DC lighting

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.

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?

audioguru said:

Why didn't you use cool, modern LEDs?

Click to expand...

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.

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.

dr pepper said:

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.

Click to expand...

At this rate I'm gonna have to buy 20" waist so they don't fall down!!

Mike.

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

Yes, perfect. Since there is little difference in price after shipping, I would go with the 10A version.

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

Pommie said:

This is probably the best solution but be aware that LEDs are polarized - they need connecting the right way around.

Click to expand...

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.

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

Forgot to mention I got them off Amazon.
Max.

chasbob said:

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

Click to expand...

Why would you even consider using something that isn't LED? - LED are much cheaper to run, give a better light, and are more reliable.

Nigel Goodwin said:

give a better light,

Click to expand...

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.

gophert said:

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"

Click to expand...

I find cool white LED's give a much better and more natural colour than incandescent

Nigel Goodwin said:

I find cool white LED's give a much better and more natural colour than incandescent

Click to expand...

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.

gophert said:

Why cool white?

Click to expand...

Also known as daylight, gives a nicer light than any incandescent bulbs - and essentially you usually only have two main options, warm white or cool white, with warm white simulating incandescent bulbs.

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.

Nigel Goodwin said:

Also known as daylight,

Click to expand...

Nigel Goodwin said:

and essentially you usually only have two main options, warm white or cool white, with warm white simulating incandescent bulbs.

Click to expand...

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.

gophert said:

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

Click to expand...

Had my cataracts done a few years ago thank you

I'm not that old yet.
And I should get in more, I'm well weather beaten.