AC/DC transformer for LED lights

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Old 02-01-18, 01:48 PM
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AC/DC transformer for LED lights

Iím interested in replacing my under cabinet 20w halogen lights with LEDís. I read several reviews for the LED lights and some people were complaining that the lights were blowing out. A response from the seller was that you need to use the correct AC/DC transformer. It must be 12v. The AC/DC transformer being used now for my halogen lights is:
USA lighting model ET-60
PRI: 120v ~ 60HZ
SEC: 12v ~ 60VA
Load with 12v max. 60w halogen bulb only.

The LED bulbs will only use a total of 18w but Iím concerned that the transformer specifically states itís for halogen bulbs. It would be a pain for me to change out the transformer so Iím wondering if I could use the transformer I have or must I find one for LEDís.

Thanks.
 
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Old 02-01-18, 02:21 PM
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Transformers made for halogen have 12V AC. LED lights require DC. It will work on AC, but it will flicker. Also, transformers made for halogen are usually not regulated. Which means, it will put out more than 12V at no or low load, which can fry LED.

You will need a regulated 12V DC (most switching power supplies do) for LED lights. Some LED lights use constant current power supply (aka LED driver) instead of constant voltage.
In my opinion constant current power supplies are better for LEDs, but the LED has to be configured to run on it for it to work. Most are just configured to run on 12V DC (LEDs wired in series to accept 12V).

Just get a 12V DC power supply that is capable of supplying the current your LED lights will require. Don't get exact match. Get one larger than you need. I'd recommend at least 1.5x more than what you need.
 
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Old 02-01-18, 03:37 PM
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ok. thanks for the information. I thought all 12v transformers were DC.
 
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Old 02-01-18, 04:44 PM
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I thought all 12v transformers were DC.
Technically, all transformers are AC. There are no such a thing as DC transformer. Transformers don't work with DC because it uses changing magnetic field, but DC doesn't have changing magnetic field.

Traditionally, DC power supplies work by using transformer to reduce AC voltage, then covert it to DC using rectifier diodes and capacitors. This still doesn't give stable voltage, so a voltage regulator IC is used to keep the voltage constant on voltage sensitive devices.

Newer power supplies are switch mode power supplies and they work by converting AC straight to DC using rectifier diodes and capacitor. This gives high voltage DC. Then using IC and MOSFET the DC is switched on and off very fast making a high frequency pulse. This pulse is then fed through transformer to reduce voltage. Change in pulse width changes output voltage. Because transformers are more efficient at higher frequency, switch mode power supplies only need a small transformer. Traditional transformer power supplies need huge and heavy transformer for same output.

Many halogen power supplies use traditional transformer because it is cheap to make. Halogen don't care if it is AC or DC and not so sensitive with voltages. Therefore, all it needs is a simple transformer. However, now switch mode power supplies can be made cheaper, especially with rise in copper price. Therefore, some halogen power supplies may be switch mode power supply and putting out DC by design.
You can measure voltage with multimeter to confirm.
 
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Old 02-01-18, 05:41 PM
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awsome information. would this work?

LE DC 12V 2A Power Supply Adapter, AC 100-240V to DC 12V Transformers, Switching Power Supply for 12V 3528/5050 LED Strip Lights, 24W Max, 12 Volt 2 Amp Power Adaptor

Thanks
 
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Old 02-01-18, 06:43 PM
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You need 18w at 12vdc and that supply is 24w at 12vdc so you should be all set.
 
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Old 02-01-18, 06:44 PM
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That will work so long as your LEDs don't use more than 1.5A. The power supply is rated for 2A, but I doubt that it actually can produce that much as most power supplies are over rated. Even if it could supply full 2A, putting constant load of 2A is likely to shorten life of the power supply.
 
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Old 02-02-18, 08:01 AM
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Thanks for all your help. Iím learning a lot about power supplies, but still learning.

I have another project Iím working on involving just two 3watt LEDs. I have an AC adaptor

Input: 100-240v 50-60hz 0.5a max
Output: 12v 1.5a
LPS (limited power source)

Since it does not state that it is specifically used for halogen lights can I assume that the Output: 12v 1.5a will work for 6watts of LED lighting.

Thanks again
 
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Old 02-02-18, 11:53 AM
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Yes that will work.
12V 6W is 0.5A .

Also, when the power supply is rated for wide range input such as 100-240, it usually means switch mode power supply because it is easy to make such a power supply with a switch mode circuit.
 
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Old 02-02-18, 12:26 PM
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Thank you all so much.

I'm slowly transitioning to LED lights (standard A/C and D/C) and Iím finding out that a lot of my old/original dimmers, motion sensors, and timers need to be upgraded to LED compatible. What a pain !!! Now I need to pay attention to AC/DC transformers that will work with LED's.
 
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Old 02-02-18, 12:39 PM
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FYI. Low voltage LEDs will not work with standard dimmer even if the dimmer is designed to work with the LED light. They are meant for AC LEDs with built in circuit.
Low voltage LEDs cannot dim unless the power supply supports dimmer feature. One that do, usually come as a package containing both power supply and the light.
 
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Old 02-02-18, 01:50 PM
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I'm using the dimmers, motion sensors, and timers on A/C only LED's. Thanks
 
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