# 90 watts, 12 volts... how many amps needed?



## Sonnie (Apr 11, 2006)

I am not sure if I am looking for the right thing or not. I want to power 5 x 18watt 12 volt DC lamps (90 watts total) via a 120 volt to 12 volt adapter. What amp size adapter do I need?

I purchased these Malibu Brightscapes Landscape Lighting Deck Lights for my step lights on the riser. They include a louvered cover with a metal housing and 7 watt bulb. I purchased some extra 11 watt and 18 watt bulbs in case 7 watt is not bright enough. Therefore, I figure I need to make sure I have a large enough adapter to power the 18 watt bulbs x 5 for a total of 90 watts.

Normally I would just buy the appropriate power pack for these lights, but they don't work like I need them to. I need to be able to wire a receptacle to a light switch and plug the adapter into the receptacle. The power packs made for these are meant to be powered on all the time and turn on and off with a sensor. I could cover the sensor and leave the lights on all the time, but I figure that is not safe and the bulbs ain't gonna last long either.

Am I looking for the right thing and in the right direction?


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## steverc (Jan 5, 2008)

Sonnie:

Power in watts is voltage times current.

P=V*I

Therefore with a little algebra

I=P/V

7.5 amps minimum.

Steve


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## Sonnie (Apr 11, 2006)

So what happens if I use a 12 volt adapter at 9 amps... will that hurt anything, blow the bulbs, etc?

Thanks!


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## Eric D (Feb 9, 2009)

You're good. The 9V is max amps delivered (think of it as quantity of water). The 12V is the amount of "water pressure" of the electricity, and as long as the lamps are rated for that voltage they'll work fine. If the max amps was low, then sufficient voltage would not be delivered.

take care,


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## Sonnie (Apr 11, 2006)

Hmmm... well it seems as though these are harder to fine than I was thinking. Max I have seen is 5 amps, short of spending $75-100. That would handle 5 of the 7 or 11 watt bulbs.


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## tonyvdb (Sep 5, 2007)

Hi Sonnie, Have you given any thought to using LED bulbs for these lights? Are they more for accent lighting or do you want to light up a large area?


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## brucek (Apr 11, 2006)

Yeah, 90 watts is a fair size for 12 volts to power. It would likely require more than a wall wart.

Maybe the LED idea is something to look into....

brucek


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## Sonnie (Apr 11, 2006)

Yeah... this is just accent lighting. It is merely to light up the step so that people can see the steps when all the lights are off. The steps are somewhat in an odd place and small, so I want to be cautious, despite the screen probably giving off enough light to see.

The issue is finding LED's to work... the only thing I have found with a louvered type cover is these DC lights. Have you guys seen something different?


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## tonyvdb (Sep 5, 2007)

Have a look at this website, They have all sorts of LED lighting. I have bought alot of things from them.


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## Jon F (Jan 31, 2009)

Hi Sonnie,

Just curious if you could use the supplied adapter plugged into a switched outlet? I assume the system came with a light sensor which I would guess you could just cover with a piece of tape so whenever you flipped the switch the light would come on. Is there a size issue or maybe a delay in the lights coming on that makes you not want to use the supplied adapter?


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## Sonnie (Apr 11, 2006)

That is what I intended on using, but it is $50 extra, and bulky. Plus it doesn't memorize it's setting... or at least I could not get it to. I was going to take it back. It also does have a short delay at turn on as well.

I just spoke with a lighting company and they have 120 volt 7 watt fluorescent fixtures that are a little under 4" tall by a tad over 7" wide with black louvered covers... made specifically for this application. I guess I just didn't look hard enough initially. They should work fine.


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## Sonnie (Apr 11, 2006)

Uh... scratch that. He just informed me they are $63 each. :unbelievable: Almost $350 with tax, plus gas to drive an hour... not gonna happen.

Back to square one.

I wonder if a lip on the steps with one of those small lamps mounted underneath might work just as well. I have seen those at Wal-Mart for little of nothing.


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## tonyvdb (Sep 5, 2007)

How about these Sonnie? Any 12 volt DC adapter will run a hole bunch of them.


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## Sonnie (Apr 11, 2006)

I would have to come up with some way to mount them without them being too bright. Their 12" strip light would work underneath a lip edge, but I have one light that was in the side of the riser wall where you first walk in the door... just to make sure you don't trip over the riser.

I noticed they have a 100 watt/8.3 amp 12 volt power supply for $50... that should work to power the ones I have now, which would be ideal since I already have it setup for those and have those purchased.


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## Sonnie (Apr 11, 2006)

Why do they have two COM and two +V outputs?


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## robbo266317 (Sep 22, 2008)

I'd guess its so you can have two runs without trying to cram too many contacts on the terminals.

Or it may be that each terminal can only handle 5 Amps max as the smaller 5 amp unit only has one set of terminals


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## Sonnie (Apr 11, 2006)

As it is I just have one wire ran to all five lights. I guess maybe if I were to go in opposite directions that would be helpful.


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## robbo266317 (Sep 22, 2008)

Just check with a meter that they are connected and make a link cable to join the two to share the current.


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## Anthony (Oct 5, 2006)

Radio Shack and others sell a 12V 10A power supply. I have one and it was $60 IIRC. I use it to power a car amplifier for my driver and crossover testing. It used to power my subwoofer. It's an option (albeit bulky and heavy).


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## Sonnie (Apr 11, 2006)

Yeah... I ordered the one that Tony had linked to and that I mentioned above since it is rather small and should be mountable in the wall, with access of course.


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## Sonnie (Apr 11, 2006)

I wonder if it can be used with a dimmer?


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## Anthony (Oct 5, 2006)

I doubt it. Those DC converters are expecting a 110V input signal. Depending on the design, it will probably try to maintain 12V out as long as possible and then quit working when the "dimmed" signal went below 100V or so.

Doing that may also damage it, but the one I have is pretty beefy so I bet not.


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## Sonnie (Apr 11, 2006)

Yeah... that does make sense. Not a big issue, just a thought anyway.


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