Thread: Connecting Driving Lights

Originally Posted by steveG

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

Originally Posted by eckwalsh

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

Steve