size.
> 
> ...


I'm not an expert in motors, magnetics or outboards but are you sure it's AC? Unless they expect an external rectifier and regulator that's pretty useless on a boat.

Assuming its DC lets run the numbers on the 33 ohms using an 80W bulb as an example. The 80 watt bulb would expect to draw 80/12=6.7A so it would have a resistance of ~12/6.7=1.8 ohms. So if you put that 33 ohm resistor in series you'll cut the current down to 12/(33+1.8)= 0.34A or 4 watts (12*0.34) but nearly 100% of those watts will be in the resistor. You'd have no light and a very hot (or very large) resistor.

So that's not a good option.

Obviously if you blew the last light bulb, assuming it was a good bulb and didn't just blow by coincidence then you had too much voltage. So yes a 24V bulb would be fine. This is much better than adding a resistor. A better option here would be an LED which are all current limited in some fashion which and likely to be rated for 12/24V operation already.

But finally, yes I agree with others. Get a battery powered LED combo nav light and carry a spare set of batteries.


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## TQA (Apr 4, 2009)

Hey maybe I need to explain further. The engine is on a dinghy. The VERY last thing I want to do is add more complicated electrickery AND a battery to simple dink. 

I want to have the engine with it/s little white light as a self contained unit. I have done this before with a Johnson and just ran the 12 volt bulb straight off the lighting circuit with no rectifier or regulator.

The output from the charging coils is ALTERNATING CURRENT.

On open circuit and nr max rpm we get about 100 volts.

So what resistor bulb combination would work. 

Would an LED work ? I did not think so as the max reverse voltage is about 5 volts and positive going is not that much higher?


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## Bruce_L (Jun 19, 2012)

TQA said:


> The output from the charging coils is ALTERNATING CURRENT.
> 
> On open circuit and nr max rpm we get about 100 volts.


YOW!!! :hothead:100 VOLTS, hope you are made of highly resistive fiberglass !!!

Seriously, 100 Volts??? Help me to understand, how/where did you make this measurement. What scale was the meter set to as well. Give us some facts and we'll try to help, but I'm all for a simple LED setup myself like others here.


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## aaronwindward (Aug 8, 2010)

I think getting a small lead acid battery might be the right thing to do, but I'm not sure if that's what's typically done. You could try a resistor, and maybe combine it with a large capacitor, but I really think a battery is going to work the best, and it will have the side benefit of giving you a little bit of light if your motor dies.

I think the right way to make a circuit is some sort of shunt thing with a diode and a resistor.


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## pvanv1 (Nov 16, 2007)

TQA said:


> I have fitted a 12 volt nav light to my Tohatsu 18hp outboard and wired it up to the charging circuit. The bulb promptly blew the first time I revved the engine. [Never had this problem with my old Johnson.]
> 
> OK I understand that I have an unregulated AC output from the lighting coil. The Tohatsu manual suggests that I use an 12v 80w bulb. This is car headlight size.
> 
> ...


What is your actual model of outboard? If you are not running a rectifier (available for the motor), you need to. Otherwise, if you have tied into just the alternator, you can produce some fairly high AC voltages. Likewise, you want a battery to stabilize the circuit.


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## asdf38 (Jul 7, 2010)

TQA said:


> Hey maybe I need to explain further. The engine is on a dinghy. The VERY last thing I want to do is add more complicated electrickery AND a battery to simple dink.
> 
> I want to have the engine with it/s little white light as a self contained unit. I have done this before with a Johnson and just ran the 12 volt bulb straight off the lighting circuit with no rectifier or regulator.
> 
> ...


The bottom line is that the voltage at the bulb when operating must be less than the voltage it's rated for. There is no trick. While you say it may reach 100V I assume that this droops significantly under load (but now much load I don't know). If you really want to do this, I still think the easiest way is just try high voltage bulbs. An incandescent bulb is resistive so adding a resistor doesn't add much value. Unless you just don't have 24V or 48V bulbs on hand. The other variable is power. Assuming that the output voltage droops under load a larger load will cause it to drop lower. So a higher wattage 12V bulb may pull the voltage down to 12V and work fine.

I take back part of what I said about LED's. With 100V AC I don't think they are likely to work but again it all comes down to whether the voltage falls to an acceptable range for the particular module.

But again, this is all guesswork without knowing the parameters of the output (voltage, frequency, impiedence (how much it drops under load)).

@aaronwindward
A cap creates an RC filter which will lower the voltage in an AC circuit. But a resistor also lowers the voltage. The RC filter would also depend on frequency of the AC power with higher frequency equating to lower voltage. This is somewhat interesting because it might help cancel out the effect of voltage increasing at higher RPM's assuming the frequency is tied to that.

What your probably thinking of is a zener diode (or transorb) and a resistor. Zeners can clamp to a particular voltage by dumping all excess current (kind of like that emergency drain at the top of your sink). You'd need back to back Zeners in this case due to AC and a resistor would help handle excess power but specifying them correctly would mean knowing more about the output.

Anyway, this is way more detail than needed but it's the result of just not knowing what that output is. When I suggested battery before I meant AA's or D's or whatever. A handheld battery module with an LED can run for hours. What about a flashlight? Still a nice easy option.


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## TQA (Apr 4, 2009)

It is a 2012 Tohatsu 18hp 2st.

The voltage was measured OPEN CIRCUIT with a sensitive digital multimeter at the charging circuit outlet under the cowl.

The manual says this oulet will produce 80 watts at 12 volts. 

I have done this sort of thing before and know that some combination of bulb voltage/wattage and resistor will run the bulb in its dinky housing on the top of the OB cowl. It is just that the trial and error method will be difficult and time consuming for me as I sit at anchor in a bay on the south coast of Grenada with no Radio Shack to hand. 

I have seen other Tohatsus with this sort of set up so it can be done!


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## ParadiseParrot (Oct 6, 2010)

Ohm's law - Wikipedia, the free encyclopedia

the math is simple

How to Lower the Voltage in a Circuit | eHow.com

What 2 do


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## asdf38 (Jul 7, 2010)

TQA said:


> It is a 2012 Tohatsu 18hp 2st.
> 
> The voltage was measured OPEN CIRCUIT with a sensitive digital multimeter at the charging circuit outlet under the cowl.
> 
> ...


http://www.tohatsu.com/tech_info/own_man_pdfs/Toh99C_15C_18D.pdf

So based on that data point use an 80W 12V bulb. I just looked up your manual and it says exactly that on page 46.

Try here:
McMaster-Carr


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## Bruce_L (Jun 19, 2012)

If you really want to use the alternator for power, why not put several 12V automotive type bulbs in series since you have limited resources where you are? Lets say, put 6 bulbs (6x12 =72) in line with each other and then subtract one at a time until you get the results you desire. This will account for loading of the alternator coils as well as heating effect changing the total resistance of the circuit. Only drawback is if one bulb burns out, you will lose all the light, just like the old Christmas tree lights from the good ole' days.


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## pvanv1 (Nov 16, 2007)

TQA said:


> It is a 2012 Tohatsu 18hp 2st.
> 
> The voltage was measured OPEN CIRCUIT with a sensitive digital multimeter at the charging circuit outlet under the cowl.
> 
> ...


We do it all the time. Forget trying to match up to the unregulated and unrectified alternator. The output at that level is difficult to harness.

Assuming you have an M18E2. (You did not say the actual model, you could also have an old 18B or 18C)... the motor (as do all outboards) requires a rectifier kit to stabilize the alternator output for charging and lighting.

You would get 346761602M RECTIFIER KIT 8B THRU 40C $108.96, from any Tohatsu/Nissan dealer, including me.

Then your output will be appropriate for battery charging and lighting.


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## asdf38 (Jul 7, 2010)

pvanv1 said:


> We do it all the time. Forget trying to match up to the unregulated and unrectified alternator. The output at that level is difficult to harness.
> 
> Assuming you have an M18E2. (You did not say the actual model, you could also have an old 18B or 18C)... the motor (as do all outboards) requires a rectifier kit to stabilize the alternator output for charging and lighting.
> 
> ...


Rectifiers convert AC to DC but have nothing to do with regulating. It's needed for charging a DC battery but does nothing to help with a polarity insensitive light.

Here is someone else talking about lighting on that output and another site specifying the alternator at 12V 80W (7A) so this is an expected use of this output.

So again, 80W is your target. A 12V 80W bulb is safe. Likewise a 24V 80W bulb is also safe and won't be as bright or draw as much current (~1/4 the current/power at 12V). Probably less than 80W, perhaps 50W 12V bulb will be ok (assuming the voltage doesn't rise too high) and 25W-50W 24V bulb would also likely work. Nominally the 24V bulb would draw 20W at 12V, but again, the voltage will rise with lower load so >20W should be your target.

TOHATSU 18 HP 2-STROKE 18 HP



> Yes, you need a rectifier to do charging; The alternator's raw output was intended to supply lighting (only) as is, and can be rectified into DC for charging.


Tohatsu M-18A Battery charging.


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## hellosailor (Apr 11, 2006)

"On open circuit and nr max rpm we get about 100 volts.
So what resistor bulb combination would work. Would an LED work ? "

As they say in Brooklyn: FUGGEDABOUDIT!

There is no way you are going to "simply" connect a 100 volt source to a 12 volt bulb without blowing things up in short order. And an LED, which is typically a 3-4.5v device, is going to blow up even more dramatically.

You need some type of regulator, NOT A RESISTOR, for this job. A resistor drops voltage but the result will vary with the load and the engine's voltage output, making your lighting somewhere between unreliable and useless.

The easy solution will be to buy a small deep cycle battery (maybe a 12v 17AH "backup" or "alarm" type battery) and use that for your lights, take it off for charging, or else try to find a regulator that can handle 100VAC input and give you a 12-14V output.

That shouldn't be impossible, someone is selling everything somewhere these days.

It sounds like the engine has a generator not an alternator (like an old motorcycle) and an old "dump regulator" is what would work. I know those are still made for motorbikes, don't know about outboard accessories for that but would expect they were similar.


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## pvanv1 (Nov 16, 2007)

asdf38 said:


> Rectifiers convert AC to DC but have nothing to do with regulating. It's needed for charging a DC battery but does nothing to help with a polarity insensitive light.
> 
> Here is someone else talking about lighting on that output and another site specifying the alternator at 12V 80W (7A) so this is an expected use of this output.
> 
> ...


Just as amps _available_ is just that, AVAILABLE, the same is true of watts. Go back to school, and get your EE degree (so you can speak knowledgeably on electronics). You do NOT need to consume all of the available 80w. But the modern Tohatsu rectifiers are also regulated, to prevent overcharging the battery. And yes, you do want at least a small battery for "cushion" when the motor is off, at low speeds, or at WOT.

FWIW, as a point of history, The motors came with alternator standard. The electric start models also had rectifiers. Most competing motors of that vintage from other manufacturers REQUIRED a battery, or the diodes in the rectifier bridge would blow from having too high a PIV.


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## SHNOOL (Jun 7, 2007)

Um, pardon me (the EE) from interjecting, but the Engineer in me says buy this:
Attwood Portable LED Navigation Light Kit

Be legal, be safe, and be done... it's $34.


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## hellosailor (Apr 11, 2006)

Good solution, shnool. Although I think AAA cells are going to be an expensive PITA to power anything. I'm in shock that the bigbox stores want a buck PER battery for AA and AAA batteries these days, and considering how much faster the AAA get consumed, I just hate the damned things.

Then again, many things baffle me. Like, how folks expect 12v or 24v bulbs to work is the engine if putting out ONE HUNDRED VOLTS on that output. Ain't gonna happen for very long if there really is 100 volts there.


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## asdf38 (Jul 7, 2010)

pvanv1 said:


> Just as amps _available_ is just that, AVAILABLE, the same is true of watts. Go back to school, and get your EE degree (so you can speak knowledgeably on electronics). You do NOT need to consume all of the available 80w. But the modern Tohatsu rectifiers are also regulated, to prevent overcharging the battery. And yes, you do want at least a small battery for "cushion" when the motor is off, at low speeds, or at WOT.
> 
> FWIW, as a point of history, The motors came with alternator standard. The electric start models also had rectifiers. Most competing motors of that vintage from other manufacturers REQUIRED a battery, or the diodes in the rectifier bridge would blow from having too high a PIV.


First, you're making assumptions. I am an EE who designs PCB's all day every day (when I'm not wasting time on sailnet). Mostly embedded digital, FPGA's etc, but also analog and every board needs power design dc-dcs and regulators. That said, every industry has it's own conventions so I try not to overstate how well what I know translates into the marine environment.

I 100% agree we don't need to consume all 80W but without more information (and as I said, motors and magnetics are not my strong suit) I'm only willing to treat two things as reasonably firm. First, the alternator maxes at 80W out, second it puts out roughly 12V at 80W. Hence why I said an 80W 12V bulb is safe, because that hits the one data point here that we're reasonably sure of. I also said that an 80W 24V bulb would be safe and pointed out that it would draw, nominally, 20W at 12V.

The rectifier regulates? You could be right but nothing I've seen, and I've checked into the manual and other sources, says so. So how do you know? The common application here would be to rectify and then hook up to a 12V battery which has the net effect of regulating the voltage. You may be confusing that end result with a function in the rectifier.

@Hellosailor, this is a legit question. The output has a finite power and a voltage/current curve. It can't put out 100V at load so if you load it properly it will drop to a voltage where it will work. You just need to chose a bulb whose load hits that curve at an acceptable voltage. I've provided suggestions that have a very high chance of working.

Not to mention the OP has done this before, I've cited the page on in the manual where it says this output is for a light and pointed to another discussion tackling this exact same problem...

EDIT: Also you should take back your personal attack, not because I happen to have the degree (that only makes it more ironic) but because it doesn't matter to begin with. You don't need a degree to talk about a light bulb and my degree (and yours if you have one) would be completely trumped by anyone who knew more about this damn output on the outboard.


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## pvanv1 (Nov 16, 2007)

As an EE and also as a Certified Tohatsu technician, I can say with authority that the newer Tohatsu rectifiers do have regulation included. Sorry about the educational smal... there are so many "armchair" engineers out there who don't understand the effects of loading on the alternators, that I sort of just flew off the handle. Again, my apologies.


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## hellosailor (Apr 11, 2006)

"I can say with authority that the newer Tohatsu rectifiers do have regulation included. "

See, this is the problem Paul. Whilst there may be many armchair engineers out here, there are also apparently many engineers who are "armchair speakers" out there.

A rectifier rectifies, it does not regulate. An alternator...oh, the technical arguments over what that is are meaningless to the consumer, but it oes not regulate either. Only a regulator regulates, and if Tohatsu makes a rectifier _with an integral regulator_ or an alternator _with an integral regulator_ the same way most of the auto industry has for the last 40 years...then they have no business calling it a rectifier.

And a real engineer should be able to call it what it is. A spade. Ergh, no wait, that's what playwrights call it. I call it YnGlitch. A technologically bent form of English that too many not-native-language technical folks know they speak quite well.

Two nations divided by a common tongue, and all that good stuff, right?


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## asdf38 (Jul 7, 2010)

I'd love to know if TQA has tried anything else or gotten it to work...


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## Bruce_L (Jun 19, 2012)

I know a lot of Japanese motorcycles have a box with heat sink fins on it called a "Regulator/Rectifier" They use both words in the name of the part. Probably has 6 internal diodes and a IC voltage regulator, hence the fins on the package to dissipate heat while its "regulating" I hated the Suzuki ones as the internal semiconductors had issues with reliability. Little buggers were pricey too !!! The device connected to the battery on the output side and the alternator coil wires on the input side. Does the Tohatsu have an all-in-one unit like this perhaps? I'm looking at a new 6HP Tohatsu sail-pro myself. Heard lots of complaints about low charge current.


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## TQA (Apr 4, 2009)

asdf38 said:


> I'd love to know if TQA has tried anything else or gotten it to work...


I am working on it. Waiting on a couple of 24v 10w bulbs and will try them in series


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## pvanv1 (Nov 16, 2007)

Bruce_L said:


> I know a lot of Japanese motorcycles have a box with heat sink fins on it called a "Regulator/Rectifier" They use both words in the name of the part. Probably has 6 internal diodes and a IC voltage regulator, hence the fins on the package to dissipate heat while its "regulating" I hated the Suzuki ones as the internal semiconductors had issues with reliability. Little buggers were pricey too !!! The device connected to the battery on the output side and the alternator coil wires on the input side. Does the Tohatsu have an all-in-one unit like this perhaps? I'm looking at a new 6HP Tohatsu sail-pro myself. Heard lots of complaints about low charge current.


The MFS6CUL (sail pro) makes up to 5A (60W) at WOT. Keep in mind that at lower RPMs, you will only get a couple of amps. That's enough to replenish running light consumption, but not a heck of a lot more. The output is low enough that even without regulation, you won't boil your battery dry.


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