ECN Forum Forums General Topics of the Trade General Discussion Area Electronic transformer/LED lighting

I'm trying to power 25 watts of LED strip lights that require 12 volts DC, from a small transformer that will fit in a 2 gang plastic nail on box.
I found an electronic transformer that will fit, it's from Seagull lighting but puts out 12 volts AC, (15 to 60 watts) but will not light the LED's.
So, I went to RadioShack and bought a Full Wave Bridge Rectifier (400V 8A); hooked it up as per the schematic and the LED's light but within minutes the rectifier is too hot to touch and starts to smoke!
Help! Any suggestions?
thanks,
Joe

Contact the manufacturer of the LED strip

If you hook up 12VAC to a rectifier, you get about 17v P/P slamming into those diodes. Try it with one putting out 9 VAC
Also be sure it is really hooked up right.
On the square bridge. AC is East and West, DC is North and South. (Look for the plus sign to get oriented)

Of course that assumes these are just diodes with a resistor. If they have a smarter PS, who knows what will happen with unfiltered DC.
It may give you the effect of the whole 17v

LEDs need a constant current source unless they have a current regulator built in. You might need a special LED driver.

You do realize that full wave silcon recifiers consist of a diode bridge -- and that when installed neet a HEAT SINK and special HEAT TRANSFER PASTE -- and the standard bolt through the center to the heat sink hardware?

This heat sink approach you will see in every power supply to speak of.

You can get any number of said heat sinks via scrapped out TVs. Being sure to discharge any capacitors involved, the exposed (switching) power supply will have a ton of power transistors or some such bolted to heat sinks all over.

The universal need for heat sinks is so widely known and presumed that their need is not brought up in the packaging -- or by the counter sales person.

While industry uses extruded, finned, aluminum in almost all cases, you can get away with drilling out a 4s cover or some such as a steel heat sink.

The key thing is that you HAVE to have the magic heat transfer paste. This is sold in one-application plastic sqeeze blisters. (dinky, they are) Normally, you have to ask for them. The typical counter sales person will not know what the heck you're talking about.

Do a Google search for such.

(Get them by mail? They would fit in a fat letter, first class.)

%%%

Addendum:

LEDs need heat sinks even more massive than diode bridges. These end up getting larger the cooler the LED has to run. The vast bulk of current LED gadgets are dedicated to their heat sinks. Just take a peak. You'll see.

This bridge should not be dissipating more than 3 watts. Warm maybe but I doubt it is "burn your finger" hot.

Unless that 17v P/P is over driving something. I am not sure why the LEDs haven't smoked

Greg, every bridge diode I've had the pleasure to purchase came with instructions that it HAD to be mounted to a heat sink.

The actual area available at the base of the package is pitifully small.

As the temperature rises, the resistance goes into a run-away mode... needing to dump 6 Watts not 3.

In every power supply I've ever torn apart, every single solid state power device had a heat sink. These were typically much larger than the package.

BTW, there is a tendency to use even cheesier diodes to save on $$$. Past experience with American production may not represent current imports.

I can't even get consistency in EMT fittings... from the same supplier. (At least, that's how they were packaged.)

http://www.mpja.com/5M-Blue-LED-Lighting-Strip-12V/productinfo/19441%20OP/
The LED strings I looked into have 3 surface mount LEDs and a surface mount resistor, and can be cut into groups of 3. I attached the link to a blue string because they have a datasheet for it. That means that they need a regulated 12V supply to run at the designed current. Overdriving them with 120Hz pulsing DC won't result in long life. Your transformer, bridge/heat sink, should be followed by input caps and a fixed regulator like the LT1085CT-12.(12V/3A fixed) An adjustable regulator like the LM350T and 2 resistors would also work. Both require heatsinks, additional caps and diodes and a circuit board. In other words, you probably need to buy a listed supply, rated conservatively higher than 25W.
Joe

Tesla, I am just talking in a practical sense. I imagine if he was using this at the rated 8 amps it would need a heat sink but this is supposed to be about 2.

There are plenty of 2a axial lead diodes that don't even get warm at that load.

Thanks everyone for your input. I finally got the manufacturer to supply the correct transformer so the problem has been resolved.
My application was lighting a closet, with the LED's mounted over the door operated by a jamb switch. The 2 gang box was in the wall near the ceiling so having diodes with heat sinks would have been a problem.