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Author Topic:   Switch wiring using a diode
AtoZ posted 03-06-2008 01:23 PM ET (US)   Profile for AtoZ   Send Email to AtoZ  
I have a switch panel that I want to use that has two SPST switches (instead of using the original pull switch. I want to use one to light my bow and stern light and the other to light my stern only.

I assume I need to use a diode to prevent power feeding my bow light when I want the stern only.

What kind/size of diode do I need?

AnnMarie posted 03-06-2008 02:46 PM ET (US)     Profile for AnnMarie  Send Email to AnnMarie     
You don't need a diode.
Since you never want to light bow only, wire one switch up to turn both lights on and then wire the second switch to turn the bow light OFF.
Chuck Tribolet posted 03-06-2008 06:29 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
AnnMarie: That's a very creative solution. Really. But the
human factors are not so hot.

AtoZ: I'd be looking for a DPDT center-off switch that fits
that panel. What's the panel and maybe somebody here knows
a switch that will fit.


Chuck

Chuck Tribolet posted 03-06-2008 06:46 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Back to the subject of diodes. Diodes have a voltage drop
which will:

A. Dim your bowlight a bit.
B. Generate heat in the diode. If it's too much heat, it will
blow the diode.

What's the bowlight bulb? From that we can figure the specs
on the diode, though due to A. I'd disrecommend it.

The world needs a really low voltage drop diode with reasonable
current flow characteristics. There are lots of uses for it.


Chuck

rayblnc posted 03-06-2008 07:49 PM ET (US)     Profile for rayblnc  Send Email to rayblnc     
I have my bow and stern lights on different circuits on the panel. A SPST toggle switch for each. Maybe this is a good idea for you too. If you have spare capacity on you panel that is.

Ray

jimh posted 03-06-2008 07:53 PM ET (US)     Profile for jimh  Send Email to jimh     
A technicality perhaps, but the lamps are certified to meet the required light output with 12-volts, not with 11-volts. Operating the certified navigation lamps with less than the specified voltage probably will cause them to fail to produce the required light output.

If you want to control navigation lamps with a single switch, why not buy the specialized switch which has been engineered and sold for years for this purpose?

AtoZ posted 03-06-2008 09:34 PM ET (US)     Profile for AtoZ  Send Email to AtoZ     
Ray, you are right... I was over complicating it.
efduffer posted 03-06-2008 09:43 PM ET (US)     Profile for efduffer  Send Email to efduffer     
I had the old two-position pull switch and hated it.

I did like Chuck suggested and put in a DPDT (on-off-on) switch and just hooked the stern up to one side and both sets to the other side. No problems. Nice because it matches all the other throw switches on the panel and can be protected with a waterproof boot, too.

swist posted 03-07-2008 06:56 AM ET (US)     Profile for swist  Send Email to swist     
With regard to diodes, if you use one with the proper current rating, it will not overheat, And while it will drop the voltage by a volt or so, most alternators put out at least 13.5 volts, so you would not be dropping the voltage at the lamp below 12 volts, at least not when the engine is running. And the longevity of the bulb would be greatly improved (it is not ususual for a light bulb to double its life with a 10% voltage drop).

I'm not saying that diodes are the solution to this particular problem, but let's at least get the facts straight.

jimh posted 03-07-2008 08:47 AM ET (US)     Profile for jimh  Send Email to jimh     
It is a well-known fact that the light output from an incandescent bulb is proportional to the voltage applied to it and the current flow that results.
Chuck Tribolet posted 03-07-2008 01:42 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Current flow in an incandescent lamp is not proportional to
voltage. Ohm's law doesn't apply because the resistance
goes up as voltage (and current) go up. See Smith, Circuits,
Devices, and Systems, page 22.


Chuck

jimh posted 03-07-2008 02:14 PM ET (US)     Profile for jimh  Send Email to jimh     
I urge Chuck to demonstrate how a navigation lamp produces less light output as the voltage increases, which would be necessary in order to refute my statement:

quote:
"Operating the certified navigation lamps with less than the specified voltage probably will cause them to fail to produce the required light output."
jimh posted 03-07-2008 02:15 PM ET (US)     Profile for jimh  Send Email to jimh     
Also, a careful reading of my comment will reveal that I did not say that the voltage and current in an incandescent lamp were always related as in a pure resistance. This was an incorrect inference made by Chuck.
jimh posted 03-07-2008 02:21 PM ET (US)     Profile for jimh  Send Email to jimh     
An additional (albeit quite technical) consideration with navigation lamps is the chromaticity of the light they emit.

If the illuminating incandescent bulb is operated at a lower voltage, the light spectra of its output will change. This will likely cause a shift in the light spectra of the navigation lamp's light. The shift may result in the certified lamp no longer emitting the proper color.

Chuck Tribolet posted 03-08-2008 05:44 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
JimH: I was refuting "It is a well-known fact that the light
output from an incandescent bulb is proportional to the
voltage applied to it and the current flow that results."

Let's factor that:

A. "It is a well-known fact that the light output from an incandescent bulb is proportional to the voltage applied to it "

and

B. "It is a well-known fact that the light output from an incandescent bulb is proportional to the current flow that results from the
voltage applied to it."

Both are incorrect.

The ENERGY output of an incandescent lamp is proportional to
the product of the voltage and the resulting current. So (and
I missed this the first time around, and it's a far bigger
factor), even if Ohm's law applied, the energy output would
be proportional to the SQUARE of the voltage. In this case,
it's a bit less than the square of the voltage, but more than
proportional to the voltage (since the current rises less
slowly than the voltage).

And we haven't even started to discuss where that energy
output appears: visible light, IR, convective heating of
the gas around the bulb, ...

I took EE 101 from Dr. Smith. He dearly loved to belabor
the class on the cool things you could do in electronic
circuits with funny non-linear devices like incandescent and
neon bulbs. The fun part for me was when we got to digital
circuits: RTL (that came before TTL).


Chuck

Chuck Tribolet posted 03-08-2008 05:51 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
BTW, the non-linear characteristics of these devices got
Hewlett-Packard going. H&P, working in a garage in Palo Alto,
had developed a audio frequency sine wave generator where one of these
devices was critical to the design. It must have been a
neon bulb because the time constants of an incandescent would
have been too long. Walt Disney was coming out with Fantasia
and bought one for every theater that was to show the movie
so he could vet their sound system. That got HP started.

That sine wave generator was still the state of the art in
1969.


Chuck

davej14 posted 03-08-2008 11:21 AM ET (US)     Profile for davej14  Send Email to davej14     
You will also find bulbs in the speaker boxes of may Bose systems. They are used for speaker protection and their pure resistance characteristic does not distort the sound. As you turn up the power, the power to the more sensitive upper frequency speakers is reduced by the non-linear resistance characteristics of the bulbs. By that time the booming base is overwhelming any reasonable listening level of the mid and high frequencies so it goes relatively unnoticed. After your kids are through cranking the music, your speakers survive and you can still enjoy the nuances of classical and jazz.
jimh posted 03-08-2008 11:36 AM ET (US)     Profile for jimh  Send Email to jimh     
Having fiddled around a bit myself trying to build low distortion audio oscillators, the incandescent lamp is a handy device to use with them. Because its resistance changes as the filament warms up, you exploit this property to advantage in an audio oscillator. Using a traditional bridge circuit, the distortion is related to the balance in the bridge segments. But if the bridge is perfectly balanced for low distortion, the circuit will not jump into oscillation at initial start up. The solution is to add a small incandescent lamp in one leg. The lamp changes resistance as its filament warms up, and this allows the oscillator to start running, yet when the filament changes to its final hot resistance it will shift the bridge balance to the lower distortion setting.

By the way, the light output from an incandescent lamp is very non-linear with applied voltage. It does tend to increase with increasing voltage, up to a point, then there is a very rapid decrease--when the filament burns out!

Regarding the meaning of the word "proportional" as I used it initially, there is no basis to assume that proportional means only strictly linear. The light output could be proportional on an exponential basis to the applied voltage. To impute that proportional means only linearly proportional is a restriction which was not intended.

But all this parsing and slicing of language aside, I don't think it is a good solution to insert steering diodes into the navigation lighting electrical system. It is far simpler to just use the proper switch to begin with.

Regarding the claim that running an incandescent lamp on lower voltage will prolong its life, I have no dispute. But, as for the life span of the incandescent miniature lamps used in navigation lamp fixtures, as far as I can tell on my own boat the c.1992 bulbs are still operating. I do acknowledge, however, that I do not do a great deal of boating in periods of restricted visibility or darkness. So I would not say that adding a diode to intentionally drop the voltage of a navigation lamp in order to prolong its life is recommended.

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