Reply
 
Thread Tools Search this Thread Display Modes
 
Old 01-10-2019, 07:26 PM   #1
Senior Member
 
Rossland's Avatar
 
City: Buffalo
Country: US
Vessel Name: Almost Perfect
Vessel Model: Kadey-Krogen 48
Join Date: Dec 2015
Posts: 194
24V light running on 12V

I have a question for you electrical experts.

I have a number of incandescent courtesy lights that mostly light stairs at night. They are labelled 3watts at 24v, but are actually connected to 12v. The are not very bright, but they are supposed to be dim.

My question is will these bulbs still draw 3watts at 12v, or will they draw 1.5w since the voltage is half.
__________________
Advertisement

Rossland is offline   Reply With Quote
Old 01-10-2019, 07:33 PM   #2
Guru
 
tiltrider1's Avatar
 
City: Seattle
Country: USA
Vessel Name: AZZURRA
Vessel Model: Ocean Alexander 54
Join Date: Aug 2017
Posts: 1,316
Watts = amps x volts. You changed the volts but nothing else changed so yes 1.5 watts.
__________________

tiltrider1 is offline   Reply With Quote
Old 01-10-2019, 07:33 PM   #3
Veteran Member
 
City: Raymond NH
Country: USA
Vessel Name: Mischief Managed II
Vessel Model: 1992 Tollycraft 44 CPMY
Join Date: Oct 2018
Posts: 89
Quote:
Originally Posted by Rossland View Post
I have a question for you electrical experts.

I have a number of incandescent courtesy lights that mostly light stairs at night. They are labelled 3watts at 24v, but are actually connected to 12v. The are not very bright, but they are supposed to be dim.

My question is will these bulbs still draw 3watts at 12v, or will they draw 1.5w since the voltage is half.

At 12 volts they only use .75 watts.
Mischief Managed is offline   Reply With Quote
Old 01-10-2019, 07:35 PM   #4
Veteran Member
 
City: Raymond NH
Country: USA
Vessel Name: Mischief Managed II
Vessel Model: 1992 Tollycraft 44 CPMY
Join Date: Oct 2018
Posts: 89
Quote:
Originally Posted by tiltrider1 View Post
Watts = amps x volts. You changed the volts but nothing else changed so yes 1.5 watts.


With half the volts, the amps cut in half too, so only 25% of the wattage, not half...
Mischief Managed is offline   Reply With Quote
Old 01-10-2019, 07:50 PM   #5
Guru
 
City: ??
Country: ??
Vessel Name: ??
Vessel Model: ??
Join Date: Oct 2007
Posts: 2,392
It may be slightly higher than the straight line calculation. I agree mostly that if you halve the voltage on a resistance device you quarter the wattage.

With light bulbs, incandescent, though the very high temperature of the Tungsten filament also produces a higher resistance than when cool. That high temp is used to keep the current to spec.

When the voltage is halved in addition the temperature will drop, the resistance will also drop so the current will rise. It may not be a hill of beans in this case but personally I would simply insert the current function of your DMM in the circuit for one bulb and measure it.

Without actually measuring the filament temperature you cannot calculate the new temp so you would be guessing somewhat.
C lectric is offline   Reply With Quote
Old 01-10-2019, 08:25 PM   #6
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 5,152
Yep, on light bulbs and heating elements, resistance changes with temp. So the "half volts equals quarter watts" rule does not usually hold very well.
Ski in NC is offline   Reply With Quote
Old 01-11-2019, 05:24 AM   #7
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 19,152
If the question was is there any danger in this half voltage , the answer is NO.
FF is offline   Reply With Quote
Old 01-11-2019, 05:38 AM   #8
Senior Member
 
Rossland's Avatar
 
City: Buffalo
Country: US
Vessel Name: Almost Perfect
Vessel Model: Kadey-Krogen 48
Join Date: Dec 2015
Posts: 194
Thanks for the replies everyone. I was not concerned with any danger, since the bulbs barely get warm. But I was wondering if it was practical to replace these incandescent with LED's. The answer is no.
Rossland is offline   Reply With Quote
Old 01-11-2019, 07:09 AM   #9
Veteran Member
 
City: Raymond NH
Country: USA
Vessel Name: Mischief Managed II
Vessel Model: 1992 Tollycraft 44 CPMY
Join Date: Oct 2018
Posts: 89
Quote:
Originally Posted by Rossland View Post
Thanks for the replies everyone. I was not concerned with any danger, since the bulbs barely get warm. But I was wondering if it was practical to replace these incandescent with LED's. The answer is no.


Agreed, and 24V bulbs at 12V will probably last longer than 12V LEDs too...
__________________

Mischief Managed is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 05:03 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012