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ac/dc conversion

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Dalork

New Member
Firstly I'll say hello as I'm new here and know very little indeed, so howdy all.

Next my question, is there a device I can buy or something easy to build to convert ac to dc without any change to the voltage. Problem is father-in-law bought a 12v ac (240v to 12v /105va) power supply and 5x 10w LED floodlights that seem to be 12v dc only (they flash extremely fast on 12v ac). It's all for outdoors garden stuff so needs to be ok for outdoor use (will be under cover though so no direct sun/water etc)
 

Dalork

New Member
Was hoping to get away somehow using what he already bought, those items are not as cheap here in Australia, most sellers of the LED lights have a built in rectifier it seems as they work on both ac/dc. Just seems the units he bought are DC only (which I believe is native for LEDs).

How many amps would 5 x 10w LED floodlights draw at max?

I was wondering if something like this **broken link removed** would work or not enough amp rating? If not I guess your right and it'll work out just as cheap to buy a new 12dc power supply or buy another 5 LED lights that run AC as both items don't cost that much really, was just trying to see if there was a cheap/easy way to use what he had already.
 

4pyros

Well-Known Member
Most Helpful Member
12 volts at 50 watts is 4.16 amps.
The converters you linked to would work but they are only rated at 3 amps so you would need more than one.
 

NorthGuy

Well-Known Member
How many amps would 5 x 10w LED floodlights draw at max?

50W I guess. You can rectify what is coming out of transformer, but you need 4 relatively big diodes and a big smoothing capacitor. With good choice of components you can get close to 12V in average, but it'll be some ripple which may or may not be harmless to your LEDs. However, these components won't be cheap.
 

tcmtech

Banned
Most Helpful Member
You can rectify what is coming out of transformer, but you need 4 relatively big diodes and a big smoothing capacitor. With good choice of components you can get close to 12V in average, but it'll be some ripple which may or may not be harmless to your LEDs. However, these components won't be cheap.

What are yo considering spendy? To me what he needs would be under $10 where I am at. 10 amp bridge rectifier is about $4 and a 4700 uf 25 volt capacitor would be at most another $4 leaving $2 for shipping.
 

NorthGuy

Well-Known Member
What are yo considering spendy? To me what he needs would be under $10 where I am at. 10 amp bridge rectifier is about $4 and a 4700 uf 25 volt capacitor would be at most another $4 leaving $2 for shipping.

My calculation was $2/diode and about $10 for 10000 uF 30V capacitor, which is about what you can buy a ready-made regulated PSU for.
 

tcmtech

Banned
Most Helpful Member
My point was the parts are pretty cheap to buy and for anyone with basic scavenging skills could be found for free.

Personally I don't consider anything under $100 to be expensive being thats the base cost for me to take the family out to eat once a week just to keep my wife off my butt. :rolleyes:

Heck $100 doesn't even buy me a full tank of gas for my pickup now either. :p
 

dougy83

Well-Known Member
I was wondering if something like this **broken link removed** would work or not enough amp rating?
That unit is only rated to 3A, and is simply an overpriced bridge rectifier. It will simply convert the AC into pulsed DC, so the 50Hz flickering you a seeing without it will be doubled to 100Hz flickering (which you may or may not notice). The 100Hz flicker will be present in "AC" LEDs too.

You can get a 10A/1000V bridge rectifier on ebay **broken link removed** for $1, incl. postage, or you can pick one up from Jaycar **broken link removed** for a couple of dollars.

If you want 12VDC without flicker, you can use three bridge rectifiers in series (with two dropping around 2V and the last dropping 1V by using just one of its diodes) with a 4700uF 16V or 25V capacitor. This will cost <$10 from Jaycar, or $4 from ebay (longer lead time). The dropping of the voltage is required because 12VAC (RMS) has a peak voltage of 17V; by connecting the capacitor after the rectifier, you get the peak voltage supplied.
 

4pyros

Well-Known Member
Most Helpful Member
Then he has to wire it all up and put it in a box and wire it between the transformer and the lights. More money and more time. Not worth the $20 to buy the right thing!
 

dougy83

Well-Known Member
Then he has to wire it all up and put it in a box and wire it between the transformer and the lights. More money and more time. Not worth the $20 to buy the right thing!
Transformers and accessories aren't particularly cheap in Australia. You are right though, it's better just to get the correct thing in the first place. You can get a 12VDC 5A supply for $30 from Jaycar.
 

Dalork

New Member
Wish I knew more, but I'm learning all the time lol. I think the transformer he bought was $70-$100ish which is why I wanted to try something cheap to help out as I'm fitting the lights and wiring them over his BBQ and table outside.

He was just trying to do what I had done, but all the LEDs I used were suitable for AC use (no flicker) we both have the same kind of garden transformer/power supply just because it's rated for outdoor use we thought might be safer/better. I think they were designed for halogen or whatever garden lighting used to use.

I have a soldering iron and am willing to learn but have no idea on how to do what dougy83 above has suggested though it sounds fairly easy-ish. I'd need a pic of how to assemble something like that to be honest though, wouldn't mind if any knew of a good website where you can learn basics or at least where to start on little projects like this and other 12v LED type stuff.
 

dougy83

Well-Known Member
If you're happy with just rectifying the AC into 100Hz pulsed DC, which I believe is all the first ebay converter unit you linked to does, then just get a bridge rectifier from the local shop. It has four terminals, each labelled with '+', '-', '~' and '~'. The two labelled with '~' connect to the 12VAC from the power supply. The '+' and '-' terminals connect to the appropriate terminals of your LEDs.
 

Dalork

New Member
You make it sound so simple? lol, I'd live to have it 12vdc without flicker or ripple, sounds like I need to do some basic research so I know or at least understand what to solder to which and where.

It may be best to just grab him a 12vdc power supply for these couple of lights for now, but I think I'd still like to learn how to convert the 12vac to stable 12vdc and besides then I might be able to put the other transformer he got to use in some other way with some more led lights. Be a shame to just throw it in the back of the shed and let it go to waste.

Thank you dougy83 and also all others for your advice and info, might have to start myself a new hobby.
 

tcmtech

Banned
Most Helpful Member
I'm thinking that once the AC is rectified the odds are the flicker rate will be well able the average persons visual sensitivity to notice.

Right now each LED is running at just over a 50% dead time which at 50 - 60 H is within our range to perceive. Once rectified it will be a 100- 120 hz flicker with a much shorter dead time that is well below our visual sensitivity. Add a few thousand microfarads of capacitance and the remaining flicker becomes tiny by our visual standards.

As far as voltage goes most 12 volt rated lighting systems are not that fussy about getting a few extra volts. As is they are obviously handing the ~17 peaks without issues.

Enclosure wise a 10 amp bridge rectifier and a standard size electrolytic capacitor are not huge devices and could more than likely be refitted into the original transformers case some where. I have a old 12 VAC 100 watt LV yard lighting power pack and that has lots of room in the compartment where the wiring goes.
 

Dalork

New Member
I already found some 4700uF 16V caps, would one of these be suitable?
If the slight increase in voltage and the flicker can't be seen/does no damage to the lamps that sounds rather simple, cheap and tidy too.
 

dougy83

Well-Known Member
Add a few thousand microfarads of capacitance and the remaining flicker becomes tiny by our visual standards.
As far as voltage goes most 12 volt rated lighting systems are not that fussy about getting a few extra volts. As is they are obviously handing the ~17 peaks without issues.
Adding the capacitor also increases the RMS voltage, which will increase the power dissipated by the LEDs. Using a 4700uF will double the power dissipated for a single 10W LED string. With 5 LED strings connected, this increase will not be as much (nor will the flicker reduction)
I already found some 4700uF 16V caps, would one of these be suitable?
If the slight increase in voltage and the flicker can't be seen/does no damage to the lamps that sounds rather simple, cheap and tidy too.
Try without first. If you do decide to add the 4700uF, you should make sure all five LED strings are attached (or use the voltage dropping technique mentioned above). The 16V caps will be working close to their rated voltage, but should be ok.
 

alec_t

Well-Known Member
Most Helpful Member
If you want to try rectifying the 12VAC, here's how you would connect things:
12V_LEDsupply.gif
The capacitor smoothes the rectifier output somewhat, to give an average voltage of about 12V across the LED array with the value shown.
There is a trade-off. A higher capacitor value would provide better smoothing (less ripple current) but would increase the average voltage (which the LEDs might not like), would cost more and would be bulkier. You could even try omitting the capacitor (which results in a 15V peak voltage but an average voltage of about 9V).
 

Dalork

New Member
Wow, that's awesome and shows me how the bits all wire and/or fit together. Thank you ever so much.

What actually is Ripple? is that the fluctuation of voltage between say 9v-15v or something else.

I'm also putting in a remote control for him to control the lights, 3 of the led flood lights on one channel and 2 on the other. Those little devices look simple enough though, so don't think I'll have any drama there. I can use a multi-meter to work the switching out.
 
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