Hi, I have a string of 1500 Christmas tree LEDs which were powered by a small mains controller which gives a number of different programmes. This unit regularly stops working (I think a safety feature cuts it out as it gets quite warm) Looking to replace it, I placed a power supply delivering 18v DC which illuminated half the LEDs, reversing the polarity illuminated the other half, hence the requirement for an AC supply. However connecting a transformer without any rectification, while illuminating all the LEDs, gives a very noticeable pulse to the light output - presumably due to the 50Hz supply delivering power to individual LEDs only 25 times per second.
Without an oscilloscope I am unable to find out if the original PSU increased the frequency in some way. Is there a simple way to deliver essentially 100hz AC to these lights in order to eliminate the flicker? All the reading I have done gives info relating to DC supplies to LEDs - I have not come across AC for LEDs before. Can anyone help me understand how the original PSU works so that I might design a simple PSU?
Many thanks in advance.

 

You can use a H-bridge to change the polarity. Use an oscilator to generate frequency high enough to drive the H-bridge

 

. However connecting a transformer without any rectification, while illuminating all the LEDs, gives a very noticeable pulse to the light output - presumably due to the 50Hz supply delivering power to individual LEDs only 25 times per second.

At 50HZ, for each string, there will be half a cycle 50 times a second. Unless you have very sensitive eyes, that is a reasonably fast frame rate. You could try connecting a 10uF capacitor with high enough working voltage in parallel with each string to smooth out the ripple a bit. Make sure you get the polarity right!

 

For me, 25Hz flicker has always been very noticeable on PAL TV in UK.

The only solution I can offer is to convert 50Hz AC supply to DC and then use a bipolar DC supply that outputs at a frequency higher than 60Hz (100Hz should do it).

A 555-timer circuit driving a half bridge or full H-bridge should work.

Edit: Stefan already said it in post #2.

 

At 50HZ, for each string, there will be half a cycle 50 times a second. Unless you have very sensitive eyes, that is a reasonably fast frame rate. You could try connecting a 10uF capacitor with high enough working voltage in parallel with each string to smooth out the ripple a bit. Make sure you get the polarity right!

I dont think you can connect the 10uF capacitor the way you want to.

 

You can connect the LED string to the outputs of a bridge rectifier at the transformer output, which will give 50Hz pulses to the LEDs,

 

You can connect the LED string to the outputs of a bridge rectifier at the transformer output, which will give 50Hz pulses to the LEDs,

Thats what he did without the rectifier. Maybe the rectifier makes it a little bit better, but it is still 50Hz

 

If you can match up pairs of LEDs such that when one is off (because of the 50 Hz power line frequency) the adjacent one will be on, thus greatly reducing the flicker. If the LEDs are multicolored it is important to match up LEDs of the same color.



No flicker, no problem since 2006

 

If you can match up pairs of LEDs such that when one is off (because of the 50 Hz power line frequency) the adjacent one will be on, thus greatly reducing the flicker. If the LEDs are multicolored it is important to match up LEDs of the same color.
View attachment 287197
View attachment 287198

No flicker, no problem since 2006

Because it isn't in a closed case (long string of christmas tree LEDs), i would at least use a transformer for safety reasons.

 

Thats what he did without the rectifier. Maybe the rectifier makes it a little bit better, but it is still 50Hz

No, without the bridge rectifier you get half-wave 25Hz pulses, since the LEDs only conduct in one direction.

 

No, without the bridge, you get half-wave 25Hz pulses, since the LED only conducts in one direction.

With the bridge only half the LED will conduct, the rest is in the wrong direction.

 

No, without the bridge rectifier you get half-wave 25Hz pulses, since the LEDs only conduct in one direction.

Also it is a 50 Hz half wave and with a bridge rectifier it will be 100 Hz.

 

No, without the bridge rectifier you get half-wave 25Hz pulses, since the LEDs only conduct in one direction.

No, each LED is lit for half of each 50 Hz cycle. That does not make it 25 Hz.

 

Also it is a 50 Hz half wave and with a bridge rectifier it will be 100 Hz.

Opps, that's true.
My math was a little off.

 

Opps, that's true.
My math was a little off.

Will a bridge not produce a voltage always positive or negative ?
does the OP now need volts to be swapping polarity, to light alternate strings of leds ?

 

Will a bridge not produce a voltage always positive or negative ?
does the OP now need volts to be swapping polarity, to light alternate strings of leds ?

You are right. The solution would not work in this application

 

does the OP now need volts to be swapping polarity, to light alternate strings of leds ?

If he does, he can use a separate bridge for each string with opposite polarity output.

 

If he does, he can use a separate bridge for each string with opposite polarity output.

I have impression its only two wires for string,
Current One direction for one string, other direction for other string.
Ive seen this used on lights before, to effectively double the flash rate,

 

At 50HZ, for each string, there will be half a cycle 50 times a second. Unless you have very sensitive eyes, that is a reasonably fast frame rate. You could try connecting a 10uF capacitor with high enough working voltage in parallel with each string to smooth out the ripple a bit. Make sure you get the polarity right!

I cannot work under std CFL's in US, the 60Hz bugs my eyes, with just the right angle I can "see" the 60Hz flicker, then headaches come.

Also to note, the 1/2 cycle that a LED turns off, it could still be emmiting light. Although most on/off times are fast compared to 60Hz, but it's a timer in the math. Rise/fall times were a concern of mine couple of years ago when building a non-multiplexed msec timer using 7 segment LED display modules.

 

I have impression its only two wires for string,
One direction for one string, other direction for other string.
Ive seen this used on lights before, to effectively double the flash rate,

These type of LED strings are wired such that the LEDs in one sub-string is connected in reverse of a second sub-string. They have a power wire and a common wire at their power connection point. Along the string there are three wires. One for the positive sub-string, one for the negative sub-string and the common. The LEDs in the entire string alternate physical positions so that consecutive LEDs light with opposite polarity. As such, they only work with AC. To work DC, the entire string would have to be rewired.