Driving LEDs on AC

[SFM-Ptgal]

Besides the blinking @the mains frequency is there any major issue about feeding leds with ac voltage?

Here are the factors:
156 leds, connected in series. If each led uses 3 volts that means that all will use 468v.
Since there's no way to get 468v I'll have to divide the 156 leds by 3 wich gives me 52 leds. Those 52leds*3v=156v. This way I'll have all the sections of the words equally lighten. remember that I'll use ac voltage.

Assuming that all I've written above is correct (correct me if not) How do I get 156v from 220? Can anyone here share some light, please? :

I'm already running 4 leds in series with 10v ac fed onto them on my Stereo 80 Harris console relay board.

 

[rrsounds]

Besides the blinking @the mains frequency is there any major issue about feeding leds with ac voltage?

Here are the factors:
156 leds, connected in series. If each led uses 3 volts that means that all will use 468v.
Since there's no way to get 468v I'll have to divide the 156 leds by 3 wich gives me 52 leds. Those 52leds*3v=156v. This way I'll have all the sections of the words equally lighten. remember that I'll use ac voltage.

Assuming that all I've written above is correct (correct me if not) How do I get 156v from 220? Can anyone here share some light, please? :

I'm already running 4 leds in series with 10v ac fed onto them on my Stereo 80 Harris console relay board.

Unless the LEDs are white or blue, the voltage drop for each will be considerably less than 3.0V. Reds even less than greens.
But if your LEDs do indeed drop 3V per light, then why not divide into two groups instead of three? 468/2=234, which is really close to 220.

Back to the real world: 220 VAC is 220V *RMS,* which means the peak voltage will be substantially higher. Do the math yourself, and you may be surprised at what you find.
Keep this in mind: LEDs don't care about RMS, and they WILL pop when the peak voltage is too high.

Another "BTW" ...since LEDs are diodes, your 50 Hz supply is going to be reduced to 25Hz flashes, which is a very noticeable blink. As you may have noticed...

One last thing: LEDs work *really* well with DC! Why not use a simple power supply, and avoid all the high voltage/AC drive problems? Otherwise, you may want to be prepared for a multitude of sacrificial parts.

Kind Regards,
David

 

[rew]

I would wire it for lower voltage. I tried something like that a few years back and it didnt work to well. If fed from the AC line a little surge on the line can wipe out a lot of diodes in just a few micro seconds. If you want it to last wire it for a lower voltage and use a DC power supply. 52 strings of 3 leds each fed with 9 volts.(If they are 3 volt leds) I dont know how much current the leds use. I am guessing around 50ma. If this is correct you will need a power supply that will furnish about 2.5 to 3 amps at around 9 volts.
Extra work and cost a little more but it will be money and time well spent if you want it to last.
Robert

 

[vetguy]

To cut down the amp rating of the power supply, many displays that have a substantial number of LED's in them, pulse the LED's on and off at a frequency/duty cycle just above what the human eye can see. Again, more work and parts but cut's down the heat, heatsink, and amps required in the power supply.

 

[rrsounds]

To cut down the amp rating of the power supply, many displays that have a substantial number of LED's in them, pulse the LED's on and off at a frequency/duty cycle just above what the human eye can see. Again, more work and parts but cut's down the heat, heatsink, and amps required in the power supply.

Right!
I remember reading somewhere that pulsing at just the right frequency will actually *appear* brighter than straight DC, while using less power.

Kind Regards,
David

 

[boiseengineer]

Traffic lights use the pulsed method, much to the displeasure of car AM listeners.

 

[Neil E.]

Driving LEDs is not just a matter of supplying a voltage across them since they are essentially current mode devices. This means that you should control the current through them, usually with a series resistor. Typically, LEDs use from 5 to 40 mA, the more current the brighter and subject to the maximum current rating.

Once the correct current is chosen, from the data sheet you can estimate the forward voltage drop. For several LEDs in series with one resistor, calculate R = (Vsupply-N*Vled)/I where I is the current and N is the number of LEDs in series. The total voltage drop across all the LEDs has to be less than the supply voltage. The resistor power rating is then calculated by I^2*R.

Without a series resistor or some means to control the current it is likely that the LEDs will be damaged.

Another issue for AC is that LEDs have a very low reverse breakdown voltage, on the order of a few volts. This is very easily exceeded in AC applications and can be handled by protective diodes, or by wiring the LEDs in pairs in parallel with the polarity reversed so one in a pair lights on the positive half cycle, and the other in the pair lights in the negative half cycle, though with this scheme the reverse breakdown voltage can still be exceeded.

I do not mean to discourage you from your project but rather to point you in the right direction so you will have success. What I described sounds pretty involved but it is not if you have some familiarity with electronics and know what to look for. Perhaps you could find someone with technical knowledge to help you if you are having trouble. I helped a friend design a motocycle LED tail lighting system which he wanted to build from scratch and it was a success. He is a ME and I an EE and together we got it done.

Neil