# LEDs that pull 12.5 Amps. How to pick a transformer.

#### AcousticBruce

Joined Nov 17, 2008
58
I was hoping to get some help with selecting a transformer. We wired up a couple grids of LEDs, on my companies Yacht, that pull a total of 12.5 Amps and is 12V. We need to buy a transformer to power these.

Is these a need for a special LED transformer?

Can we dim the LED's with the 12V DC output or again, is there a special transformer option for this?

I figured 250 to 300 Watt would be ok for this. What other info do I need to make this decision?

#### wayneh

Joined Sep 9, 2010
16,128
Normal LEDs require a regulated CURRENT as opposed to a regulated voltage. If your grid contains no current limiting (resistors or an active DC-DC converter), then a typical 12V regulated voltage supply is not appropriate.

Without more information it's hard to tell. Can you provide a schematic or a photo?

Oh, and a transformer is only one piece of a power supply. What power source are you starting with on the yacht? Going straight to the batteries may be the most efficient.

#### AcousticBruce

Joined Nov 17, 2008
58
They are regulated and take 12V. They come in a strands of 50 units. 50 units pull 3Amps. You hook directly to 12V.

The yacht has 120V 60Hz. The transformer needs to convert to DC and provide enough power to supply all LED's.

I hooked them to a multimeter and it was pulling 12.5 Amps total after testing each section. I am going to run each section (of 3 Amps) parallel into the transformer.

We need to be able to dim them from a switch also.

#### AnalogKid

Joined Aug 1, 2013
8,229
Transformer is the right concept in that it will step wall voltage down. But what you want is 12V DC, not AC. Head off to Mouser, Digi-Key, Allied, etc. for a 12V output DC power supply, 200 W or more. Most industrial units adjust only 10% or 20%, so finding one with a wide enough adjustment range to get you the dimming range you want limits your choices.

For more money, get a bench or lab power supply. These adjust down to 0 V, have current limiting, and adjustment know, output voltage meter, etc. Unless you're just dying to build something, its everything you want, for a price.

ak

#### Plasmahunt3r

Joined Feb 6, 2014
28
FYI: Home Depot has 100 Watt 120v to 12v Led Drive Transformers. The brand name is Sea Gull which sounds nautical.

#### AcousticBruce

Joined Nov 17, 2008
58
FYI: Home Depot has 100 Watt 120v to 12v Led Drive Transformers. The brand name is Sea Gull which sounds nautical.
LOL. nice. I really need 250 to 300 Watt. Otherwise I will need more than 2 per side :/

#### AcousticBruce

Joined Nov 17, 2008
58
I was thinking just simply putting a potentiometer on the 12V output might be a problem because it changes the load on the transformer and this changes some calculations. Is this correct?

Is this a subject that I can look up by certain key words?

#### AnalogKid

Joined Aug 1, 2013
8,229
A potentiometer than can handle 12.5 A would be...large?

ak

#### CVMichael

Joined Aug 3, 2007
419
The yacht has 120V 60Hz.
I have no idea how a yacht works... but I am thinking that the 120V 60Hz comes from an inverter, so can't you connect the LEDs directly to the battery? (if the battery is 12V of course)

#### AcousticBruce

Joined Nov 17, 2008
58
I have no idea how a yacht works... but I am thinking that the 120V 60Hz comes from an inverter, so can't you connect the LEDs directly to the battery? (if the battery is 12V of course)
Well, that would require drilling holes and running even more wire. And the captain or owner would not be too happy about new wires coming into the engine room. There is a 120V source right there behind the counter.

#### wayneh

Joined Sep 9, 2010
16,128
It would be worth your time to investigate direct-to-battery more, in my opinion. The lights will place quite a load on the ship's inverter and the conversions from DC to AC and back to DC waste precious power. Neither of these are deal killers for your project, but definitely worth some effort to avoid.

My friend's sailboat uses 4x12V batteries in series. If you also have 48V available, this might allow you to put strings in series instead of parallel, and thereby reduce the current requirement.

When you say the LED strings take 12V, does this mean they can accept a range of voltages typically seen in a "12V" environment? For instance 16V during charging. If so, this solves a lot of problems for you.

Also, you'll want a switch-mode-power-supply (SMPS), not a big heavy (expensive) transformer. Something like a power brick for a laptop, only more wattage.

#### cork_ie

Joined Oct 8, 2011
428
A yacht will always have issues with mains voltage fluctuation in offbeat marinas and with onboard generators.

If it were my yacht I would simply fit a slave 12 battery of about 40 a/h (small car), or greater, if you have the room and keep that charged up with a switched mode power supply.
Switched mode has a much greater tolerence of mains voltage fluctuation and far better regulation than a transformer based power supply. If you want to buy something ready made, then I suggest you have a look at http://www.sterling-power.com/ they have lots of good info on marine electrical systems and there products are quite good.
If you want to go the DIY route many PC power supplies have a power rating of well over 200W and are all switched mode There is lots of info on the internet on how to modify these for use as a power supply. Unfortunately All About Circuits terms and conditions expressly forbid discussion on building or modifying transformerless power supplies, so you will have to do some searching elsewhere.

#### burger2227

Joined Feb 3, 2014
194
Heck, might as well toss in a solar panel to charge the batteries too! After all most boats sit more than they are used.

#### wayneh

Joined Sep 9, 2010
16,128
But most sit in a marina where they plug into the grid.

#### burger2227

Joined Feb 3, 2014
194
Solar does not need a marina plug. Boats may sit for months unattended. I know, we had enough of them. Bigger waste of money than swimming pools! At least the pool is next to the house normally.

#### wayneh

Joined Sep 9, 2010
16,128
Solar does not need a marina plug.
And you don't need solar if you have a marina plug.

#### Bernard

Joined Aug 7, 2008
5,335
Miscelaneous data:
1050 W SMPS, 3 X 12 V @ 30 A, $125, Electronic Goldmine Transformer, 230V:24V @ 20A,$ 65 All Electronics ?
PS PR40, 13.8 V @40 A, Mouser, $282 Transformer, VPT 24-6670, Triad, 12V @ 13.34A,$55.00, Mouser
" VPT 24-10420 " 12V @ 20A \$76.79 "
Wind your own on microwave oven transformer core.
Use PWM to control V & dimming?