Connecting Driving Lights

[Carslil Jim]

Hi,
What is the best source apart from the battery to pick up the positive to power the driving lights. I was about to install a cable from battery to relay, but wondered if there is another good spot a bit closer to the front (eg starter motor).

I am istalling two by 100W aircraft landing lights, I have two batteries and a 50Amp alternator...I know these are the questions you will have! (and its an 85 V8 County for the really curious).

Regards

Jim.

[JDNSW]

I would take it from the alternator output terminal - after all, this is where the power is going to be coming from, and is closest. Apart from that, the battery terminal on the starter relay - which is the other end of the wire from the alternator, and further away.

John

[Hoges]

Apart from the attraction of their novelty value and/or possibly getting them for "free":D, landing lights are much less efficient than a decent set of automotive lights and are more prone to damage from constant road vibration ...which is different from the odd "bump" on landing.
I'm presuming they are sealed beam? Their popularity for road use faded some years ago when their shortcomings became most obvious... :(
But if you get a kick out of making it work :D good luck to you;)

[Loubrey]

I've had an old set out of an AT-6 Harvard (450W, 24 Volt) on an old farm runabout SIII and my current set of Lightforce 170 Strikers are easily twice as bright as they were. Couple that with the nearly 100C temperature difference when operating and there is really no comparison...

[BertR]

I've had an old set out of an AT-6 Harvard (450W, 24 Volt) on an old farm runabout SIII and my current set of Lightforce 170 Strikers are easily twice as bright as they were. Couple that with the nearly 100C temperature difference when operating and there is really no comparison...


If you run 24v light on a 12v system they will be dull...and just to add to the confusion on a 12v system 450W would drag 37.5 amps or 18.75 amps on a 24v. It's Ohms law:wasntme:

[steveG]

If you run 24v light on a 12v system they will be dull...and just to add to the confusion on a 12v system 450W would drag 37.5 amps or 18.75 amps on a 24v. It's Ohms law:wasntme:

But isn't a light rated at 450W at 24V only going to run at half the wattage on 12v - hence why its dull?

Steve

[Carslil Jim]

I am picking up the main positive supply from the back of the alternator and that goes directly to the relay, to be switched to the lights.

I suspect I should put a decent fuse between the alternator and the relay?

[Carslil Jim]

The answer is that the supply should be from behind the regulator as the alternator could put out as much as 16V and that may blow the lights.

In the end I picked up 12V supply from a second fuse box I have installed in the bottom of the main console between the seats. It is hooked to the dual batteries with a thumping thick cable.

So for the record, yes a fuse is required and the power must be no more than 12V and 20mp (4mm) wire should be used for the light circuit.

[Loubrey]

If you run 24v light on a 12v system they will be dull...and just to add to the confusion on a 12v system 450W would drag 37.5 amps or 18.75 amps on a 24v. It's Ohms law:wasntme:

Ex-military SIII running on 24 volt, but the lights were still pretty useless and very hot...

Back to Hoges' comment that unless the landing lights are of novelty or sentimental value, you'll do a lot better with modern halogens.

[Wal Rat]

What is the best source apart from the battery to pick up the positive to power the driving lights.I ran a heavy batt cable to the drivers side inner guard and mounted a 8\eight possition fuse box and used that for all the power to the standard lights and the driving/fog lights also gave me power for the electric fan.
amazing the difference it makes to head lights when they get full voltage

[eckwalsh]

But isn't a light rated at 450W at 24V only going to run at half the wattage on 12v - hence why its dull?

Steve

Ohms law is actually P=VI
where P = watts.V= voltage and I= current(amps)
To work out the current created (I) we have to transpose the formula.
therefore I=P/V
Since we know W=450w and V(voltage) is either 12 or 24
so you have either 450/12 or 450/24
so 18.75amps at 24v and 37.5amps at 12v
That is how the current is worked out.

or to put it another way. If you had an object that weighed 450kg 12 people would have to work twice as hard as 24 people to move it.

there will be a few extra losses etc in cable length/size/voltage drop, but this will be close enough
Regards
Eck

[steveG]

Ohms law is actually P=VI
where P = watts.V= voltage and I= current(amps)
To work out the current created (I) we have to transpose the formula.
therefore I=P/V
Since we know W=450w and V(voltage) is either 12 or 24
so you have either 450/12 or 450/24
so 18.75amps at 24v and 37.5amps at 12v
That is how the current is worked out.

or to put it another way. If you had an object that weighed 450kg 12 people would have to work twice as hard as 24 people to move it.

there will be a few extra losses etc in cable length/size/voltage drop, but this will be close enough
Regards
Eck

Thats a bit like saying if it takes one ship 3 days to cross the ocean, then 3 ships will get across in one day, as the number of ship-days is constant.

The incorrect assumption that you are making is that a 24V/450W bulb will still produce 450W if connected to a 12V supply. It wont.
Its a resistive circuit, and the element in the bulb is the resistor. The resistance isn't exactly constant, but near enough for this discussion.
If you take the current you worked out above at 24V (18.75A), and put it into Ohms law (R=V/I), then you get:
R=24/18.75
R=1.28 Ohms

Since resistance is constant (unless we physically change the bulb), then recalculate at 12V
Transpose Ohms law to make it easier:
I=V/R
I=12/1.28
I=9.375 Amps.

Now if you calculate the power using your formula above, we get:
P=VI
P=12*9.375
P=112.5W

So if you run the 24V light on 12V, the current is half, and the power is 1/4.

I've probably stuffed up the calcs there somewhere, but from practical observation it looks about right.
The inverse is also true. Take a 12V bulb and run it on 24V and it will be very bright (briefly) before it burns out due to the higher current vaporising the filament. You get a faster result if you hook a 1.5V bulb up to a 240V supply :twisted:

Steve