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BRIDGE RECTIFIER ? used electronic transformers 240AC in, 65va max - 12AC out,, to power LED strip lights? Answered

Hi, I need help to understand what I would need to use a bunch of these (free) old transformers to power various 12vdc LED strip lights in my garage.          I have read-up on the subject of using a bridge rectifier in each circuit to convert the AC to DC,,, BUT,, being completely useless at formula / algebra / maths, I cannot work out which bridge rectifier would work for me. I can adjust the length of LED strip to suit, but the longer - the better!  The LED strips I have are 5m long, 300leds, & the specs say 5amp for the full length. So, is there a bridge rectifier available to buy, what number is it? (same if I need anything else)  Please help,, and try not to baffle me with techy science.  Many thanks.


I would be interested to see what happens if you just apply 12V AC to the strips....

It will probably work, and would definitely work if two strips were connected in anti-parallel

I like this answer for its Zen simplicity. Don't use an external rectifier. Just let the LEDs turn on exactly half of the time, assuming the AC waveform driving them is also, positive half the time, negative the other half.

Sometimes less is more.

Something that would personally bother me would be the flickering of that arrangement, so if nothing else, getting a big premade bridge rectifier and a huge filtering capacitor would be the quick and dirty solution, although really I would instead just buy a switching power supply or LED inverter for this exact purpose.

That might be the ideal way to do things; i.e. just find some 12 VDC power bricks. I guess it would be ones rated at 5A max current, or more, for powering a full length LED string, at 5A.

I seem to recall, the old Xbox 360(r), was a particularly beefy 12 VDC power brick, capable of max current of like 12A, or 16A, or something like that. Also old computer power supplies, like the metal boxy, ATX type, kind, are also capable of suppling regulated 12 VDC, with similar maximum current.

Yet, I get the impression that OP really wanted to make use of these electronic transformer gizmos, like the one in the picture.

I don't know how many of these things OP has, "a bunch", according to the question/topic.

In my mind, I am imagining he or she, has huge quantities of them, all stacked on top of each other like poker chips. Or maybe stacked like the way Scrooge McDuck has room-sized stacks of cartoon money and gold coins. Ha!

So it would be a crying shame if the "best answer" was to use some other thing.

+2 Wow Steve

Sounds like the best solution and no need for an iffy rectifier at that frequency !!

1 and 2- :(
Tried that and the strips fail very quickly as the LED's don't like the negative push at all.

So if DA and DB are fast small junction area diodes there should be NO negative voltage against any LED...

Please click the pic to see the whole image..


You are correct here, I was refering to just the normal strips you get online.
So far I never had any strip with protection diodes included.
With DA and DB it should work just fine, without them the strips won't last long.

Checked one of the spare LED transformers I have sitting around.
Mains input, 11.5V output, load: min 15W max 35W.
With a 20W light buld the output is indeed 11.5V but as an AC at around 40kHz.
No sine wave of course, not a square either, a dirty something if you ask me LOL
Reducing the load to 10W and you get still 11.5V but with spikes going as high as 20V and the output waveform looks like it has been chopped by a cheap dimmer right after the spike.
Guess that explains why my single 7W lights died so quickly...
Rectifying and filtering the output might work to some extent IMHO not worth it, at least not with the cheap models like this one.
Another LED transformer I had around was from a panel light.
Here the output is labeld as 36V DC with a current limit of 40mA.
Was an expensive light when new but the transformer certainly was not...
Just added a bunch of resistors to get around 30mA load and the output was nothing I would call clean.
No wonder people get all sorts of interference when using LED systems extensivly around their home....

If you want to prevent premature death of your strips by single LED's failing then go for a 12V downlight transformer.
The LED types are already rectified and provide 11.5 instead of 12V output.
The loss of brightness can be neglected but the LED's last much longer.
Since you want to use several strips anyway these transformers won't complain about not having a high enough load connected.
If you want to use old 12V transformers you have then it is best to check with a multimeter how much they actually throw out.
Use a car break light bulb or reversing light bulb to give the transformer some load and check the voltage on that bulb.
If 12V or higher then forget about it, anything from 10.8 to 11.5V will be fine.
Another option is to use a so called LED Driver, these are little boards that provide a fixed output voltage and offer current limitation as well.
Most have a wide input range for the 12V versions that allow anything from around 12.2 to 24V to be used as input - double checking before ordering strongly recommended.

If your LED strips are the kind I think they are, they are the kind that want to be fed with constant voltage, specifically a constant voltage roughly equal to 12.0 volts DC, at some amount of current, around 5.0 A in the case of one full length strip.

Also note the amount of current drawn by this kind of LED strip is proportional to the length of the strip, like 50 mA per group of 3 LEDs.

(which works out to 5000 mA = 5.0 A, in the case of 100 groups of 3 LEDs, or 300 LEDs, the full length of the strip)

Anyway, I think math is unavoidable for this problem. However math like this is pretty easy for me.

I don't want to actually show you the math, because you say you don't like math.

However, my conclusion is this: The voltage output from this gizmo is going to be close to what the LED strips want, but likely a little to high for LED strips to be comforatable. Specifically I calculate:

Vout = 15.8 VDC

(15.8 is 12 times square root of 2, minus 2 times 0.6)
(15.8 = 12*(2^(1/2)) - 2*0.6 = 17.0 - 1.2)

I am guessing this estimate of, Vout = 15.8 V DC, is roughly correct, and that you want to do something to make Vout smaller, closer to 12.0 V DC.

Moreover, I claim, it really does not matter which bridge rectifier you use. The choice of bridge rectifier does not have a strong influence on the magnitude (i.e number of volts DC) of the DC voltage output.

However, there is a relatively easy way to throttle down the output voltage a little bit, and the method I have in mind, is by using a lamp dimmer. After all, this electronic transformer claims to be dimmable. It says so right on the package.

If you put a lamp dimmer on this thing's input power, its output voltage will no longer be 12 VAC. It will be a number less than that, like 10 VAC, or 6 VAC, or 2 VAC, or whatever you want it to be, in the range between 12 VAC and 0 VAC.

Also there seems to be kind of a happy coincidence between the power rating on this tranformer, namely 65 VA max, and the expected power draw of one full length LED strip, namely (12 V)*(5A) = 60 W. So I am guessing one transformer gizmo + one bridge rectifier + one filter capacitor, will comfortably power one full length LED strip.

So yeah. That is what I would try. That, and throttle the input power through a lamp dimmer, and also watch the output voltage from the rectifier with your voltmeter, when testing this out for the first time.

By the way, if what I have written is unclear, I can also provide you with a hand drawn wiring diagram, if you reply to this post, and say, "Hey, could you draw me a picture of this please?"

Regarding the specs for the bridge rectifier, that can anything with voltage rating higher than 12 volts, and current rating greater than 5 amperes. Try not to pick exactly 5 A, because that is too close to the rated current of the full LED strip, which is exactly 5A. 8 A or 10 A would be better.

Not sure how big of a filter capacitor is wanted, because I do not know how fast (in hertz, i.e. cycles per second) is the output of this electronic gizmo. It might be 400 Hz. It might be 1000 Hz. It might be, who knows?

Leaving out the filter cap is a possibility, but the LEDs will flicker at 400 Hz or 1000 Hz, or whatever, although that might not be a big deal if the flicker is too fast to see.

Also there exists the possibility that the lamp dimmer will not be needed,and the way that possibility will show itself, is if you turn the knob up to 100%, yet the LEDs and transformer still run cool and comfortable. Also if the voltage the voltmeter says it is measuring, across the LED strip, is close to 12 volts, well that is a good indicator too.

Or who knows, your LED strips might be just as happy with 16 VDC as they are with 12 VDC?

So it might be the case that no dimmer is necessary. However, my initial calculation suggests the output voltage from the rectifier is going to be little higher than 12 VDC, (i.e 15.8 VDC) which is why I am suggesting putting a lamp dimmer on the input power to throttle things down a little bit.

If as you correctly surmised the output frequency is that high.. I taught maybe non-sine wave but then they would not worry about the power factor..

Upshot is it might just work without any changes or any capacitor just as you said with no noticeable flicker.. And the average non-filtered current is reduced through pulsating DC as per my top image..

I think for these little "electronic transformer" gizmos, the output is some kind of AC, but it is AC whose frequency is determined by an oscillator inside the gizmo, in contrast to a real, like, lump of laminated steel plates and copper windings, transformer, whose output frequency is the exact same as its input. I think I read 400 Hz somewhere, today, while looking at various search results, trying to learn more about the "electronic transformer", but I am not sure if that number is common to all electronic transformers.

Also, it looks I completely forgot a factor of 2 in the flicker frequency for the LEDs. The flicker rate for the LEDs is naturally going to be twice the frequency of the input voltage to the bridge rectifier.

Regarding the quote about power factor scribbled on the nameplate, i.e. "cos(phi)=0.99", I think it makes sense if it is referring to AC on the input side, which is mains power, which we expect to be sinusoidal.

You know, I kind of feel sorry for OP, if he or she is reading this, because the initial question said OP didn't like math, and didn't want to baffled by "techy science". Yet all the discussion here so far seems to have lots of math and "techy science" stuff in it.


True we are not regarding his numerical aversion but think of the fifteen other viewers and what they are gleaning from this question..

Regarding that quote about power factor scribbled on the nameplate, i.e. "cos(phi)=0.99" It does make sense it is a primary presented PF because where it resides on the nameplate although I do not see how that would affect CRISDER application utility in any way..

Hey, do you want to see something weird? I just found this page,


and I think it is a collection of other "12 VAC", "electronic transformers", from the same family as OP's electronic transformers. I am guessing these are related, i.e. little plastic covered power converters, intended for powering 12 VAC halogen lamps.

What is really interesting about this page, is the author gives us, side by side, a picture of the plastic case of the gizmo itself, with a picture from an oscilloscope of what its output voltage waveform looks like.

And, oh wow! It is so much uglier than I thought it would be! For most of these, the waveform is like a few 100s of KHz, amplitude modulated by like a 100 Hz envelope.

But that's nothing that bridge rectifier can't fix. Maybe...

Also, another thing I keep seeing on that page, is power factor quotes, and a lot of them are close to unity, e.g. 0.98, 0.99, etc. So maybe that is something they, the makers of these electronic transformers gizmos, like to brag about, for some reason.

Cool site !

I get concerned when the switching Fq approaches 100Kc.

At those frequencies the bridge PN large junctions may not respond and become leaky capacitors..

It interests me in plotting the weight VA ratio vs Fq to see if there is a corolation..


10 months ago

Any 8 Amp 24 or more volt DC Bridge Rectifier.. Here is an 8A 100V for $1.49


Two important things starting with [1] the voltamp VA=65 (resistive -> cosL=0.99)

Means A=65/12 = 5.4 amperes which is more then enough for a full length of your 300 LED string...

Second [2] the rectified output DC peak voltage is 1.414 x VAC = 16.9 VDC !!

This fact will cause your LED string to draw more current possibly overload and damage your CRISDER !! Especially if you add the capacitor to prevent LED flashing at 100Hz...

There are several remedies for reducing the DC voltage down to 12V DC in ease of application order..

a] Some Electronic Transformers come with a trim the output adjustment.

b] Add a voltage regulator LM7812

c] Put a bunch of 5A diodes in series with the LED string..