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Help with wireing 12volt lights please???

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cxcxcxvcvcvc

New Member
G,day everyone!
I am currently doing a little project for the wife but have run into a bit of a problem! I am making her one of those American style mirrors with all the little lights around them, (the ones the old style movie stars have) and i am using 12volt 21watt bulbs that i got from the wreckers (25 in total) that i pulled out of the stop lamps of cars (i only went this route as it cost me $5 from the wreckers and shops wanted $30+ for that many bulbs lol) anyway i wired them up and threw a 12volt DC pack on them and they are very dull (almost useless) anyway is there some way i can get around this? if i wire the bulbs up on @ a time they are very bright but the more i add the duller they get:( anyway any help would be great!
Thankyou:)
 

colin55

Well-Known Member
The problem you are facing is the fact that a globe takes about 5 times more current to "start-up."
Thus each globe takes about 10 amps to start up and when the filament is bright, the current reduces to about 2 amp. This means you need a power supply capable of delivering 5 times more current to get the chain of globes to illuminate.
 

cxcxcxvcvcvc

New Member
The problem you are facing is the fact that a globe takes about 5 times more current to "start-up."
Thus each globe takes about 10 amps to start up and when the filament is bright, the current reduces to about 2 amp. This means you need a power supply capable of delivering 5 times more current to get the chain of globes to illuminate.
Thanks colin for the reply!
So @ the moment i am useing a 12vdc power pack with a 1 amp? output, so what should i be looking to use? i tried a 12vdc with a 2.5 amp output but the bulbs still stayed very dim and where flashing?
Thanks again for the help:)
 

Sceadwian

Banned
10 12 volt bulbs in series will run off 120 volt mains nicely, get 5 more bulbs and make it an even 30, then you can make three strings. Only thing bad about this is if one bulb blows an entire string will go out and you have to figure out which one's the bad one.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks colin for the reply!
So @ the moment i am useing a 12vdc power pack with a 1 amp? output, so what should i be looking to use? i tried a 12vdc with a 2.5 amp output but the bulbs still stayed very dim and where flashing?
Thanks again for the help:)
Do the simple maths:

21W x 25 bulbs = 525 watts.

525W divided by 12V = 43.75A

Rather a little more than your puny 2.5A supply.
 

colin55

Well-Known Member
Try the series connection as suggested above. It is the only realistic solution.
Globes are much more tricky than you think.
 

crutschow

Well-Known Member
Most Helpful Member
It would seem that 525W of incandescent lights around the mirror would be blinding. I would think you would want each bulb to be more like 5 to 10 watts.
 

cxcxcxvcvcvc

New Member
Thanks everyone for all the comments,
I am just considering pulling the bulbs out and getting something like Decking/Christmas lights (but bigger) from Bunnings with a power pack supplied as i really don't know a lot about some (most of this stuff)!
Overall i would also like to add that this is a great site and looks very useful and i appreciate the reply's to this post!
Regards Chris.
 

Ross Craney

New Member
10 12 volt bulbs in series will run off 120 volt mains nicely, get 5 more bulbs and make it an even 30, then you can make three strings. Only thing bad about this is if one bulb blows an entire string will go out and you have to figure out which one's the bad one.
Automotive bulbs are designed for DC operation & will not operate for any length of time on AC. This is due to filiament rattle caused by the AC voltage & the hellical wind of the filiament. Even full wave rectification is too much for most of them unless it is smoothed. It might last 5 minutes or it might last a couple of hours but that will be about it. Why not use dommestic downlights.
 

Sceadwian

Banned
I did not know that Ross, I'll keep that in mind. Makes sense when you explain it.
 
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tcmtech

Banned
Most Helpful Member
I've ran lights off of small audio power amps before using them as a dummy load.
And some will sing for you!
I would believe that with the right size and length of filiament they could shake themselves apart given the right frequency.
 

Sceadwian

Banned
Ahh, harmonics 101 =)
 

cxcxcxvcvcvc

New Member
Automotive bulbs are designed for DC operation & will not operate for any length of time on AC. This is due to filiament rattle caused by the AC voltage & the hellical wind of the filiament. Even full wave rectification is too much for most of them unless it is smoothed. It might last 5 minutes or it might last a couple of hours but that will be about it. Why not use dommestic downlights.
Thanks for that information! Very interesting indeed:)
I cannot use down lights as i want the lights to go around the mirror as a feature, but i got some nice lights from Bunnings for $40 but the are multi colored and im not sure if i want to go a multi colored setup, i was really wanting to go a clear! It may be possible for me to scrape the colouring off the bulbs, Will see how it turns out!
 

Hero999

Banned
Automotive bulbs are designed for DC operation & will not operate for any length of time on AC. This is due to filiament rattle caused by the AC voltage & the hellical wind of the filiament. Even full wave rectification is too much for most of them unless it is smoothed. It might last 5 minutes or it might last a couple of hours but that will be about it. Why not use dommestic downlights.
Sorry I find that hard to believe, the thermal time constant of the thick filament is likely to be much longer than half an AC cycle.

I've powered car lights of AC with no problems. Some smaller motorcycles even power their headlight of AC directly from the alternator; they don't use special bulbs either, just normal car headlights.

Most incandescent lamps are better off run from AC than DC because more metal tends to evaporate from the negative side of the filament than the positive and reversing the polarity helps to balance this out.
 

Thunderchild

New Member
just rig the lights in series and power them from the mains, if its 25 bulbs then make it 26 and have 13 on each, if your that bothered about AC versus DC then rectify the mains and use a smoothing condenser
 

Ross Craney

New Member
Sorry I find that hard to believe, the thermal time constant of the thick filament is likely to be much longer than half an AC cycle.

I've powered car lights of AC with no problems. Some smaller motorcycles even power their headlight of AC directly from the alternator; they don't use special bulbs either, just normal car headlights.

Most incandescent lamps are better off run from AC than DC because more metal tends to evaporate from the negative side of the filament than the positive and reversing the polarity helps to balance this out.
And the thermal stress is of no consequence ?
What motorbikes would these be ?
And the evaporated metal magically condences back to where it came from ?

Come on - pull the other leg.
If you find it hard to believe then that's fine , I only post to help NOT to argue !
 

Nigel Goodwin

Super Moderator
Most Helpful Member
What motorbikes would these be ?
Almost every small motor bike, scooter, moped, and off-road bike does this.

There are two windings on the alternator, the smaller one feeds a rectifier and charges the battery (which feeds indicators and brake light). The larger winding feeds directly to the headlight bulb only, feeding it with variable frequency AC and (to some limited extent) a variable voltage as well.

I had a Yamaha DT400 (400cc trial bike), it was my only means of transport, and I did a LOT of miles on it - I never had a headlight bulb fail.

I'm not going to get involved in AC vs DC, but it seems to make very little difference, and certainly for this application AC would seem by far the preferred method - if an odd bulb fails every year or two, so what?.
 

Sceadwian

Banned
Ross, like hero was saying the thermal stress is negligible, the bulb doesn't cooled down appreciably during the zero cross because it happens so fast. And the vaporized metal will deposit on the filament to some degree, this is the entire reason for the existence of Halogen bulbs, but they're constructed to better take advantage of the process. How much inductance does the filament really have and is it enough to actually set up a vibration significant enough to hurt the life expectancy? I don't doubt you I just need to see the actual reasons behind it.
 

Thunderchild

New Member
Almost every small motor bike, scooter, moped, and off-road bike does this.

There are two windings on the alternator, the smaller one feeds a rectifier and charges the battery (which feeds indicators and brake light). The larger winding feeds directly to the headlight bulb only, feeding it with variable frequency AC and (to some limited extent) a variable voltage as well.

I had a Yamaha DT400 (400cc trial bike), it was my only means of transport, and I did a LOT of miles on it - I never had a headlight bulb fail.

I'm not going to get involved in AC vs DC, but it seems to make very little difference, and certainly for this application AC would seem by far the preferred method - if an odd bulb fails every year or two, so what?.
needless to mention the mopeds with no battery at all and the constant variable voltage current and frequency
 
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