Need to convert 12VAC to 12VDC & cheap = good

Factual Questions
1

I have some 12VDC colorchanging LED’s and I have a available 12VAC source, I need a way to combine the 2.

I could do this with a single diode but I don’t want any flicker effect.

I know there is a simple circuit I could build with diodes and a capacitor which is a option, but would need help in getting the correct ones for this purpose.

Or is there a simple and cheap premade device to do exactly that, which would be ideal?

Also what would happen if I hooked up the LED’s direct to the 12VAC as they are diodes already, it would seem like it may work but have the flicker effect?

2

Use an LM7812-type regulator, a bridge rectifier and 2 capacitors.

ETA: here’s a schematic - ignore the transformer (you already have that).

EETA: Remember to put the regulator on a heatsink if you are going to dissipate more than a Watt or so (power dissipated by the regulator is Vdrop from input to output of the regulator X the current the LEDs are using).

3

With full-wave regulation and capacitor filtering, a 12 VAC transformer will generate a no-load voltage of around 15.6 VDC. The voltage will quickly drop from this value as the load resistance is decreased. If the input voltage to the LM7812 drops below 14 VDC the LM7812 will stop working. That’s not much of a margin. A bigger capacitor can help, but only up to a point. A 14 VAC to 18 VAC transformer would be more suitable.

4

Go to a thrift store. Look at toys and computer printers. You may find a 12VDC adapter ready to go.

5

Or this place might work http://www.goldmine-elec-products.com/prodinfo.asp?number=G17259

6

Where the heck are you getting a 12VAC source?

A rectifier and capacitor would work, but you’d never get really clean output. Your best bet might be a full-wave rectifier and something like this. It handles anywhere from 3-34 VDC input and 4-35 VDC output. Depending on the input frequency and output current, the unit’s internal capacitors may be enough to handle the low-voltage periods; otherwise, an extra capacitor would do the trick.

7

Careful.
That is not a buck/boost converter. It’s boost-only, which means that the input voltage must be lower than 12v. If it’s higher, the output voltage will rise.

8

Good point. If there’s enough capacitance, that shouldn’t be a problem, plus there will be some voltage drop from the rectifier. But if you wanted to be extra careful you could put an LM7809 or some such on the input.

9

It’s from a low voltage landscape lighting system, which I would like to also add in these color changing LED’s but the system operates at 12VAC. I don’t want to run another wire just for these LED’s, and also would like the LED’s to light up when the other lights come on and not have 2 different timers.

10

How much current do your new LEDs require?

11

It appears like 20mA x 5 LED’s, so if I am reading this right 100mA’s for all.

12

Well, it depends on whether they’re wired in series or parallel. However, if these are RGB color changers, then they’ll need at least 3 volts or so, which all told exceeds 12 VDC. So there’s a good bet the rest of the voltage is handled via current-limiting resistors.

With that low of a current rating, I’d just stick with a full-wave rectifier and a capacitor. You wouldn’t need much of a cap to handle that low of a current.

13

The LED’s came prewired individually (not linked together) , they are for now each hooked up to 12VDC for testing - right now using a transformer though that’s not how I want to run them in the garden as I want to use this existing 12VAC source. So I assume there is a resistor.

Thanks for the help so far, but these questions remain.

Should I get a rectifier or make one? Doing a quick search it looks like rectifiers are made to attach to a PCB instead of being wired, also some of them appear to be 3 wire instead of 4 and I’m not sure how that would work.

What happens if I hook up the LEDs directly to the 12VAC? Will I damage them?

14

Looking at the Radio Shack site I see this rectifier:

Would that do what I need it to and would I need a capacitor or is it part of the rectifier?

15

you would need a capacitor in addition to that rectifier.

16

You NEED a regulator!
If you rectify and filter 12v, you will generated nearly 17v! (You are capturing the peaks of the AC sinewave). This will likely destroy your LEDs.

17

A 12 VDC regulator is best, obviously. But as I mentioned above, using something like an LM7812 with a 12 VAC transformer might not work due to the regulator’s dropout voltage.

If the OP does not want to use a regulator chip, there may be some ways of getting around it (albeit less efficient and less elegant). Because I do not know the precise specs for the transformer and load, I will not guarantee any of these will work right off the bat; some experimentation is probably in order:

Circuit Idea 1
Circuit Idea 2
Circuit Idea 3

18

There are a lot of different types of power supply circuits.

You could just stick a capacitor on the end of your single diode, like this circuit:
http://www.electronic-circuits-for-hobbyists.com/images/half-wave-rectifier-circuit-diagram.JPG

This circuit actually works fine for some applications, and it has the advantage of being very simple and very cheap. The disadvantage is that the single diode makes what is called a half-wave rectifier, meaning that it simply chops off and ignores the negative half of the AC sine wave. This makes it less efficient than a full wave rectifier and it will also be able to supply less DC power than a full wave rectifier, all other things being equal. The filter capacitor also has to be larger than the filter capacitor in a full wave rectifier, since it has to hold up the voltage for a longer period of time.

It also has a disadvantage in that there is no voltage regulation at all in it. AC power in your house isn’t 120 volts exactly (or 220 depending where you are in the world). If the power company guarantees 120 +/- 5 percent, then the voltage could be anywhere from 112 to 126 volts. This would mean your AC output of your transformer would be anywhere from 11.2 to 12.6 volts. Some circuits don’t mind that much variation. Other circuits want that 12 volts to be very close to 12 volts.

You can also use four diodes to make a full wave rectifier, like this circuit:

This has the advantage of using the entire AC wave, which means it can supply more power and it requires a smaller filter capacitor than the half wave rectifier above. It still has the disadvantage of not having any regulation in it, so the output voltage will still vary with the input AC voltage.

If you want your 12 volts to be 12 volts, and not somewhere between 11 and 13 volts, then you can add a voltage regulator, like the top circuit here:
http://www.dwengo.org/sites/default/files/domotics_circuit_0.png

You have to be careful with this, as most voltage regulators will not regulate all they way “to the rail” (up to the supply voltage). Most of them need to be a volt or two above their output voltage. If you try to drive a 12 volt regulator off of a voltage that varies between 12 and 14 volts you are going to find that it doesn’t work properly.

About capacitors: An electrolytic capacitor is good for the bulk filtering, but they don’t work very well at high frequencies. Ceramic disk capacitors work very well at high frequencies, but they are too small to be used for bulk filtering. Many power supply circuits will therefore use both types of capacitors in parallel, with the big electrolytic doing the bulk of the work and the small ceramic making sure you don’t get any high frequency weirdness.

Also keep in mind that a full wave or half wave rectifier alone, with a capacitor filter, is going to have a voltage fairly close to the peak voltage, not the average (RMS) voltage. So if you have a 12 VAC transformer output, the peak output is up close to 17 volts. Factoring in the voltage drop of your diodes and the ripple in your filter capacitor and the output voltage will probably be hovering somewhere around 15 to 16 volts. ETA: But again, without a regulator this voltage will be higher or lower depending on the AC supply voltage, so it could be anywhere between maybe 14 to 17 volts or so.

19

Eh… as best I can tell the voltage drop of that rectifier is 1.4v, so you’re really talking 15.5v. If the base voltage drop of the LED is 3.3v, then that implies a 440 ohm limiting resistor. If at 15.5v, the higher current implies a voltage of 3.5v, then the current has gone up to only 27 mA. That’s very unlikely to smoke an LED, or even significantly shorten its lifespan.

I had to guess on the voltages since I don’t have the datasheet, but I know they aren’t too far off. And if he’s still worried, he can just stick 3 LM4004s in series after the rectifier to get the voltage to drop to closer to 12.

Still, a regulator would be best. It’s a bit more expensive than the one I linked to upthread, but this supports both up and down regulation. You’ll still need a rectifier and a cap, though.

20

One thing to consider that hasn’t been mentioned yet is that small AC transformers in particular have a relatively high internal resistance, and output voltage is specified at full load, so the no load (and light load, as in the OP’s use, assuming the transformer isn’t really small) output voltage will be higher; I have seen no-load voltages as high as 20v (after rectification) on 12v transformers. Also, instead of using standard 1N400x diodes, you could use schottky diodes like 1N5819s and gain about a volt (voltage drop is more like 1 volt at load using standard diodes and 0.5v for schottky diodes).

In any case, if the load is relatively constant or doesn’t vary much (is the power going to LEDs only or is there additional circuitry being powered?), resistors may be sufficient, chosen for the middle of the expected voltage range (e.g use 16v for 18v no-load, 14v full load).

Also, while it may be overkill, if any significant current is needed, I’d make a simple switching regulator to convert to a regulated 12 v (or even a bit lower if the LEDs can run at lower voltage); one can even be built using a couple transistors, zener diode, inductor and some resistors and capacitors (i.e no IC needed).