How many watts in a candela?

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Mirelle

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Sorry, I am a bit scientifically challenged, here!

Our boat has, as well as the usual tricolour, two sidelights. Each one has a 22cd bulb, on a 24v circuit. We have a step up transformer from 12v to serve these two bulbs only. When I switch them on, the ammeter shows 10A drawn.

10A at 12v is 5A at 24v...is 120watts....is 44 cd, so 30 watts is 11cd, so 1 cd is about 2.7watts....?


So a 22cd bulb is a 55w bulb??

Is this right?
 
...possibly, but there again, possibly not. You are mixing inputs and outputs here. The watts you are talking about measure the rate at which electrical energy is being fed into the bulbs, the candela (cd) is a measure of luminous intensity - the rate at which the bulb is producing light. The link between the two is the efficiency of the bulb. Different types of bulb, different efficiencies.

The other factor to be consider is the "transformer". This will be converting DC to AC and then back to DC. This process will not be 100% efficent, so not all of your 10 amps will actually be arriving at the bulbs.

BTW the numbers involved seem large for a yacht - Aqua Signal sidelights for vessels up to 20mtrs use 25w bulbs.
 
Thanks. Yes, allowance for losses in the transformer and in the (I think quite poor) efficiency of incandescent bulbs must be allowed for.

They are not yacht sidelights; to cut a very long story short they really belong on a small harbour tug, but they suit our particular boat, a fairly hefty old gaffer, and they are only used when under power and/or in harbours or port approaches.
 
On a different tack, have you tried to fit 12v bulbs and do away with the transformer? There is almost certainly a 12v bulb available but you might have to search - try RS or Farnell.
Dickh
 
Thanks for that idea. They are German bulbs with a bayonet cap which is not the usual yacht one (I have a lifetime's supply of 24v bulbs as I laid in a stock!) I will see what I can find.

The current drain is fairly awesome, at about 160w with 25w steaming and stern lights as well, but we are certainly easy to see!
 
Not much help with your big bulbs, but I've seen some figures for conventional (not halogen) car bulbs similar to those we use. A 10 watt bulb gives around 4-5 candela, 25 watts around 30 - 40. They can deliver vastly more if the light is focussed, as with the fresnel lens in many navigation lights.
 
You are not comparing Like with Like.
The Watts in, consist of:-
1. The inefficiency of the Converter. A transformer won't work on DC, you need an inverter to convert to AC then a transformer to up the voltage then a rectifier to change back to DC.
2. The efficiency of the Bulb is the amount of light it produces compared to the heat. (more light less heat = more efficient).

Candella is purely a measure of the light output.

Your system would be far more efficient if you replaced with 12v bulbs.

Regards,
Brian

Brian
http://homepage.ntlworld.com/brunyard
 
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