# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: simon66 on November 17, 2014, 04:52:12 pm

Title: Transformer Question
Post by: simon66 on November 17, 2014, 04:52:12 pm
Hi guys,
I ran into a problem that I'm not too sure what to do. I have about 100 LED desk lamps with broken power supplies. My only problem is that the transformer makes no sense! I don't have a picture with me right now but this is what its written on it:

Input Voltage: 120VAC 0.46a
Output Voltage: 12v 50W MAX

The transformer looks like this
http://www.rfwiz.com/Maxon/Images/PortableAccessories/QPA-1413_400ph.jpg (http://www.rfwiz.com/Maxon/Images/PortableAccessories/QPA-1413_400ph.jpg)

So I've looked around the internet I can't find a wall power supply (Like the one in the picture above) with the same rating as the broken ones I got. I'm searching for 12v at 4a (Which equals to 48W). The reason I'm so confused is why does it matter that the input voltage is 0.46a?!?! Did they mean the output voltage is 0.46a?
Also, these are LED lamps, shouldn't they use much lower power than 50W?

Thanks!
Title: Re: Transformer Question
Post by: mariush on November 17, 2014, 05:46:24 pm
The input voltage is 120v AC and the current is 0.46A ... this just tells you that the adapter may use up to  120v x 0.46a = 55.2w to produce the energy that comes out of it. The energy coming out is 50w at a voltage of 12v ... that means the output current is 50w / 12v = about 4.2A.

In practice, if your led lamp actually uses all those 50w, chances are the adapter will use a bit more than 55 watts from the mains because conversion from a higher voltage to a lower voltage is not perfect, there are some losses.

All you need to care about is to find an adapter that is capable of 12v output, and capable of providing at least 4.2A of current... if the adapter lists a higher current amount, that's OK, it just means the adapter is more powerful than needed.
The only other thing you have to be aware of is the connector diameter... there's various sizes but the large majority are two standard sizes, 2.1 and 2.5. Just make sure there.

12v is a somewhat standard voltage, so you should be able to easily find a laptop adapter or something similar, for example I'm thinking Asus eePc laptop adapters (but these are only rated for 36w) ... anyway, just on a quick search on Amazon after "power supply 12v" I found these ... (not sure about quality) :

Title: Re: Transformer Question
Post by: Dongulus on November 17, 2014, 05:53:44 pm
What mariush said.

Also, a 50W power supply is not alarming for a LED lamp. A LED lamp would likely draw much less than 50W, but the rating on the power supply is how much power it can potentially deliver; the actual power delivered depends on how much the load draws.
Title: Re: Transformer Question
Post by: Seekonk on November 17, 2014, 05:59:43 pm
Maybe they used that transformer just for weight.  That is the only thing that keeps many of them from tipping over.  I would hook up the lamp to 12V and actually measure the current, everyone has a car battery.  I can't imagine a 50W LED lamp on a desk.  I have a 30W Philips DLM lamp.  That has a three pound aluminum heatsink attached to it!
Title: Re: Transformer Question
Post by: simon66 on November 17, 2014, 07:07:48 pm
Wow! thanks for the really good answers! I was literally expecting someone to bash me haha. Ok so I'm not going crazy and it indeed needs a 50W output transformer.

My other concern are the laptop power bricks. I dont trust them and since the floor here at my place is carpeted, Its a fire hazard. Those that plug to the wall (Like the photo I posted) are the ideal ones. Are there any you guys would recommend (Not the laptop brick ones)?

Thanks!

PS: Also, what happens if I get a 12v with lower amperage? My guess is that the lamp will be "weaker" but hopefully nothing will catch on fire (Finding those wall mounted PSU are hard!).
Title: Re: Transformer Question
Post by: mariush on November 17, 2014, 08:16:27 pm
The reason you won't find lots of adapters that plug directly into the socket is because it's a bit hard to make reliable adapters capable of providing so much power in such a small package that's also light enough to not affect the socket you plug it in mechanically.... you don't want the weight of the adapter pulling the adapter out or bending the internal contacts of the socket.

These adapters are generally sealed, so that no humidity and air will get inside (to protect people and house against shorts and other things). This however has a side effect... the heat generated inside by losses in power conversion has a very hard time getting out of the box... the air and components inside will slowly heat up and the only way is through the mains leads and the plastic of the case (poor conductor of heat).

With an efficiency of about 85-90%, so you can imagine that if the adapter produces 50 watts of power, about 5-10 watts of power are wasted as heat. A small adapter like the ones that go straight in the mains socket aren't really capable of making those 5-10w of power go outside the case so eventually the adapters will fail.
I may be wrong, but I personally wouldn't trust an adapter that goes straight in the mains socket and advertises more than 20 watts output power.

Quality adapters in the style of those brick laptop adapters have more room for a circuit board inside which allows for more complex circuits which in turn can be much more efficient (some reaching 95-97%) so they don't produce as much heat inside even though they're capable of generating more power. So for example, if you buy an adapter rated for 12v 6a (72w)  it will work just fine if your lamp uses only 50w and shouldn't be that warm since it will not used to its maximum capability.
But again, it's important to be brand name, quality adapters, there's lot of fake Chinese laptop adapters out there that aren't even capable of what they advertise.

You could simply take the power cord and tie it to the plastic case of the adapter reducing its length, and then the adapter will simply hang between the socket and the floor... it will look ugly maybe but it would work.

Here's some suggestions of quality adapters from Digikey (a reliable electronics store that doesn't sell fakes), you have there adapters that output 12v and 45-100w :

http://www.digikey.com/product-search/en?FV=fff40009%2Cfff8004a%2C8000d%2C8000e%2C80052%2C80056%2C80061%2C80078%2C80216%2C80285%2C803a5%2C8043c%2C804af%2C806af%2Cc00002&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=1&pbfree=0&rohs=0&quantity=10&ptm=0&fid=0&pageSize=25 (http://www.digikey.com/product-search/en?FV=fff40009%2Cfff8004a%2C8000d%2C8000e%2C80052%2C80056%2C80061%2C80078%2C80216%2C80285%2C803a5%2C8043c%2C804af%2C806af%2Cc00002&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=1&pbfree=0&rohs=0&quantity=10&ptm=0&fid=0&pageSize=25)