See the pics in post #17.
Update
I went with post #17, link #2 but the voltage was only 9.x after the series of diodes so I removed them to just a rectifier and capacitor and the voltage is 10.x, which the LEDs seem to be happy enough with.
Thank you for you help.
Also just to add there are 4x12VAC IC lights and nothing but the LED’s on the DC side on this circuit. The transformer is about at 1/2 it’s rated max capacity.
Hi
I would like to do exactly what you did, but the LED lights strip(this is strip of 150 LEDS lights) I have came with a DC power source rated as 12V and 3 Amps. I cannot use the Power source that came with the lights as I do not have 120v AC outlet on the pergola where the lights are being installed. I only have 12V AC there currently.
Would the setup you have will work with my configuration?
The input voltage coming from the Landscape transformer is 12 V AC.
Thank you in advance for any help in this matter.
Regards,
Sam
Sam your power usage is exactly inline with mine, but you have 150 LED’s instead of 5 and using 30 times more power. Your 12VAC source needs at least 36 watts spare capacity even to consider this. And that doesn’t take into account conversion loss, so I would assume you would want at least 40 watts spare, but that is just a guess.
You would need diodes rated for that power load and also a much larger capacitor, as for how much larger I don’t know but if it flickers you need a larger one. This might be a better case for a voltage regulator.
Thanks Kanicbird for the quick reply.
I have a 150 watt power 12V AC transformer and I am currently using at the most 45 Watts for other lights. After accounting another 40 W for the LED string, I’ll still have plenty of spare watts.
I am thinking of using the 4A rectifier here RadioShack.com Official Site - America's Technology Store
Not sure how big a capacitor and diodes I need. Would these Rectifier Diodes work? They are rated at 6 Amps. (RadioShack.com Official Site - America's Technology Store)Hopefully somebody will answer that question.
I’m not a expert in this, but it seems like it should work and I don’t think you are going to harm anything if it doesn’t except a $2.50 rectifier. You would only need a capacitor, the rectifier will contain, well they are, the diodes.
Will these work in the configuration mentioned in post# 17
Full Wave Rectifier:
Diodes : RadioShack.com Official Site - America's Technology Store
Capacitor:
Thanks Kanicbird. Just saw you reply after my post.
They would, but you’d want a more powerful rectifier if you are going to draw 40 watts from it (3.3 amps at 12 volts, but capacitive loads in particular need derating, say to 50% , including a safety margin - plus you need a heat sink); also, I wouldn’t recommend RadioShack because they are MUCH more expensive than other places (except for their non-copper-clad perfboards, but you don’t really need one for your purpose); for example, you can buy a similar capacitor (with superior specs to what is probably a generic capacitor, if a bit lower voltage, but still enough) and 8 amp bridge rectifier for $2 total from Jameco (if you’re buying them online). Of course, I guess that isn’t an issue unless you buy parts by the thousand as I do (some stuff can be as cheap as 99 cents for 100, also the minimum quantity because they cost so little). Also, you’d probably need more than once capacitor to smooth the ripple out, although it may not matter for lights (voltage may also be an issue, since 12 volts AC is more like 15 volts DC after being rectified, especially if only a part of the transformer’s rating is used; this depends on how the LED lights are arranged, as a small change in voltage will cause a big change in current if they have several LEDs in series; e.g dropping 10 volts with 2 volts across a resistor, so 14 volts would approximately double the current flow).
Of course, I wouldn’t even consider using an AC transformer for those power levels but instead build a switchmode power supply which would produce a well-regulated nearly ripple-free (millivolts) output (but that isn’t something that everybody knows how to make; alternatively, I’d use a switching regulator if a lower/regulated voltage was needed, because a linear regulator would dissipate considerable power).
What I did was very low cost, so I didn’t mind if it didn’t work or I blew something out. Though it worked and still works great and BTW the LED’s work and have been working under water also - to great effect, causes a multicolored waterfall 
.
In your case you have what seems as something more expensive so you may be less willing to be playing around with it and taking chances. I don’t know the price points of the 150 LED’s or the time you are putting into it but depending on the setup it may be worth it to run a separate line or use a more robust power converter then a simple rectifier circuit. Then again it may be all you need.
Another quick question, do I have to install a resistor to limit the current? The adapter that came with the lights was supplying 3 amps. Not sure if I need to limit the current to 3 amps too.
Thanks in advance.
Yes, LEDs always need resistors unless the power supply was designed to output a limited current (i.e. powers supplies designed to drive LEDs, or in some cheap LED flashlights, a high internal resistance button cell battery), or they will have a very short life (up to instantaneously burning out). Unless the LEDs have built-in resistors (that is, not individual LEDs but lamp assemblies; some individual LEDs do have internal resistors); check the specs to see what they say.
It is also better, if you are using individual LEDs, to wire them in series and use a resistor for the whole string, with the number of LEDs per string dependent on the voltage drop (anywhere from about 1.7 volts for red to 4 volts for blue/white; this varys with color and even with individual LEDs of the same color/type, although I have found them to be pretty consistent if they were bought together). For example, for 12 volts and LEDs with a 3.5 volt drop, 3 LEDs can be put in series for 10.5 volts, with a 75 ohm resistor for 20 mA of current (otherwise, if each LED were connected separately, you’d need three times as much current and waste over 5 times as much power in the resistors). Remember though, if you are rectifying 12 vac, it will be more like 15 volts after rectification, which will triple the current from that expected at 12 volts (12 - 10.5 = 1.5 v vs 15 - 10.5 = 4.5 v, across the resistor), so the single LED per resistor would perform better with an unregulated power supply (12 - 3.5 = 8.5 vs 15 - 3.5 = 11.5, a 35% increase in current).
Also, the current rating on the adapter is just what it can supply without overheating/blowing internal fuses, just as your service panel might be rated at 200 amps but only the power you need is drawn. 3 amps would be the current drawn by 150 LEDs at 20 mA each (or 1 amp if they were in 50 series strings of 3 LEDs each).
I am looking to do similar. I am changing out 10 x 12V 4W incandescent Moonrays bulbs for 12VDC 1.5W LEDs. Their transformer is 48W 12VAC
I am going to give this 12VAC-to-12VDC adapter a try: www.superbrightleds.com 12V AC to DC Converter Module $12.95 It is rated 3 amps & waterproof for outdoor use…Ideal for this purpose!
LED’s are driven off carefully controlled currents, as the LED’s have no resistance…
You can send it various voltage … 10 to 14… and get the same brightness.
So diode and capacitor is all you need.
Well what happens is that the ripples produced upset the switch mode power supply in the LED, so you want it well regulated , but it doesn’t really matter what the voltage is. But you do need the voltage quite smooth… so a one diode, and huge capacitor could do it.
Or bridge rectifier and large capacitor
Or rectifier, medium capacitor, regulator … But you need to ensure the regulator voltage is small enough that its below the ripple.
it’s not that they have no resistance, it’s that they’re negative temperature coefficient (NTC) devices. their resistance drops as they heat up, and burn out via thermal runaway in short order.
Correct.
If you put a stiff voltage source across an LED, the LED will eventually burn up. This is due to the LED’s NTC and positive feedback. As the LED heats up, its resistance decreases. As the resistance decreases, the power dissipation increases. It heats up even more, the resistance decreases even more, etc. etc.
The best way to power an LED is with a constant current. My favorite circuit for doing so is an LM317 configured as a constant current source.
You often see LEDs powered with a voltage source and series resistor. With this design, the LED is not powered by a constant current and it is not powered by a constant voltage - it is sort of “in between” these two. When the voltage of the voltage source is relatively high and the value of the resistor is also high, it roughly approximates a constant current source, and it is a safe design. When the voltage of the voltage source is relatively low and the value of the resistor is also low, it roughly approximates a constant voltage source, and thermal runaway could occur.