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Help, limiting AC Voltage after bridge rectifier

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zeox

New Member
Hello guys,

I have in my posession some 3W LED's that are designed to work on mains voltage (230V) in series with a resistor (1 resistor per 1 LED).
I've done a few test boards with them and they light up very well, but they have some noticeable flicker.
Another problem is, that one LED paired with a 1,6 kOhm resistor will be 2.8W at 220V but will be just 2.1W at 210V (just an example, there are different resistor values for different voltages and desired power outputs). And where I am using these LEDs, the voltage is very unstable, sometimes goes as low as 190V, and then the LEDs work at only ¬1.2W
I think that by adding a bridge rectifier, that would greatly reduce the flicker, and will increase the voltage way over 230V.
And I need a way to limit the voltage after the rectifier to steady 220V (or 210V doesn't matter as long as it is stable, I can use the correct resistor ratings to get the desired output).

What would be the best way to limit the Voltage after the bridge rectifier?
 

spec

Well-Known Member
Most Helpful Member
Hi Z,

It is very important to keep problems and solutions completely separate.

So what I recommend that you do is inform us of what exactly your LED is and how it is wired up.

For sure- you do not have a single LED (you may have a LED array) dissipating 3W and being being driven from 230V via a 1.6K resistor.

One thing to bear in mind is that, because the eyes impression of light (logarithmic), it takes a relatively large change in LED power to be noticeable as a change in light output.

spec
 
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zeox

New Member
Hi, spec, and thanks for your response.

So the LEDs I am using are these ones:
http://octa-electronics.com/web/files/product_groups/53/files/Data Sheet BA2 v1.2__.pdf

They are not the standard LEDs as you see.
Here is another document for the recommended resistor values to use with them:
http://octa-electronics.com/web/files/product_groups/53/files/BA2_Recommended Resistors Values-V1.1.pdf

My experiments with those show slightly different results, but generally 1,5/1,6/1,8 kOhm are fine.

The wiring is pretty simple. A resistor is connected to the anode, and you plug the whole thing to mains power. An array is made by connecting more of these blocks in parallel.

I've made a couple of small lights (using just 3 LED's) to light up my CNC router and work table. I also made one larger board with 24 of those LEDs.
I don't have a picture of them now. But I've found a picture of the first board a created for 3 of those LEDs.
platka.jpg
As you see it is pretty simple. The two leads from mains power go to the bottom left and right. The lumpy piece of copper left in the middle has been left out to serve as heat spreader. The bottoms of the LEDs are soldered to it.
 

AnalogKid

Well-Known Member
Most Helpful Member
Assuming there is not a bridge already inside the LED device, a bridge rectifier alone will not increase the peak voltage at all, and will double the frequency of the LED flashing. The bad news is that the power dissipation in both the LED and the resistor will double from what is on the charts.

ak
 

zeox

New Member
Hm, So it is not as I thought...

But doubling the power dissipation may not be as bad as it sounds.
If I nontheless add a rectifier bridge, and some sort of Voltage limit, that would supply only 190V to the LED's (even if the mains Voltage is 220V), I could use a 480-500 Ohm resistor, to give me ~1,5W of power dissipation, which when doubled would make a comfortible 3W.
And perhaps a parallel wired non polar capacitor would further help to reduce the flicker
 

audioguru

Well-Known Member
Most Helpful Member
Didn't anybody look at the schematic of the LED in its datasheet? It shows two strings of LEDs with the polarity of one string the opposite to the polarity of the other string so that with the strings in parallel they flash at 100Hz when the mains is 50Hz. If you feed the LED with rectified AC then one string will flash at 50Hz and the other string will not light so you get more flashing and at half brightness.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

Yes i agree, you cant use a rectifier with this LED. It's made for AC not DC so you must feed it with AC if you want to get the full brightness.
If you did use DC you'd have to use a capacitor with that, and then you'd have to be careful not to over power it with too high of an average DC voltage. Max power then would be about 2.5 watts but only half of the LEDs inside would be lighting up. So best bet is to just follow the recommended hook up which is with just one resistor, looks like 1640 ohms at 230vac. 1800 ohms would allow it to run a little bit cooler. Might get away with a 1/2 watt resistor but check that first.
 

AnalogKid

Well-Known Member
Most Helpful Member
Saw the schematic on page 4, missed the one on page 8. oops. So, no bridge, and no filter capacitor. The good news is that it looks like a TRIAC dimmer will work with this part.

ak
 

zeox

New Member
Thanks guys.
I myself didn't see this small schematic on page 8 :) I thought that I shouldn't connect those LEDs in series, but I'll give it a try. Strange why the resistors are not shown on this schematic. Does it mean that I should power them without a resistor?
And yes, TRIAC dimmers do work with this :)
As for resistors, It was hard, but I finally found some 3W SMD resistors, they are pretty expensive though. I should probably give it a try with 1W resistors, if it works it will be a lot cheaper.

And as for the low voltage issue.... Can I place something to limit the voltage to 190V? This way I would get equal light output, when Hooked up to a propper 230V mains, or a unstable one that fluctuates from day to day.
 

audioguru

Well-Known Member
Most Helpful Member
The schematic on the datasheet shows what is in the LED. They wrongly show it connected to L and N without a current-limiting resistor. The resistor must be added by you and since it gets hot then it is not included in the LED.
If you limit the voltage to 190V then the LED will be dim all the time but you can contact the manufacturer to ask for the list of recommended resistor values.
 

zeox

New Member
Yes, It will be dim. I already saw this effect on the crappy mains in my storage yard.
But I can get it to work at higher brightness with a lower Ohm resistor.

The idea is to make these lamps work equally well on 190V and 230V. So if I am able to limit the voltage to 190V and the LEDs have resistors for this voltage, the lamps should light up equally well on 190V/200V/210V mains and on 230V mains.

And what can I use to limit the flicker? Even at 230V/50Hz it is well noticeable. And when hooked to 190V with slightly lower frequency (~42Hz) the flicker is awful!

P.S. Here is my design for the PCB with the 24 LEDs. the traecs and the solder mask. The large circles in three rows (top, middle and bottom) are for bolting holes.
PCB.jpg solder mask.jpg
 
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audioguru

Well-Known Member
Most Helpful Member
The spec's for the LED show its range of brightness with 200VAC as the minimum voltage. Maybe you will be lucky and find better than typical LEDs or buy thousands of them, test all of them and pick out the best ones.
 

zeox

New Member
Well, they light up at 190 VAC with 1,8 kOhm resistors. That is tested and confirmed.
 

spec

Well-Known Member
Most Helpful Member
Hi, spec, and thanks for your response.

So the LEDs I am using are these ones:
http://octa-electronics.com/web/files/product_groups/53/files/Data Sheet BA2 v1.2__.pdf

They are not the standard LEDs as you see.
Here is another document for the recommended resistor values to use with them:
http://octa-electronics.com/web/files/product_groups/53/files/BA2_Recommended Resistors Values-V1.1.pdf

My experiments with those show slightly different results, but generally 1,5/1,6/1,8 kOhm are fine.

The wiring is pretty simple. A resistor is connected to the anode, and you plug the whole thing to mains power. An array is made by connecting more of these blocks in parallel.

I've made a couple of small lights (using just 3 LED's) to light up my CNC router and work table. I also made one larger board with 24 of those LEDs.
I don't have a picture of them now. But I've found a picture of the first board a created for 3 of those LEDs.
View attachment 104062
As you see it is pretty simple. The two leads from mains power go to the bottom left and right. The lumpy piece of copper left in the middle has been left out to serve as heat spreader. The bottoms of the LEDs are soldered to it.
Hi Zeox,

Thanks for the above information- very useful.:)

I have looked at various ways to do what you want and it seems to boil down to two main approaches:
(1) Replace the resistor in series with the LED string with a constant current generator
(2) Drive the LED strings (with resistors in series as normal) from an inverter which would provide a constant 240V AC power line.

As far as I can tell at this stage, option (1) would require 2 x MOSFETS, 2 x BJT (normal transistors), 2 x rectifier diodes, 2 x Zener diodes, and 6 resistors.

Option (2) can have a few implementations but the simplest/easiest would be to have a mains to DC power supply to produce a stable 12V. This 12V would then power an inverter which would produce a constant 250V AC to power your LED strings.

Both the power supply and inverter are freely available standard items at a reasonable price.

A square wave inverter should eliminate the LED flicker that you mentioned.

If you are interested in either of these options please let us know.

spec
 
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aardyvarky

Member
Zeox,

Do you have lots of these LED arrays already and are looking to do something with them?
Are you trying to create a home-made light (or lights) using one pcb with 3 arrays on each (or can we vary the number of LED arrays for each light)?
With the 1K8 resistor in series, please measure the r.m.s. voltage across the supply and the LED array.
Do same measurements with a 2K resistor.
I'm wondering if the simplest solution would be to use a 1k 5W rheostat in series with a 1k resistor and set your brightness as required at the time?
 

zeox

New Member
Hi I have, about 500 of those. I also have lots of cheap chinese DC leds, but I am quite intrigued by those. I'm doing different PCB's with them, for different applications.
So far I've done a couple a small lights for the inside and one larger projector for the outside.

spec, The idea for a constant current generator is interresting.
A 45mA constant current split between 3 parallel wired LEDS, should give each of them ~15mA without a resistor. I pretty much suck at AC calculations. But I have lots of those, so I can experiment :D
 

audioguru

Well-Known Member
Most Helpful Member
How will you split the 45mA for equal 15mA in each of the three LEDs? Like all LEDs, these ones have a range of forward voltage, the lowest voltage one will hog most of the current. But you might be lucky and get matched LEDs.
 

spec

Well-Known Member
Most Helpful Member
WARNING: The circuit in this post involves dangerous voltages. You must observed safety precautions. Never touch the circuit when the power is applied. If you are not experienced in high voltage electronics do not attempt to build this circuit.

POST ISSUE 3 of 2017_02_07

Hi Zeox,

Below is the schematic for a constant current LED driver:

2017_02_06_ISS1_ETO_CONSTANT_CURRENT_LED_STRING_DRIVER_V2.png
NOTES
(1) The circuit above will drive a constant 24 mA into one single LED unit.
(2) The two LED strings shown on the schematic represent the innards of one of your LED units.
(3) Constant current is a technical term and does not mean that the current will always be present but the circuit will ensure that each LED string in the LED unit will get a constant current and longer during each half cycle of the mains supply.
(4) The circuit will provide a constant current irrespective of the mains voltage variations, within limits of course.
(5) The constant current is defined by the formula, Ik = 0.6V/R2, where Ik is in Amps and R3 is in Ohms.
(6) The NMOSFET will get warm so should be in free cool air. A heatsink is advisable.
(7) The NMOSFET is a high-voltage, low-threshold voltage type- no other type of MOSFET should be used without checking with the designer.
(8) There are safety aspects to the circuit and the fuse must not be omitted.
(9) Q2 can be pretty much any small-signal transistor but the higher the voltage, VCEo, the better. In addition to the ideal transistor (BC546) shown on the schematic, other transistors can be used: BC547/8/9, 2N2222, PN2222, BC337, BC182/3/4, BC107/8/9
(10) If you would like to drive more than one LED unit in parrallel with this circuit we can discuss how that can be done, but ideally one constant current driver for each LED unit would give the best performance.

DATASHEETS
 

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zeox

New Member
How will you split the 45mA for equal 15mA in each of the three LEDs? Like all LEDs, these ones have a range of forward voltage, the lowest voltage one will hog most of the current. But you might be lucky and get matched LEDs.
Well I am not quite sure, but at least I can try. This works well for DC LEDs, it wont hurt me a lot if I burn out a couple of those (Actually I've already burned some:D).

spec, Thanks for the schematic. I'll give it a try soon. I'll only change LEd arrangement to parallel, as I think those don't work well in series (but then again I could try that too :) )
 

tcmtech

Banned
Most Helpful Member
As with you I played around with line powered LED strings for a long time.
My preference was to use simple capacitive impedance type power circuits being they are by far the most efficient and typically pretty forgiving to moderately wide input voltage swings.

All you need is a non polar (AC rated) capacitor of a specific value and bridge rectifier and electrolytic capacitor and a small low value resistor (all items easily scavenged from most old CFL power supplies) to make a solid reliable flicker free LED driver circuit. ;)

https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwjLkbvI4PzRAhUn54MKHfP0DOUQjRwIBw&url=http://www.electroschematics.com/5678/capacitor-power-supply/&bvm=bv.146094739,d.cGw&psig=AFQjCNFcJOH89S9yvXB_Eol5vyiCBDuqhg&ust=1486514592350068

Basically this (ZD, R1, MOV, and 4 A fuse are optional) with the right C1 and C2 values to match the supply current to your LED strings forward voltage drop based on your line voltage and frequency.
 
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