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DC lighting

chasbob

New Member
Hi, my wife is building a diorama model if Bethlehem for our church Christmas services, and I have just put some lighting in for her. I am no electrician, but I thought I understood the basics of DC circuits. I used 25 'grain of wheat bulbs', each rated at 12v / 80mA, which I wired in parallel to a 12v / 2A regulated power supply (mains transformer). 25 x 80mA = 2A, so I thought the load would be OK. It worked fine for a few hours (switching on and off several times as I tested various sections). Then some time after the lighting was completed, it suddenly it stopped working and we weren't doing anything to the electrics at the time. The bulbs are still working and the circuit is still intact - I tested the circuit using a 9v battery and the lamps still lit, but putting a meter across the power supply output to check the output DC voltage, it only registered about 0.1v. I assume this means that the power supply has failed. Does this suggest a faulty power supply or could there be anything in the way I have connected the bulbs that could have caused it to fail? I ran a wire (speaker wire) from each bulb to 2 terminal blocks, connected the individual terminals in each block together and then connected the terminal blocks to the power supply. The average length of each wire is probably about half a metre. Could the resistance in the wires have increased the load enough to blow the power supply? Thanks in anticipation of any help anyone can give me.
 

audioguru

Well-Known Member
Most Helpful Member
The resistance of the wires reduces the current.
The old fashioned incandescent light bulbs use up to 10 times the rated current when they are cold (room temperature of less). When white hot at 2000 degrees C then they are using their rated current. So of course the overloaded power supply burnt out.

You should use a 12V power supply that has its current limited to 5A or 10A. Then as the light bulbs warm up they reduce their current to normal.
Why didn't you use cool, modern LEDs?
 

Pommie

Well-Known Member
Most Helpful Member
Why didn't you use cool, modern LEDs?
This is probably the best solution but be aware that LEDs are polarized - they need connecting the right way around.

Did the existing supply get very hot? It may have thermal shutdown which may reset itself with time.

Mike.
 

dr pepper

Well-Known Member
Most Helpful Member
Unfortunately these days products like power supplies tend to be optimistically rated, 2a may have been more than it coud take.
a 5a might be a better choice.
 

Pommie

Well-Known Member
Most Helpful Member
Unfortunately these days products like power supplies tend to be optimistically rated, 2a may have been more than it coud take.
a 5a might be a better choice.
At this rate I'm gonna have to buy 20" waist so they don't fall down!!

Mike.
 

chasbob

New Member
Thanks everyone for your help. Do you think the 10A version of this power supply would do the business:-


12V 1/2/3/4/5/6/8/10A Power Supply AC to DC Adapter 5050 3528 LED Strip Light UK

100% OEM Compatible

Over-charge protection

Short Circuit Protection

Over Voltage Protection

Electromagnetic Compatibility

Built-in over voltage, over current,and short circuit protection

Specification:

Output Voltage: 12V

Current: 1A 2A 3A 5A 6A 8A 10A

AC Input: 100~240V / 50-60HZ

Wattage: 12W TO 120W
 

gophert

Well-Known Member
Most Helpful Member
Yes, perfect. Since there is little difference in price after shipping, I would go with the 10A version.
 

chasbob

New Member
Thanks Gophert, I've ordered this (I'm in the UK, so no shipping and quick delivery). Let's hope it does the trick!
 

MaxHeadRoom78

Well-Known Member
This is probably the best solution but be aware that LEDs are polarized - they need connecting the right way around.
I just purchased LED 12v AC/DC lamps for my yard lights to replace 12v AC that are becoming hard to find here in Canada.
The supply is just a 12v power transformer secondary . running now much lower current!
Max.
 
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chasbob

New Member
Thanks to all who have suggested LEDs. I will certainly take note of that for any future projects. Thanks everyone for all your help.
 

gophert

Well-Known Member
Most Helpful Member
give a better light,
Better is a relative term and people that deal with color matching will knock you on the floor if you tell them LED lighting "gives better light" the color rendering index of LEDs bounces all over and people tend to hat when their blue company logo looks blue in sunlight but purple under LED.
 
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gophert

Well-Known Member
Most Helpful Member
I find cool white LED's give a much better and more natural colour than incandescent :D
Why cool white? Look for LEDs as close to sunlight as possible which is 5000K (neither cool nor warm) and CRI of 85 or above. The human eye has been tuned to sunlight over the past few years. You may as well take advantage of that.
 

dr pepper

Well-Known Member
Most Helpful Member
Agreed with nige on that one.
Whne you buy leds you can choose the colour temp, 6000 is daylight I think, colour rendering at this level of white is very good.
I fitted led panels in the rest room where I work, and it now feels much better.
 

gophert

Well-Known Member
Most Helpful Member
Also known as daylight,
and essentially you usually only have two main options, warm white or cool white, with warm white simulating incandescent bulbs.
Ha, you guys should get out more. Maybe shop at a store besides the dollar store (pound store). "Neutral white" is actually called sunlight and 500K, not 6000K is the norm. Agree or not, it is simple science and you are welcome to live in the world of Marketeers of cheap LED bulbs. If you have the perception that "cool white" LEDs look like sunlight, you may want to have the yellow cataracts removed from your eyes or you may need to buy Eyeglasses that are not tinted yellow (about $5/lens more but well worth it).

Cheers.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Ha, you guys should get out more. Maybe shop at a store besides the dollar store (pound store). "Neutral white" is actually called sunlight and 500K, not 6000K is the norm. Agree or not, it is simple science and you are welcome to live in the world of Marketeers of cheap LED bulbs. If you have the perception that "cool white" LEDs look like sunlight, you may want to have the yellow cataracts removed from your eyes or you may need to buy Eyeglasses that are not tinted yellow
Had my cataracts done a few years ago thank you :D
 

dr pepper

Well-Known Member
Most Helpful Member
I'm not that old yet.
And I should get in more, I'm well weather beaten.
 

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