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View Full Version : Need to convert 12VAC to 12VDC & cheap = good


kanicbird
04-27-2012, 01:16 PM
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?

beowulff
04-27-2012, 01:18 PM
Use an LM7812-type regulator, a bridge rectifier and 2 capacitors.

ETA: here’s (http://www.phoenixcomputerlabs.com/All-About-Mixers/images/LM7812-1Amp-12v-Power-Supply.gif) 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).

Crafter_Man
04-27-2012, 04:46 PM
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.

figure9
04-27-2012, 05:27 PM
Go to a thrift store. Look at toys and computer printers. You may find a 12VDC adapter ready to go.

figure9
04-27-2012, 05:32 PM
Or this place might work http://www.goldmine-elec-products.com/prodinfo.asp?number=G17259

Dr. Strangelove
04-27-2012, 05:50 PM
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 (http://www.ebay.com/itm/LM2577-DC-DC-Converter-3-34V-to-4-35V-15W-Step-up-Power-Supply-Boost-Module-/150803933025?pt=LH_DefaultDomain_0&hash=item231c9d6761#ht_4468wt_1196). 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.

beowulff
04-27-2012, 06:16 PM
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 (http://www.ebay.com/itm/LM2577-DC-DC-Converter-3-34V-to-4-35V-15W-Step-up-Power-Supply-Boost-Module-/150803933025?pt=LH_DefaultDomain_0&hash=item231c9d6761#ht_4468wt_1196). 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.

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

Dr. Strangelove
04-27-2012, 06:20 PM
Careful.
That is not a buck/boost converter. Itís boost-only, which means that the input voltage must be lower than 12v. If itís higher, the output voltage will rise.

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.

kanicbird
04-27-2012, 06:24 PM
Where the heck are you getting a 12VAC source?

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.

beowulff
04-27-2012, 06:39 PM
How much current do your new LEDs require?

kanicbird
04-27-2012, 07:09 PM
How much current do your new LEDs require?

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

Dr. Strangelove
04-27-2012, 07:13 PM
It appears like 20mA x 5 LED's, so if I am reading this right 100mA's for all.

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.

kanicbird
04-28-2012, 07:05 AM
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?

kanicbird
04-28-2012, 07:13 AM
Looking at the Radio Shack site I see this rectifier:

http://www.radioshack.com/product/index.jsp?productId=2062581

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

johnpost
04-28-2012, 07:22 AM
you would need a capacitor in addition to that rectifier.

beowulff
04-28-2012, 08:38 AM
Looking at the Radio Shack site I see this rectifier:

http://www.radioshack.com/product/index.jsp?productId=2062581

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

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.

Crafter_Man
04-28-2012, 09:33 AM
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 (http://i1103.photobucket.com/albums/g471/ohio_rifleman/Slide1.jpg)
Circuit Idea 2 (http://i1103.photobucket.com/albums/g471/ohio_rifleman/Slide2.jpg)
Circuit Idea 3 (http://i1103.photobucket.com/albums/g471/ohio_rifleman/Slide3.jpg)

engineer_comp_geek
04-28-2012, 01:17 PM
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:
http://upload.wikimedia.org/wikipedia/commons/thumb/a/a3/RC_Filter.png/400px-RC_Filter.png

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.

Dr. Strangelove
04-28-2012, 01:30 PM
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.

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 (http://www.ebay.com/itm/Auto-Buck-Boost-Solar-Power-Voltage-Regulator-3-35V-to-1-2-30V-1A-DC-Converter-/280867874670?pt=LH_DefaultDomain_0&hash=item416507a76e#ht_5304wt_902) supports both up and down regulation. You'll still need a rectifier and a cap, though.

Michael63129
04-28-2012, 02:13 PM
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).

Crafter_Man
04-28-2012, 04:59 PM
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.
See the pics in post #17.

kanicbird
04-30-2012, 05:11 PM
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.

sameershah
06-21-2012, 01:53 PM
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

kanicbird
06-21-2012, 02:31 PM
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.

sameershah
06-21-2012, 02:50 PM
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 http://www.radioshack.com/product/index.jsp?productId=2062580

Not sure how big a capacitor and diodes I need. Would these Rectifier Diodes work? They are rated at 6 Amps. (http://www.radioshack.com/product/index.jsp?productId=2062591)Hopefully somebody will answer that question.

kanicbird
06-21-2012, 03:12 PM
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 http://www.radioshack.com/product/index.jsp?productId=2062580

Not sure how big a capacitor and diodes I need. Would these Rectifier Diodes work? They are rated at 6 Amps. (http://www.radioshack.com/product/index.jsp?productId=2062591)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.

sameershah
06-21-2012, 03:16 PM
Will these work in the configuration mentioned in post# 17
Full Wave Rectifier:
http://www.radioshack.com/product/index.jsp?productId=2062580

Diodes : http://www.radioshack.com/product/index.jsp?productId=2062591

Capacitor:

http://www.radioshack.com/product/index.jsp?productId=2102508&filterName=Brand&filterValue=RadioShack

sameershah
06-21-2012, 03:18 PM
Thanks Kanicbird. Just saw you reply after my post.

Michael63129
06-21-2012, 03:53 PM
Will these work in the configuration mentioned in post# 17
Full Wave Rectifier:
http://www.radioshack.com/product/index.jsp?productId=2062580

Diodes : http://www.radioshack.com/product/index.jsp?productId=2062591

Capacitor:

http://www.radioshack.com/product/index.jsp?productId=2102508&filterName=Brand&filterValue=RadioShack

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 (http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_607814_-1) (with superior specs to what is probably a generic capacitor, if a bit lower voltage, but still enough) and 8 amp bridge rectifier (http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_1545416_-1) 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).

kanicbird
06-21-2012, 04:22 PM
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.

sameershah
06-27-2012, 02:53 AM
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.

Michael63129
06-27-2012, 03:44 PM
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).